Docs/2.12

The LTTng Documentation

Last update: 28 November 2023

Welcome!

Welcome to the LTTng Documentation!

The Linux Trace Toolkit: next generation is an open source software toolkit which you can use to trace the Linux kernel, user applications, and user libraries at the same time.

LTTng consists of:

  • Kernel modules to trace the Linux kernel.

  • Shared libraries to trace C/C++ user applications.

  • Java packages to trace Java applications which use java.util.logging or Apache log4j 1.2.

  • A Python package to trace Python applications which use the standard logging package.

  • A kernel module to trace shell scripts and other user applications without a dedicated instrumentation mechanism.

  • Daemons and a command-line tool, lttng, to control the LTTng tracers.

Open source documentation

Note:This is an open documentation: its source is available in a public Git repository.

Should you find any error in the content of this text, any grammatical mistake, or any dead link, we would be very grateful if you would file a GitHub issue for it or, even better, contribute a patch to this documentation by creating a pull request.

Target audience

The material of this documentation is appropriate for intermediate to advanced software developers working in a Linux environment and interested in efficient software tracing. LTTng is also worth a try for students interested in the inner mechanics of their systems.

If you do not have a programming background, you may wish to skip everything related to instrumentation, which often requires at least some programming language skills.

What’s in this documentation?

The LTTng Documentation is divided into the following sections:

  • Nuts and bolts explains the rudiments of software tracing and the rationale behind the LTTng project.

    Skip this section if you’re familiar with software tracing and with the LTTng project.

  • Installation describes the steps to install the LTTng packages on common Linux distributions and from their sources.

    Skip this section if you already properly installed LTTng on your target system.

  • Quick start is a concise guide to getting started quickly with LTTng kernel and user space tracing.

    We recommend this section if you’re new to LTTng or to software tracing in general.

    Skip this section if you’re not new to LTTng.

  • Core concepts explains the concepts at the heart of LTTng.

    It’s a good idea to become familiar with the core concepts before attempting to use the toolkit.

  • Components of LTTng describes the various components of the LTTng machinery, like the daemons, the libraries, and the command-line interface.

  • Instrumentation shows different ways to instrument user applications and the Linux kernel.

    Instrumenting source code is essential to provide a meaningful source of events.

    Skip this section if you don’t have a programming background.

  • Tracing control is divided into topics which demonstrate how to use the vast array of features that LTTng 2.12 offers.

  • Reference contains reference tables.

  • Glossary is a specialized dictionary of terms related to LTTng or to the field of software tracing.

Convention

Function names, parameter names, variable names, command names, argument names, file system paths, file names, and other literal strings are written using a monospace typeface in this document. An italic word within such a block is a placeholder, usually described in the following sentence.

Practical tips and notes are given throughout the document using the following style:

Tip:Read the tips.

Terminal boxes are used to show command lines:

$
#
echo Command line as a regular user
echo Command line as a the `root` user

Command lines which you need to execute as a priviledged user start with the # prompt or with sudo. Other command lines start with the $ prompt.

Acknowledgements

A few people made the online LTTng Documentation possible.

Philippe Proulx wrote most of the content, created the diagrams, and formatted the document. He’s the current maintainer of the LTTng Documentation.

Daniel U. Thibault, from the DRDC, wrote “LTTng: The Linux Trace Toolkit Next Generation — A Comprehensive User’s Guide (version 2.3 edition)” which was used to complete parts of the “Core concepts” and “Components of LTTng” sections and for a few passages here and there.

The entire EfficiOS team made essential reviews of the whole document.

We sincerely thank everyone who helped enhance the quality of this documentation.

What’s new in LTTng 2.12?

LTTng 2.12 bears the name Ta Meilleure, a Northeast IPA beer brewed by Lagabière. Translating to “Your best one”, this beer gives out strong aromas of passion fruit, lemon, and peaches. Tastewise, expect a lot of fruit, a creamy texture, and a smooth lingering hop bitterness.

New features and changes in LTTng 2.12:

Tracing control
  • Clear the contents of one or more tracing sessions without having to destroy and reconfigure them with the new lttng-clear(1) command.

    This is especially useful to clear the tracing data of a tracing session between attempts to reproduce a problem.

    See Clear a tracing session.

  • Before LTTng 2.12, the lttng-track(1) and lttng-untrack(1) commands used to add and remove process IDs (PIDs) to a whitelist so that LTTng would only trace processes with specific PIDs.

    LTTng 2.12 adds Unix user IDs (UIDs) and Unix group IDs (GIDs) to the available process attributes to track. You can specify numeric user/group IDs and user/group names to track, for example:

    $
    lttng track --userspace --vuid=http,999 --vgid=mysql,9

    While you can also track UIDs and GIDs with the --filter option of the enable-event command, this dedicated process attribute tracking approach reduces tracing overhead and prevents the creation of sub-buffers for the users and groups which LTTng doesn’t track.

    In the command manual pages, the term “whitelist” is renamed to “inclusion set” to clarify the concept.

  • The relay daemon can now maintain many files virtually opened without using as many file descriptors (FD). It does so by closing and reopening FDs as needed.

    This feature is meant as a workaround for users who can’t bump the system limit because of permission restrictions.

    The new --fd-pool-size relay daemon option sets the maximum number of simultaneously opened file descriptors (using the soft RLIMIT_NOFILE resource limit of the process by default; see getrlimit(2)).

  • By default, the relay daemon writes its traces under a predefined directory hierarchy, $LTTNG_HOME/lttng-traces/host/session/domain, with:

    host

    Remote hostname.

    session

    Tracing session name.

    domain

    Tracing domain name (ust or kernel).

    Change this hierarchy to group traces by tracing session name rather than by hostname ($LTTNG_HOME/lttng-traces/session/host/domain) with the new --group-output-by-session option of the relay daemon.

    This feature is especially useful if you’re tracing two or more hosts, having different hostnames, which share the same tracing session name as part of their configuration. In this scenario, you can use a descriptive tracing session name (for example, connection-hang) across a fleet of machines streaming to a single relay daemon.

  • The relay daemon has a new --working-directory option to override its working directory.

Linux kernel tracing
  • New instrumentation hooks to trace the entry and exit tracepoints of the network reception code paths of the Linux kernel.

    Use the resulting event records to identify the bounds of a network reception and link the events that occur in the interim (for example, wake-ups) to a specific network reception instance. You can also analyze the latency of the network stack thanks to those event records.

  • The thread field of the irqaction structure, which specifies the process to wake up when a threaded interrupt request (IRQ) occurs, is now part of the lttng_statedump_interrupt event record.

    Use this information to discover which processes handle the various IRQs. You can also associate the events occurring in the context of those processes with their respective IRQ.

  • New lttng_statedump_cpu_topology tracepoint to record the active CPU/NUMA topology.

    Use this information to discover which CPUs are SMT siblings or part of the same socket. As of LTTng 2.12, only the x86 architecture is supported since all architectures describe their topologies differently.

    The architecture field of the tracepoint is statically defined and exists for all architecture implementations. Analysis tools can therefore anticipate the layout of the event record.

    Event record example:

    lttng_statedump_cpu_topology:
      architecture: x86
      cpu_id: 0
      vendor: GenuineIntel
      family: 6
      model: 142
      model_name: Intel(R) Core(TM) i7-7600U CPU @ 2.80GHz
      physical_id: 0
      core_id: 0
      cores: 2
    
  • New product UUID metadata environment field, product_uuid, which LTTng copies from the Desktop Management Interface (DMI).

    Use this environment field to uniquely identify a machine (virtual or physical) in order to correlate traces from multiple virtual machines.

Nuts and bolts

What is LTTng? As its name suggests, the Linux Trace Toolkit: next generation is a modern toolkit for tracing Linux systems and applications. So your first question might be: what is tracing?

What is tracing?

As the history of software engineering progressed and led to what we now take for granted—complex, numerous and interdependent software applications running in parallel on sophisticated operating systems like Linux—the authors of such components, software developers, began feeling a natural urge to have tools that would ensure the robustness and good performance of their masterpieces.

One major achievement in this field is, inarguably, the GNU debugger (GDB), an essential tool for developers to find and fix bugs. But even the best debugger won’t help make your software run faster, and nowadays, faster software means either more work done by the same hardware, or cheaper hardware for the same work.

A profiler is often the tool of choice to identify performance bottlenecks. Profiling is suitable to identify where performance is lost in a given software. The profiler outputs a profile, a statistical summary of observed events, which you may use to discover which functions took the most time to execute. However, a profiler won’t report why some identified functions are the bottleneck. Bottlenecks might only occur when specific conditions are met, conditions that are sometimes impossible to capture by a statistical profiler, or impossible to reproduce with an application altered by the overhead of an event-based profiler. For a thorough investigation of software performance issues, a history of execution is essential, with the recorded values of variables and context fields you choose, and with as little influence as possible on the instrumented software. This is where tracing comes in handy.

Tracing is a technique used to understand what goes on in a running software system. The software used for tracing is called a tracer, which is conceptually similar to a tape recorder. When recording, specific instrumentation points placed in the software source code generate events that are saved on a giant tape: a trace file. You can trace user applications and the operating system at the same time, opening the possibility of resolving a wide range of problems that would otherwise be extremely challenging.

Tracing is often compared to logging. However, tracers and loggers are two different tools, serving two different purposes. Tracers are designed to record much lower-level events that occur much more frequently than log messages, often in the range of thousands per second, with very little execution overhead. Logging is more appropriate for a very high-level analysis of less frequent events: user accesses, exceptional conditions (errors and warnings, for example), database transactions, instant messaging communications, and such. Simply put, logging is one of the many use cases that can be satisfied with tracing.

The list of recorded events inside a trace file can be read manually like a log file for the maximum level of detail, but it is generally much more interesting to perform application-specific analyses to produce reduced statistics and graphs that are useful to resolve a given problem. Trace viewers and analyzers are specialized tools designed to do this.

In the end, this is what LTTng is: a powerful, open source set of tools to trace the Linux kernel and user applications at the same time. LTTng is composed of several components actively maintained and developed by its community.

Alternatives to LTTng

Excluding proprietary solutions, a few competing software tracers exist for Linux:

dtrace4linux

A port of Sun Microsystems' DTrace to Linux.

The dtrace tool interprets user scripts and is responsible for loading code into the Linux kernel for further execution and collecting the outputted data.

eBPF

A subsystem in the Linux kernel in which a virtual machine can execute programs passed from the user space to the kernel.

You can attach such programs to tracepoints and kprobes thanks to a system call, and they can output data to the user space when executed thanks to different mechanisms (pipe, VM register values, and eBPF maps, to name a few).

ftrace

The de facto function tracer of the Linux kernel.

Its user interface is a set of special files in sysfs.

perf

A performance analysis tool for Linux which supports hardware performance counters, tracepoints, as well as other counters and types of probes.

The controlling utility of perf is the perf command line/text UI tool.

strace

A command-line utility which records system calls made by a user process, as well as signal deliveries and changes of process state.

strace makes use of ptrace to fulfill its function.

sysdig

Like SystemTap, uses scripts to analyze Linux kernel events.

You write scripts, or chisels in the jargon of sysdig, in Lua and sysdig executes them while it traces the system or afterwards. The interface of sysdig is the sysdig command-line tool as well as the text UI-based csysdig tool.

SystemTap

A Linux kernel and user space tracer which uses custom user scripts to produce plain text traces.

SystemTap converts the scripts to the C language, and then compiles them as Linux kernel modules which are loaded to produce trace data. The primary user interface of SystemTap is the stap command-line tool.

The main distinctive features of LTTng is that it produces correlated kernel and user space traces, as well as doing so with the lowest overhead amongst other solutions. It produces trace files in the CTF format, a file format optimized for the production and analyses of multi-gigabyte data.

LTTng is the result of more than 10 years of active open source development by a community of passionate developers. LTTng is currently available on major desktop and server Linux distributions.

The main interface for tracing control is a single command-line tool named lttng. The latter can create several tracing sessions, enable and disable events on the fly, filter events efficiently with custom user expressions, start and stop tracing, and much more. LTTng can record the traces on the file system or send them over the network, and keep them totally or partially. You can view the traces once tracing becomes inactive or in real-time.

Install LTTng now and start tracing!

Installation

LTTng is a set of software components which interact to instrument the Linux kernel and user applications, and to control tracing (start and stop tracing, enable and disable event rules, and the rest). Those components are bundled into the following packages:

LTTng-tools

Libraries and command-line interface to control tracing.

LTTng-modules

Linux kernel modules to instrument and trace the kernel.

LTTng-UST

Libraries and Java/Python packages to instrument and trace user applications.

Most distributions mark the LTTng-modules and LTTng-UST packages as optional when installing LTTng-tools (which is always required). In the following sections, we always provide the steps to install all three, but note that:

  • You only need to install LTTng-modules if you intend to trace the Linux kernel.

  • You only need to install LTTng-UST if you intend to trace user applications.

Ubuntu: LTTng Stable 2.12 PPA

The LTTng Stable 2.12 PPA offers the latest stable LTTng 2.12 packages for Ubuntu 16.04 Xenial Xerus, Ubuntu 18.04 Bionic Beaver, Ubuntu 20.04 Focal Fossa, and Ubuntu 22.04 Jammy Jellyfish.

To install LTTng 2.12 from the LTTng Stable 2.12 PPA:

  1. Add the LTTng Stable 2.12 PPA repository and update the list of packages:

    #
    #
    apt-add-repository ppa:lttng/stable-2.12
    apt-get update
  2. Install the main LTTng 2.12 packages:

    #
    #
    #
    apt-get install lttng-tools
    apt-get install lttng-modules-dkms
    apt-get install liblttng-ust-dev
  3. If you need to instrument and trace Java applications, install the LTTng-UST Java agent:

    #
    apt-get install liblttng-ust-agent-java
  4. If you need to instrument and trace Python 3 applications, install the LTTng-UST Python agent:

    #
    apt-get install python3-lttngust

Debian

To install LTTng 2.12 on Debian 11 bullseye:

  1. Install the main LTTng 2.12 packages:

    #
    #
    #
    apt-get install lttng-modules-dkms
    apt-get install liblttng-ust-dev
    apt-get install lttng-tools
  2. If you need to instrument and trace Java applications, install the LTTng-UST Java agent:

    #
    apt-get install liblttng-ust-agent-java
  3. If you need to instrument and trace Python applications, install the LTTng-UST Python agent:

    #
    apt-get install python3-lttngust

Alpine Linux

To install LTTng-tools 2.12 and LTTng-UST 2.12 on Alpine Linux 3.12, Alpine Linux 3.13, Alpine Linux 3.14, or Alpine Linux 3.15:

  1. Add the LTTng packages:

    #
    #
    apk add lttng-tools
    apk add lttng-ust-dev
  2. Download, build, and install the latest LTTng-modules 2.12:

    $
     
     
     
     
     
     
    cd $(mktemp -d) &&
    wget http://lttng.org/files/lttng-modules/lttng-modules-latest-2.12.tar.bz2 &&
    tar -xf lttng-modules-latest-2.12.tar.bz2 &&
    cd lttng-modules-2.12.* &&
    make &&
    sudo make modules_install &&
    sudo depmod -a

Buildroot

To install LTTng 2.12 on Buildroot 2020.08, Buildroot 2020.11, Buildroot 2021.02, Buildroot 2021.05, Buildroot 2021.08, or Buildroot 2021.11:

  1. Launch the Buildroot configuration tool:

    $
    make menuconfig
  2. In Kernel, check Linux kernel.

  3. In Toolchain, check Enable WCHAR support.

  4. In Target packages → Debugging, profiling and benchmark, check lttng-modules and lttng-tools.

  5. In Target packages → Libraries → Other, check lttng-libust.

OpenEmbedded and Yocto

LTTng 2.12 recipes are available in the openembedded-core layer for Yocto Project 3.2 Gatesgarth and Yocto Project 3.3 Hardknott under the following names:

  • lttng-tools

  • lttng-modules

  • lttng-ust

With BitBake, the simplest way to include LTTng recipes in your target image is to add them to IMAGE_INSTALL_append in conf/local.conf:

IMAGE_INSTALL_append = " lttng-tools lttng-modules lttng-ust"

If you use Hob:

  1. Select a machine and an image recipe.

  2. Click Edit image recipe.

  3. Under the All recipes tab, search for lttng.

  4. Check the desired LTTng recipes.

Build from source

To build and install LTTng 2.12 from source:

  1. Using the package manager of your distribution, or from source, install the following dependencies of LTTng-tools and LTTng-UST:

  2. Download, build, and install the latest LTTng-modules 2.12:

    $
     
     
     
     
     
     
    cd $(mktemp -d) &&
    wget http://lttng.org/files/lttng-modules/lttng-modules-latest-2.12.tar.bz2 &&
    tar -xf lttng-modules-latest-2.12.tar.bz2 &&
    cd lttng-modules-2.12.* &&
    make &&
    sudo make modules_install &&
    sudo depmod -a
  3. Download, build, and install the latest LTTng-UST 2.12:

    $
     
     
     
     
     
     
     
    cd $(mktemp -d) &&
    wget http://lttng.org/files/lttng-ust/lttng-ust-latest-2.12.tar.bz2 &&
    tar -xf lttng-ust-latest-2.12.tar.bz2 &&
    cd lttng-ust-2.12.* &&
    ./configure &&
    make &&
    sudo make install &&
    sudo ldconfig

    Add --disable-numa to ./configure if you don’t have numactl.

    Java and Python application tracing

    Important:If you need to instrument and trace Java applications, pass the --enable-java-agent-jul, --enable-java-agent-log4j, or --enable-java-agent-all options to the configure script, depending on which Java logging framework you use.

    If you need to instrument and trace Python applications, pass the --enable-python-agent option to the configure script. You can set the PYTHON environment variable to the path to the Python interpreter for which to install the LTTng-UST Python agent package.

    Note:By default, LTTng-UST libraries are installed to /usr/local/lib, which is the de facto directory in which to keep self-compiled and third-party libraries.

    When linking an instrumented user application with liblttng-ust:

    • Append /usr/local/lib to the LD_LIBRARY_PATH environment variable.

    • Pass the -L/usr/local/lib and -Wl,-rpath,/usr/local/lib options to gcc(1), g++(1), or clang(1).

  4. Download, build, and install the latest LTTng-tools 2.12:

    $
     
     
     
     
     
     
     
    cd $(mktemp -d) &&
    wget http://lttng.org/files/lttng-tools/lttng-tools-latest-2.12.tar.bz2 &&
    tar -xf lttng-tools-latest-2.12.tar.bz2 &&
    cd lttng-tools-2.12.* &&
    ./configure &&
    make &&
    sudo make install &&
    sudo ldconfig

Tip:The vlttng tool can do all the previous steps automatically for a given version of LTTng and confine the installed files in a specific directory. This can be useful to test LTTng without installing it on your system.

Linux kernel module signature

Linux kernel modules require trusted signatures in order to be loaded when any of the following is true:

  • The system boots with Secure Boot enabled.

  • The Linux kernel which boots is configured with CONFIG_MODULE_SIG_FORCE.

  • The Linux kernel boots with a command line containing module.sig_enforce=1.

Example:root user running lttng-sessiond which fails to load a required kernel module due to the signature enforcement policies.

#
 
 
 
 
lttng-sessiond
Warning: No tracing group detected
modprobe: ERROR: could not insert 'lttng_ring_buffer_client_discard': Key was rejected by service
Error: Unable to load required module lttng-ring-buffer-client-discard
Warning: No kernel tracer available

There are several methods to enroll trusted keys for signing modules that are built from source. The precise details vary from one Linux version to another, and distributions may have their own mechanisms. For example, DKMS may autogenerate a key and sign modules, but the key isn’t automatically enrolled.

See Kernel module signing facility and the documentation of your distribution to learn more about signing Linux kernel modules.

Quick start

This is a short guide to get started quickly with LTTng kernel and user space tracing.

Before you follow this guide, make sure to install LTTng.

This tutorial walks you through the steps to:

  1. Trace the Linux kernel.

  2. Trace a user application written in C.

  3. View and analyze the recorded events.

Trace the Linux kernel

The following command lines start with the # prompt because you need root privileges to trace the Linux kernel. You can also trace the kernel as a regular user if your Unix user is a member of the tracing group.

  1. Create a tracing session which writes its traces to /tmp/my-kernel-trace:

    #
    lttng create my-kernel-session --output=/tmp/my-kernel-trace
  2. List the available kernel tracepoints and system calls:

    #
    #
    lttng list --kernel
    lttng list --kernel --syscall
  3. Create event rules which match the desired instrumentation point names, for example the sched_switch and sched_process_fork tracepoints, and the open(2) and close(2) system calls:

    #
    #
    lttng enable-event --kernel sched_switch,sched_process_fork
    lttng enable-event --kernel --syscall open,close

    Create an event rule which matches all the Linux kernel tracepoints with the --all option (this will generate a lot of data when tracing):

    #
    lttng enable-event --kernel --all
  4. Start tracing:

    #
    lttng start
  5. Do some operation on your system for a few seconds. For example, load a website, or list the files of a directory.

  6. Destroy the current tracing session:

    #
    lttng destroy

    The lttng-destroy(1) command doesn’t destroy the trace data; it only destroys the state of the tracing session.

    The lttng-destroy(1) command also runs the lttng-stop(1) command implicitly (see Start and stop a tracing session). You need to stop tracing to make LTTng flush the remaining trace data and make the trace readable.

  7. For the sake of this example, make the recorded trace accessible to the non-root users:

    #
    chown -R $(whoami) /tmp/my-kernel-trace

See View and analyze the recorded events to view the recorded events.

Trace a user application

This section steps you through a simple example to trace a Hello world program written in C.

To create the traceable user application:

  1. Create the tracepoint provider header file, which defines the tracepoints and the events they can generate:

    hello-tp.h

    #undef TRACEPOINT_PROVIDER
    #define TRACEPOINT_PROVIDER hello_world
    
    #undef TRACEPOINT_INCLUDE
    #define TRACEPOINT_INCLUDE "./hello-tp.h"
    
    #if !defined(_HELLO_TP_H) || defined(TRACEPOINT_HEADER_MULTI_READ)
    #define _HELLO_TP_H
    
    #include <lttng/tracepoint.h>
    
    TRACEPOINT_EVENT(
        hello_world,
        my_first_tracepoint,
        TP_ARGS(
            int, my_integer_arg,
            char*, my_string_arg
        ),
        TP_FIELDS(
            ctf_string(my_string_field, my_string_arg)
            ctf_integer(int, my_integer_field, my_integer_arg)
        )
    )
    
    #endif /* _HELLO_TP_H */
    
    #include <lttng/tracepoint-event.h>
    
  2. Create the tracepoint provider package source file:

    hello-tp.c

    #define TRACEPOINT_CREATE_PROBES
    #define TRACEPOINT_DEFINE
    
    #include "hello-tp.h"
    
  3. Build the tracepoint provider package:

    $
    gcc -c -I. hello-tp.c
  4. Create the Hello World application source file:

    hello.c

    #include <stdio.h>
    #include "hello-tp.h"
    
    int main(int argc, char *argv[])
    {
        int x;
    
        puts("Hello, World!\nPress Enter to continue...");
    
        /*
         * The following getchar() call is only placed here for the purpose
         * of this demonstration, to pause the application in order for
         * you to have time to list its tracepoints. It's not needed
         * otherwise.
         */
        getchar();
    
        /*
         * A tracepoint() call.
         *
         * Arguments, as defined in hello-tp.h:
         *
         * 1. Tracepoint provider name   (required)
         * 2. Tracepoint name            (required)
         * 3. my_integer_arg             (first user-defined argument)
         * 4. my_string_arg              (second user-defined argument)
         *
         * Notice the tracepoint provider and tracepoint names are
         * NOT strings: they are in fact parts of variables that the
         * macros in hello-tp.h create.
         */
        tracepoint(hello_world, my_first_tracepoint, 23, "hi there!");
    
        for (x = 0; x < argc; ++x) {
            tracepoint(hello_world, my_first_tracepoint, x, argv[x]);
        }
    
        puts("Quitting now!");
        tracepoint(hello_world, my_first_tracepoint, x * x, "x^2");
    
        return 0;
    }
    
  5. Build the application:

    $
    gcc -c hello.c
  6. Link the application with the tracepoint provider package, liblttng-ust, and libdl:

    $
    gcc -o hello hello.o hello-tp.o -llttng-ust -ldl

Here’s the whole build process:

Build steps of the user space tracing tutorial.

To trace the user application:

  1. Run the application with a few arguments:

    $
    ./hello world and beyond

    You see:

    Hello, World!
    Press Enter to continue...
  2. Start an LTTng session daemon:

    $
    lttng-sessiond --daemonize

    Note that a session daemon might already be running, for example as a service that the service manager of the distribution started.

  3. List the available user space tracepoints:

    $
    lttng list --userspace

    You see the hello_world:my_first_tracepoint tracepoint listed under the ./hello process.

  4. Create a tracing session:

    $
    lttng create my-user-space-session
  5. Create an event rule which matches the hello_world:my_first_tracepoint event name:

    $
    lttng enable-event --userspace hello_world:my_first_tracepoint
  6. Start tracing:

    $
    lttng start
  7. Go back to the running hello application and press Enter. The program executes all tracepoint() instrumentation points and exits.

  8. Destroy the current tracing session:

    $
    lttng destroy

    The lttng-destroy(1) command doesn’t destroy the trace data; it only destroys the state of the tracing session.

    The lttng-destroy(1) command also runs the lttng-stop(1) command implicitly (see Start and stop a tracing session). You need to stop tracing to make LTTng flush the remaining trace data and make the trace readable.

By default, LTTng saves the traces in $LTTNG_HOME/lttng-traces/name-date-time, where name is the tracing session name. The LTTNG_HOME environment variable defaults to $HOME if not set.

See View and analyze the recorded events to view the recorded events.

View and analyze the recorded events

Once you have completed the Trace the Linux kernel and Trace a user application tutorials, you can inspect the recorded events.

There are tools you can use to read LTTng traces:

Babeltrace 2

A rich, flexible trace manipulation toolkit which includes a versatile command-line interface (babeltrace2), a C library, and Python 3 bindings so that you can easily process or convert an LTTng trace with your own script.

The Babeltrace 2 project ships with a plugin which supports the format of the traces which LTTng produces, CTF.

Trace Compass

A graphical user interface for viewing and analyzing any type of logs or traces, including those of LTTng.

Note:This section assumes that LTTng saved the traces it recorded during the previous tutorials to their default location, in the $LTTNG_HOME/lttng-traces directory. The LTTNG_HOME environment variable defaults to $HOME if not set.

Use the babeltrace2 command-line tool

The simplest way to list all the recorded events of an LTTng trace is to pass its path to babeltrace2 without options:

$
babeltrace2 ~/lttng-traces/my-user-space-session*

babeltrace2 finds all traces recursively within the given path and prints all their events, sorting them chronologically.

Pipe the output of babeltrace2 into a tool like grep(1) for further filtering:

$
babeltrace2 /tmp/my-kernel-trace | grep _switch

Pipe the output of babeltrace2 into a tool like wc(1) to count the recorded events:

$
babeltrace2 /tmp/my-kernel-trace | grep _open | wc --lines

Use the Babeltrace 2 Python bindings

The text output of babeltrace2 is useful to isolate events by simple matching using grep(1) and similar utilities. However, more elaborate filters, such as keeping only event records with a field value falling within a specific range, are not trivial to write using a shell. Moreover, reductions and even the most basic computations involving multiple event records are virtually impossible to implement.

Fortunately, Babeltrace 2 ships with Python 3 bindings which make it easy to read the event records of an LTTng trace sequentially and compute the desired information.

The following script accepts an LTTng Linux kernel trace path as its first argument and prints the short names of the top five running processes on CPU 0 during the whole trace:

top5proc.py

import bt2
import sys
import collections

def top5proc():
    # Get the trace path from the first command-line argument.
    it = bt2.TraceCollectionMessageIterator(sys.argv[1])

    # This counter dictionary will hold execution times:
    #
    #     Task command name -> Total execution time (ns)
    exec_times = collections.Counter()

    # This holds the last `sched_switch` timestamp.
    last_ts = None

    for msg in it:
        # We only care about event messages.
        if type(msg) is not bt2._EventMessageConst:
            continue

        # Event of the event message.
        event = msg.event

        # Keep only `sched_switch` events.
        if event.cls.name != 'sched_switch':
            continue

        # Keep only events which occurred on CPU 0.
        if event.packet.context_field['cpu_id'] != 0:
            continue

        # Event timestamp (ns).
        cur_ts = msg.default_clock_snapshot.ns_from_origin

        if last_ts is None:
            # We start here.
            last_ts = cur_ts

        # (Short) name of the previous task command.
        prev_comm = str(event.payload_field['prev_comm'])

        # Initialize an entry in our dictionary if not yet done.
        if prev_comm not in exec_times:
            exec_times[prev_comm] = 0

        # Compute previous command execution time.
        diff = cur_ts - last_ts

        # Update execution time of this command.
        exec_times[prev_comm] += diff

        # Update last timestamp.
        last_ts = cur_ts

    # Print top 5.
    for name, ns in exec_times.most_common(5):
        print('{:20}{} s'.format(name, ns / 1e9))


if __name__ == '__main__':
    top5proc()

Run this script:

$
python3 top5proc.py /tmp/my-kernel-trace/kernel

Output example:

swapper/0           48.607245889 s
chromium            7.192738188 s
pavucontrol         0.709894415 s
Compositor          0.660867933 s
Xorg.bin            0.616753786 s

Note that swapper/0 is the “idle” process of CPU 0 on Linux; since we weren’t using the CPU that much when tracing, its first position in the list makes sense.

Core concepts

From a user’s perspective, the LTTng system is built on a few concepts, or objects, on which the lttng command-line tool operates by sending commands to the session daemon. Understanding how those objects relate to eachother is key in mastering the toolkit.

The core concepts are:

Tracing session

A tracing session is a stateful dialogue between you and a session daemon. You can create a new tracing session with the lttng create command.

Most of what you do when you control LTTng tracers happens within a tracing session. In particular, a tracing session:

  • Has its own name.

  • Has its own set of trace files.

  • Has its own state of activity (started or stopped).

  • Has its own mode (local, network streaming, snapshot, or live).

  • Has its own channels to which are associated their own event rules.

  • Has its own process attribute tracking inclusion sets.

A tracing session contains channels that are members of tracing domains and contain event rules.

Those attributes and objects are completely isolated between different tracing sessions.

A tracing session is analogous to a cash machine session: the operations you do on the banking system through the cash machine do not alter the data of other users of the same system. In the case of the cash machine, a session lasts as long as your bank card is inside. In the case of LTTng, a tracing session lasts from the lttng create command to the lttng destroy command.

Each Unix user has its own set of tracing sessions.

Tracing session mode

LTTng can send the generated trace data to different locations. The tracing session mode dictates where to send it. The following modes are available in LTTng 2.12:

Local mode

LTTng writes the traces to the file system of the machine it traces (target system).

Network streaming mode

LTTng sends the traces over the network to a relay daemon running on a remote system.

Snapshot mode

LTTng doesn’t write the traces by default.

Instead, you can request LTTng to take a snapshot, that is, a copy of the current sub-buffers of the tracing session, and to write it to the file system of the target or to send it over the network to a relay daemon running on a remote system.

Live mode

This mode is similar to the network streaming mode, but a live trace viewer can connect to the distant relay daemon to view event records as LTTng generates them.

Tracing domain

A tracing domain is a namespace for event sources. A tracing domain has its own properties and features.

There are currently five available tracing domains:

  • Linux kernel

  • User space

  • java.util.logging (JUL)

  • log4j

  • Python

You must specify a tracing domain when using some commands to avoid ambiguity. For example, since all the domains support named tracepoints as event sources (instrumentation points that you manually insert in the source code), you need to specify a tracing domain when creating an event rule because all the tracing domains could have tracepoints with the same names.

You can create channels in the Linux kernel and user space tracing domains. The other tracing domains have a single default channel.

Channel and ring buffer

A channel is an object which is responsible for a set of ring buffers. Each ring buffer is divided into multiple sub-buffers. When an LTTng tracer emits an event, it can record it to one or more sub-buffers. The attributes of a channel determine what to do when there’s no space left for a new event record because all sub-buffers are full, where to send a full sub-buffer, and other behaviours.

A channel is always associated to a tracing domain. The java.util.logging (JUL), log4j, and Python tracing domains each have a default channel which you can’t configure.

A channel also owns event rules. When an LTTng tracer emits an event, it records it to the sub-buffers of all the enabled channels with a satisfied event rule, as long as those channels are part of active tracing sessions.

Per-user vs. per-process buffering schemes

A channel has at least one ring buffer per CPU. LTTng always records an event to the ring buffer associated to the CPU on which it occurs.

Two buffering schemes are available when you create a channel in the user space tracing domain:

Per-user buffering

Allocate one set of ring buffers—one per CPU—shared by all the instrumented processes of each Unix user.

Per-user buffering scheme.
Per-process buffering

Allocate one set of ring buffers—one per CPU—for each instrumented process.

Per-process buffering scheme.

The per-process buffering scheme tends to consume more memory than the per-user option because systems generally have more instrumented processes than Unix users running instrumented processes. However, the per-process buffering scheme ensures that one process having a high event throughput won’t fill all the shared sub-buffers of the same user, only its own.

The Linux kernel tracing domain has only one available buffering scheme which is to allocate a single set of ring buffers for the whole system. This scheme is similar to the per-user option, but with a single, global user “running” the kernel.

Overwrite vs. discard event record loss modes

When an event occurs, LTTng records it to a specific sub-buffer (yellow arc in the following animations) of the ring buffer of a specific channel. When there’s no space left in a sub-buffer, the tracer marks it as consumable (red) and another, empty sub-buffer starts receiving the following event records. A consumer daemon eventually consumes the marked sub-buffer (returns to white).

In an ideal world, sub-buffers are consumed faster than they are filled, as it is the case in the previous animation. In the real world, however, all sub-buffers can be full at some point, leaving no space to record the following events.

By default, LTTng-modules and LTTng-UST are non-blocking tracers: when no empty sub-buffer is available, it is acceptable to lose event records when the alternative would be to cause substantial delays in the execution of the instrumented application. LTTng privileges performance over integrity; it aims at perturbing the target system as little as possible in order to make tracing of subtle race conditions and rare interrupt cascades possible.

Since LTTng 2.10, the LTTng user space tracer, LTTng-UST, supports a blocking mode. See the blocking timeout example to learn how to use the blocking mode.

When it comes to losing event records because no empty sub-buffer is available, or because the blocking timeout is reached, the event record loss mode of the channel determines what to do. The available event record loss modes are:

Discard mode

Drop the newest event records until the tracer releases a sub-buffer.

This is the only available mode when you specify a blocking timeout.

Overwrite mode

Clear the sub-buffer containing the oldest event records and start writing the newest event records there.

This mode is sometimes called flight recorder mode because it’s similar to a flight recorder: always keep a fixed amount of the latest data.

Which mechanism you should choose depends on your context: prioritize the newest or the oldest event records in the ring buffer?

Beware that, in overwrite mode, the tracer abandons a whole sub-buffer as soon as a there’s no space left for a new event record, whereas in discard mode, the tracer only discards the event record that doesn’t fit.

In discard mode, LTTng increments a count of lost event records when an event record is lost and saves this count to the trace. Since LTTng 2.8, in overwrite mode, LTTng writes to a given sub-buffer its sequence number within its data stream. With a local, network streaming, or live tracing session, a trace reader can use such sequence numbers to report lost packets. In overwrite mode, LTTng doesn’t write to the trace the exact number of lost event records in those lost sub-buffers.

Trace analyses can use saved discarded event record and sub-buffer (packet) counts of the trace to decide whether or not to perform the analyses even if trace data is known to be missing.

There are a few ways to decrease your probability of losing event records. Sub-buffer count and size shows how to fine-tune the sub-buffer count and size of a channel to virtually stop losing event records, though at the cost of greater memory usage.

Sub-buffer count and size

When you create a channel, you can set its number of sub-buffers and their size.

Note that there is noticeable CPU overhead introduced when switching sub-buffers (marking a full one as consumable and switching to an empty one for the following events to be recorded). Knowing this, the following list presents a few practical situations along with how to configure the sub-buffer count and size for them:

  • High event throughput: In general, prefer bigger sub-buffers to lower the risk of losing event records.

    Having bigger sub-buffers also ensures a lower sub-buffer switching frequency.

    The number of sub-buffers is only meaningful if you create the channel in overwrite mode: in this case, if a sub-buffer overwrite happens, the other sub-buffers are left unaltered.

  • Low event throughput: In general, prefer smaller sub-buffers since the risk of losing event records is low.

    Because events occur less frequently, the sub-buffer switching frequency should remain low and thus the overhead of the tracer shouldn’t be a problem.

  • Low memory system: If your target system has a low memory limit, prefer fewer first, then smaller sub-buffers.

    Even if the system is limited in memory, you want to keep the sub-buffers as big as possible to avoid a high sub-buffer switching frequency.

Note that LTTng uses CTF as its trace format, which means event data is very compact. For example, the average LTTng kernel event record weights about 32 bytes. Thus, a sub-buffer size of 1 MiB is considered big.

The previous situations highlight the major trade-off between a few big sub-buffers and more, smaller sub-buffers: sub-buffer switching frequency vs. how much data is lost in overwrite mode. Assuming a constant event throughput and using the overwrite mode, the two following configurations have the same ring buffer total size:

  • Two sub-buffers of 4 MiB each: Expect a very low sub-buffer switching frequency, but if a sub-buffer overwrite happens, half of the event records so far (4 MiB) are definitely lost.

  • Eight sub-buffers of 1 MiB each: Expect four times the overhead of the tracer as the previous configuration, but if a sub-buffer overwrite happens, only the eighth of event records so far are definitely lost.

In discard mode, the sub-buffers count parameter is pointless: use two sub-buffers and set their size according to the requirements of your situation.

Switch timer period

The switch timer period is an important configurable attribute of a channel to ensure periodic sub-buffer flushing.

When the switch timer expires, a sub-buffer switch happens. Set the switch timer period attribute when you create a channel to ensure that LTTng consumes and commits trace data to trace files or to a distant relay daemon periodically in case of a low event throughput.

This attribute is also convenient when you use big sub-buffers to cope with a sporadic high event throughput, even if the throughput is normally low.

Read timer period

By default, the LTTng tracers use a notification mechanism to signal a full sub-buffer so that a consumer daemon can consume it. When such notifications must be avoided, for example in real-time applications, use the read timer of the channel instead. When the read timer fires, the consumer daemon checks for full, consumable sub-buffers.

Trace file count and size

By default, trace files can grow as large as needed. Set the maximum size of each trace file that a channel writes when you create a channel. When the size of a trace file reaches the fixed maximum size of the channel, LTTng creates another file to contain the next event records. LTTng appends a file count to each trace file name in this case.

If you set the trace file size attribute when you create a channel, the maximum number of trace files that LTTng creates is unlimited by default. To limit them, set a maximum number of trace files. When the number of trace files reaches the fixed maximum count of the channel, the oldest trace file is overwritten. This mechanism is called trace file rotation.

Important:Even if you don’t limit the trace file count, you can’t assume that LTTng doesn’t manage any trace file.

In other words, there is no safe way to know if LTTng still holds a given trace file open with the trace file rotation feature.

The only way to obtain an unmanaged, self-contained LTTng trace before you destroy the tracing session is with the tracing session rotation feature (available since LTTng 2.11).

Instrumentation point, event rule, event, and event record

An event rule is a set of conditions which must be all satisfied for LTTng to record an occuring event.

You set the conditions when you create an event rule.

You always attach an event rule to a channel when you create it.

When an event passes the conditions of an event rule, LTTng records it in one of the sub-buffers of the attached channel.

The available conditions, as of LTTng 2.12, are:

  • The event rule is enabled.

  • The type of the instrumentation point is T.

  • The name of the instrumentation point (sometimes called event name) matches N, but isn’t E.

  • The log level of the instrumentation point is as severe as L, or is exactly L.

  • The fields of the payload of the event satisfy a filter expression F.

As you can see, all the conditions but the dynamic filter are related to the status of the event rule or to the instrumentation point, not to the occurring events. This is why, without a filter, checking if an event passes an event rule isn’t a dynamic task: when you create or modify an event rule, all the tracers of its tracing domain enable or disable the instrumentation points themselves once. This is possible because the attributes of an instrumentation point (type, name, and log level) are defined statically. In other words, without a dynamic filter, the tracer doesn’t evaluate the arguments of an instrumentation point unless it matches an enabled event rule.

Note that, for LTTng to record an event, the channel to which a matching event rule is attached must also be enabled, and the tracing session owning this channel must be active (started).

Logical path from an instrumentation point to an event record.

Components of LTTng

The second T in LTTng stands for toolkit: it would be wrong to call LTTng a simple tool since it is composed of multiple interacting components. This section describes those components, explains their respective roles, and shows how they connect together to form the LTTng ecosystem.

The following diagram shows how the most important components of LTTng interact with user applications, the Linux kernel, and you:

Control and trace data paths between LTTng components.

The LTTng project incorporates:

Tracing control command-line interface

The tracing control command-line interface.

The lttng(1) command-line tool is the standard user interface to control LTTng tracing sessions. The lttng tool is part of LTTng-tools.

The lttng tool is linked with liblttng-ctl to communicate with one or more session daemons behind the scenes.

The lttng tool has a Git-like interface:

$
lttng <GENERAL OPTIONS> <COMMAND> <COMMAND OPTIONS>

The Tracing control section explores the available features of LTTng using the lttng tool.

Tracing control library

The tracing control library.

The LTTng control library, liblttng-ctl, is used to communicate with a session daemon using a C API that hides the underlying details of the protocol. liblttng-ctl is part of LTTng-tools.

The lttng command-line tool is linked with liblttng-ctl.

Use liblttng-ctl in C or C++ source code by including its “master” header:

#include <lttng/lttng.h>

Some objects are referenced by name (C string), such as tracing sessions, but most of them require to create a handle first using lttng_create_handle().

As of LTTng 2.12, the best available developer documentation for liblttng-ctl is its installed header files. Every function and structure is thoroughly documented.

User space tracing library

The user space tracing library.

The user space tracing library, liblttng-ust (see lttng-ust(3)), is the LTTng user space tracer. It receives commands from a session daemon, for example to enable and disable specific instrumentation points, and writes event records to ring buffers shared with a consumer daemon. liblttng-ust is part of LTTng-UST.

Public C header files are installed beside liblttng-ust to instrument any C or C++ application.

LTTng-UST agents, which are regular Java and Python packages, use their own library providing tracepoints which is linked with liblttng-ust.

An application or library doesn’t have to initialize liblttng-ust manually: its constructor does the necessary tasks to properly register to a session daemon. The initialization phase also enables the instrumentation points matching the event rules that you already created.

User space tracing agents

The user space tracing agents.

The LTTng-UST Java and Python agents are regular Java and Python packages which add LTTng tracing capabilities to the native logging frameworks. The LTTng-UST agents are part of LTTng-UST.

In the case of Java, the java.util.logging core logging facilities and Apache log4j 1.2 are supported. Note that Apache Log4 2 isn’t supported.

In the case of Python, the standard logging package is supported. Both Python 2 and Python 3 modules can import the LTTng-UST Python agent package.

The applications using the LTTng-UST agents are in the java.util.logging (JUL), log4j, and Python tracing domains.

Both agents use the same mechanism to trace the log statements. When an agent initializes, it creates a log handler that attaches to the root logger. The agent also registers to a session daemon. When the application executes a log statement, the root logger passes it to the log handler of the agent. The log handler of the agent calls a native function in a tracepoint provider package shared library linked with liblttng-ust, passing the formatted log message and other fields, like its logger name and its log level. This native function contains a user space instrumentation point, hence tracing the log statement.

The log level condition of an event rule is considered when tracing a Java or a Python application, and it’s compatible with the standard JUL, log4j, and Python log levels.

LTTng kernel modules

The LTTng kernel modules.

The LTTng kernel modules are a set of Linux kernel modules which implement the kernel tracer of the LTTng project. The LTTng kernel modules are part of LTTng-modules.

The LTTng kernel modules include:

  • A set of probe modules.

    Each module attaches to a specific subsystem of the Linux kernel using its tracepoint instrument points. There are also modules to attach to the entry and return points of the Linux system call functions.

  • Ring buffer modules.

    A ring buffer implementation is provided as kernel modules. The LTTng kernel tracer writes to the ring buffer; a consumer daemon reads from the ring buffer.

  • The LTTng kernel tracer module.

  • The LTTng logger module.

    The LTTng logger module implements the special /proc/lttng-logger (and /dev/lttng-logger since LTTng 2.11) files so that any executable can generate LTTng events by opening and writing to those files.

    See LTTng logger.

Generally, you don’t have to load the LTTng kernel modules manually (using modprobe(8), for example): a root session daemon loads the necessary modules when starting. If you have extra probe modules, you can specify to load them to the session daemon on the command line. See also Linux kernel module signature.

The LTTng kernel modules are installed in /usr/lib/modules/release/extra by default, where release is the kernel release (see uname --kernel-release).

Session daemon

The session daemon.

The session daemon, lttng-sessiond(8), is a daemon responsible for managing tracing sessions and for controlling the various components of LTTng. The session daemon is part of LTTng-tools.

The session daemon sends control requests to and receives control responses from:

  • The user space tracing library.

    Any instance of the user space tracing library first registers to a session daemon. Then, the session daemon can send requests to this instance, such as:

    • Get the list of tracepoints.

    • Share an event rule so that the user space tracing library can enable or disable tracepoints. Amongst the possible conditions of an event rule is a filter expression which liblttng-ust evalutes when an event occurs.

    • Share channel attributes and ring buffer locations.

    The session daemon and the user space tracing library use a Unix domain socket for their communication.

  • The user space tracing agents.

    Any instance of a user space tracing agent first registers to a session daemon. Then, the session daemon can send requests to this instance, such as:

    • Get the list of loggers.

    • Enable or disable a specific logger.

    The session daemon and the user space tracing agent use a TCP connection for their communication.

  • The LTTng kernel tracer.

  • The consumer daemon.

    The session daemon sends requests to the consumer daemon to instruct it where to send the trace data streams, amongst other information.

  • The relay daemon.

The session daemon receives commands from the tracing control library.

The root session daemon loads the appropriate LTTng kernel modules on startup. It also spawns a consumer daemon as soon as you create an event rule.

The session daemon doesn’t send and receive trace data: this is the role of the consumer daemon and relay daemon. It does, however, generate the CTF metadata stream.

Each Unix user can have its own session daemon instance. The tracing sessions which different session daemons manage are completely independent.

The root user’s session daemon is the only one which is allowed to control the LTTng kernel tracer, and its spawned consumer daemon is the only one which is allowed to consume trace data from the LTTng kernel tracer. Note, however, that any Unix user which is a member of the tracing group is allowed to create channels in the Linux kernel tracing domain, and thus to trace the Linux kernel.

The lttng command-line tool automatically starts a session daemon when using its create command if none is currently running. You can also start the session daemon manually.

Consumer daemon

The consumer daemon.

The consumer daemon, lttng-consumerd, is a daemon which shares ring buffers with user applications or with the LTTng kernel modules to collect trace data and send it to some location (on disk or to a relay daemon over the network). The consumer daemon is part of LTTng-tools.

You don’t start a consumer daemon manually: a consumer daemon is always spawned by a session daemon as soon as you create an event rule, that is, before you start tracing. When you kill its owner session daemon, the consumer daemon also exits because it is the child process of the session daemon. Command-line options of lttng-sessiond(8) target the consumer daemon process.

There are up to two running consumer daemons per Unix user, whereas only one session daemon can run per user. This is because each process can be either 32-bit or 64-bit: if the target system runs a mixture of 32-bit and 64-bit processes, it is more efficient to have separate corresponding 32-bit and 64-bit consumer daemons. The root user is an exception: it can have up to three running consumer daemons: 32-bit and 64-bit instances for its user applications, and one more reserved for collecting kernel trace data.

Relay daemon

The relay daemon.

The relay daemon, lttng-relayd(8), is a daemon acting as a bridge between remote session and consumer daemons, local trace files, and a remote live trace viewer. The relay daemon is part of LTTng-tools.

The main purpose of the relay daemon is to implement a receiver of trace data over the network. This is useful when the target system doesn’t have much file system space to record trace files locally.

The relay daemon is also a server to which a live trace viewer can connect. The live trace viewer sends requests to the relay daemon to receive trace data as the target system emits events. The communication protocol is named LTTng live; it is used over TCP connections.

Note that you can start the relay daemon on the target system directly. This is the setup of choice when the use case is to view events as the target system emits them without the need of a remote system.

Instrumentation

There are many examples of tracing and monitoring in our everyday life:

  • You have access to real-time and historical weather reports and forecasts thanks to weather stations installed around the country.

  • You know your heart is safe thanks to an electrocardiogram.

  • You make sure not to drive your car too fast and to have enough fuel to reach your destination thanks to gauges visible on your dashboard.

All the previous examples have something in common: they rely on instruments. Without the electrodes attached to the surface of your body skin, cardiac monitoring is futile.

LTTng, as a tracer, is no different from those real life examples. If you’re about to trace a software system or, in other words, record its history of execution, you better have instrumentation points in the subject you’re tracing, that is, the actual software.

Various ways were developed to instrument a piece of software for LTTng tracing. The most straightforward one is to manually place instrumentation points, called tracepoints, in the source code of the software. It is also possible to add instrumentation points dynamically in the Linux kernel tracing domain.

If you’re only interested in tracing the Linux kernel, your instrumentation needs are probably already covered by the built-in Linux kernel tracepoints of LTTng. You may also wish to trace a user application which is already instrumented for LTTng tracing. In such cases, skip this whole section and read the topics of the Tracing control section.

Many methods are available to instrument a piece of software for LTTng tracing. They are:

User space instrumentation for C and C++ applications

The procedure to instrument a C or C++ user application with the LTTng user space tracing library, liblttng-ust, is:

  1. Create the source files of a tracepoint provider package.

  2. Add tracepoints to the source code of the application.

  3. Build and link a tracepoint provider package and the user application.

If you need quick, printf(3)-like instrumentation, skip those steps and use tracef() or tracelog() instead.

Important:You need to install LTTng-UST to instrument a user application with liblttng-ust.

Create the source files of a tracepoint provider package

A tracepoint provider is a set of compiled functions which provide tracepoints to an application, the type of instrumentation point supported by LTTng-UST. Those functions can emit events with user-defined fields and serialize those events as event records to one or more LTTng-UST channel sub-buffers. The tracepoint() macro, which you insert in the source code of a user application, calls those functions.

A tracepoint provider package is an object file (.o) or a shared library (.so) which contains one or more tracepoint providers. Its source files are:

A tracepoint provider package is dynamically linked with liblttng-ust, the LTTng user space tracer, at run time.

User application linked with liblttng-ust and containing a tracepoint provider.

Note:If you need quick, printf(3)-like instrumentation, skip creating and using a tracepoint provider and use tracef() or tracelog() instead.

Create a tracepoint provider header file template

A tracepoint provider header file contains the tracepoint definitions of a tracepoint provider.

To create a tracepoint provider header file:

  1. Start from this template:

    Tracepoint provider header file template (.h file extension).

    #undef TRACEPOINT_PROVIDER
    #define TRACEPOINT_PROVIDER provider_name
    
    #undef TRACEPOINT_INCLUDE
    #define TRACEPOINT_INCLUDE "./tp.h"
    
    #if !defined(_TP_H) || defined(TRACEPOINT_HEADER_MULTI_READ)
    #define _TP_H
    
    #include <lttng/tracepoint.h>
    
    /*
     * Use TRACEPOINT_EVENT(), TRACEPOINT_EVENT_CLASS(),
     * TRACEPOINT_EVENT_INSTANCE(), and TRACEPOINT_LOGLEVEL() here.
     */
    
    #endif /* _TP_H */
    
    #include <lttng/tracepoint-event.h>
    
  2. Replace:

    • provider_name with the name of your tracepoint provider.

    • "tp.h" with the name of your tracepoint provider header file.

  3. Below the #include <lttng/tracepoint.h> line, put your tracepoint definitions.

Your tracepoint provider name must be unique amongst all the possible tracepoint provider names used on the same target system. We suggest to include the name of your project or company in the name, for example, org_lttng_my_project_tpp.

Tip:Use the lttng-gen-tp(1) tool to create this boilerplate for you. When using lttng-gen-tp, all you need to write are the tracepoint definitions.

Create a tracepoint definition

A tracepoint definition defines, for a given tracepoint:

  • Its input arguments. They are the macro parameters that the tracepoint() macro accepts for this particular tracepoint in the source code of the user application.

  • Its output event fields. They are the sources of event fields that form the payload of any event that the execution of the tracepoint() macro emits for this particular tracepoint.

Create a tracepoint definition by using the TRACEPOINT_EVENT() macro below the #include <lttng/tracepoint.h> line in the tracepoint provider header file template.

The syntax of the TRACEPOINT_EVENT() macro is:

TRACEPOINT_EVENT() macro syntax.

TRACEPOINT_EVENT(
    /* Tracepoint provider name */
    provider_name,

    /* Tracepoint name */
    tracepoint_name,

    /* Input arguments */
    TP_ARGS(
        arguments
    ),

    /* Output event fields */
    TP_FIELDS(
        fields
    )
)

Replace:

  • provider_name with your tracepoint provider name.

  • tracepoint_name with your tracepoint name.

  • arguments with the input arguments.

  • fields with the output event field definitions.

This tracepoint emits events named provider_name:tracepoint_name.

Event name length limitation

Important:The concatenation of the tracepoint provider name and the tracepoint name must not exceed 254 characters. If it does, the instrumented application compiles and runs, but LTTng throws multiple warnings and you could experience serious issues.

The syntax of the TP_ARGS() macro is:

TP_ARGS() macro syntax.

TP_ARGS(
    type, arg_name
)

Replace:

  • type with the C type of the argument.

  • arg_name with the argument name.

You can repeat type and arg_name up to 10 times to have more than one argument.

Example:TP_ARGS() usage with three arguments.

TP_ARGS(
    int, count,
    float, ratio,
    const char*, query
)

The TP_ARGS() and TP_ARGS(void) forms are valid to create a tracepoint definition with no input arguments.

The TP_FIELDS() macro contains a list of ctf_*() macros. Each ctf_*() macro defines one event field. See lttng-ust(3) for a complete description of the available ctf_*() macros. A ctf_*() macro specifies the type, size, and byte order of one event field.

Each ctf_*() macro takes an argument expression parameter. This is a C expression that the tracer evalutes at the tracepoint() macro site in the source code of the application. This expression provides the source of data of a field. The argument expression can include input argument names listed in the TP_ARGS() macro.

Each ctf_*() macro also takes a field name parameter. Field names must be unique within a given tracepoint definition.

Here’s a complete tracepoint definition example:

Example:Tracepoint definition.

The following tracepoint definition defines a tracepoint which takes three input arguments and has four output event fields.

#include "my-custom-structure.h"

TRACEPOINT_EVENT(
    my_provider,
    my_tracepoint,
    TP_ARGS(
        const struct my_custom_structure*, my_custom_structure,
        float, ratio,
        const char*, query
    ),
    TP_FIELDS(
        ctf_string(query_field, query)
        ctf_float(double, ratio_field, ratio)
        ctf_integer(int, recv_size, my_custom_structure->recv_size)
        ctf_integer(int, send_size, my_custom_structure->send_size)
    )
)

Refer to this tracepoint definition with the tracepoint() macro in the source code of your application like this:

tracepoint(my_provider, my_tracepoint,
           my_structure, some_ratio, the_query);

Note:The LTTng tracer only evaluates tracepoint arguments at run time if they satisfy an enabled event rule.

Use a tracepoint class

A tracepoint class is a class of tracepoints which share the same output event field definitions. A tracepoint instance is one instance of such a defined tracepoint class, with its own tracepoint name.

The TRACEPOINT_EVENT() macro is actually a shorthand which defines both a tracepoint class and a tracepoint instance at the same time.

When you build a tracepoint provider package, the C or C++ compiler creates one serialization function for each tracepoint class. A serialization function is responsible for serializing the event fields of a tracepoint to a sub-buffer when tracing.

For various performance reasons, when your situation requires multiple tracepoint definitions with different names, but with the same event fields, we recommend that you manually create a tracepoint class and instantiate as many tracepoint instances as needed. One positive effect of such a design, amongst other advantages, is that all tracepoint instances of the same tracepoint class reuse the same serialization function, thus reducing cache pollution.

Example:Use a tracepoint class and tracepoint instances.

Consider the following three tracepoint definitions:

TRACEPOINT_EVENT(
    my_app,
    get_account,
    TP_ARGS(
        int, userid,
        size_t, len
    ),
    TP_FIELDS(
        ctf_integer(int, userid, userid)
        ctf_integer(size_t, len, len)
    )
)

TRACEPOINT_EVENT(
    my_app,
    get_settings,
    TP_ARGS(
        int, userid,
        size_t, len
    ),
    TP_FIELDS(
        ctf_integer(int, userid, userid)
        ctf_integer(size_t, len, len)
    )
)

TRACEPOINT_EVENT(
    my_app,
    get_transaction,
    TP_ARGS(
        int, userid,
        size_t, len
    ),
    TP_FIELDS(
        ctf_integer(int, userid, userid)
        ctf_integer(size_t, len, len)
    )
)

In this case, we create three tracepoint classes, with one implicit tracepoint instance for each of them: get_account, get_settings, and get_transaction. However, they all share the same event field names and types. Hence three identical, yet independent serialization functions are created when you build the tracepoint provider package.

A better design choice is to define a single tracepoint class and three tracepoint instances:

/* The tracepoint class */
TRACEPOINT_EVENT_CLASS(
    /* Tracepoint provider name */
    my_app,

    /* Tracepoint class name */
    my_class,

    /* Input arguments */
    TP_ARGS(
        int, userid,
        size_t, len
    ),

    /* Output event fields */
    TP_FIELDS(
        ctf_integer(int, userid, userid)
        ctf_integer(size_t, len, len)
    )
)

/* The tracepoint instances */
TRACEPOINT_EVENT_INSTANCE(
    /* Tracepoint provider name */
    my_app,

    /* Tracepoint class name */
    my_class,

    /* Tracepoint name */
    get_account,

    /* Input arguments */
    TP_ARGS(
        int, userid,
        size_t, len
    )
)
TRACEPOINT_EVENT_INSTANCE(
    my_app,
    my_class,
    get_settings,
    TP_ARGS(
        int, userid,
        size_t, len
    )
)
TRACEPOINT_EVENT_INSTANCE(
    my_app,
    my_class,
    get_transaction,
    TP_ARGS(
        int, userid,
        size_t, len
    )
)
Assign a log level to a tracepoint definition

Assign a log level to a tracepoint definition with the TRACEPOINT_LOGLEVEL() macro.

Assigning different levels of severity to tracepoint definitions can be useful: when you create an event rule, you can target tracepoints having a log level as severe as a specific value.

The concept of LTTng-UST log levels is similar to the levels found in typical logging frameworks:

  • In a logging framework, the log level is given by the function or method name you use at the log statement site: debug(), info(), warn(), error(), and so on.

  • In LTTng-UST, you statically assign the log level to a tracepoint definition; any tracepoint() macro invocation which refers to this definition has this log level.

You must use TRACEPOINT_LOGLEVEL() after the TRACEPOINT_EVENT() or TRACEPOINT_INSTANCE() macro for a given tracepoint.

The syntax of the TRACEPOINT_LOGLEVEL() macro is:

TRACEPOINT_LOGLEVEL() macro syntax.

TRACEPOINT_LOGLEVEL(provider_name, tracepoint_name, log_level)

Replace:

  • provider_name with the tracepoint provider name.

  • tracepoint_name with the tracepoint name.

  • log_level with the log level to assign to the tracepoint definition named tracepoint_name in the provider_name tracepoint provider.

    See lttng-ust(3) for a list of available log level names.

Example:Assign the TRACE_DEBUG_UNIT log level to a tracepoint definition.

/* Tracepoint definition */
TRACEPOINT_EVENT(
    my_app,
    get_transaction,
    TP_ARGS(
        int, userid,
        size_t, len
    ),
    TP_FIELDS(
        ctf_integer(int, userid, userid)
        ctf_integer(size_t, len, len)
    )
)

/* Log level assignment */
TRACEPOINT_LOGLEVEL(my_app, get_transaction, TRACE_DEBUG_UNIT)
Create a tracepoint provider package source file

A tracepoint provider package source file is a C source file which includes a tracepoint provider header file to expand its macros into event serialization and other functions.

Use the following tracepoint provider package source file template:

Tracepoint provider package source file template.

#define TRACEPOINT_CREATE_PROBES

#include "tp.h"

Replace tp.h with the name of your tracepoint provider header file name. You may also include more than one tracepoint provider header file here to create a tracepoint provider package holding more than one tracepoint providers.

Add tracepoints to the source code of an application

Once you create a tracepoint provider header file, use the tracepoint() macro in the source code of your application to insert the tracepoints that this header defines.

The tracepoint() macro takes at least two parameters: the tracepoint provider name and the tracepoint name. The corresponding tracepoint definition defines the other parameters.

Example:tracepoint() usage.

The following tracepoint definition defines a tracepoint which takes two input arguments and has two output event fields.

Tracepoint provider header file.

#include "my-custom-structure.h"

TRACEPOINT_EVENT(
    my_provider,
    my_tracepoint,
    TP_ARGS(
        int, argc,
        const char*, cmd_name
    ),
    TP_FIELDS(
        ctf_string(cmd_name, cmd_name)
        ctf_integer(int, number_of_args, argc)
    )
)

Refer to this tracepoint definition with the tracepoint() macro in the source code of your application like this:

Application source file.

#include "tp.h"

int main(int argc, char* argv[])
{
    tracepoint(my_provider, my_tracepoint, argc, argv[0]);

    return 0;
}

Note how the source code of the application includes the tracepoint provider header file containing the tracepoint definitions to use, tp.h.

Example:tracepoint() usage with a complex tracepoint definition.

Consider this complex tracepoint definition, where multiple event fields refer to the same input arguments in their argument expression parameter:

Tracepoint provider header file.

/* For `struct stat` */
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>

TRACEPOINT_EVENT(
    my_provider,
    my_tracepoint,
    TP_ARGS(
        int, my_int_arg,
        char*, my_str_arg,
        struct stat*, st
    ),
    TP_FIELDS(
        ctf_integer(int, my_constant_field, 23 + 17)
        ctf_integer(int, my_int_arg_field, my_int_arg)
        ctf_integer(int, my_int_arg_field2, my_int_arg * my_int_arg)
        ctf_integer(int, sum4_field, my_str_arg[0] + my_str_arg[1] +
                                     my_str_arg[2] + my_str_arg[3])
        ctf_string(my_str_arg_field, my_str_arg)
        ctf_integer_hex(off_t, size_field, st->st_size)
        ctf_float(double, size_dbl_field, (double) st->st_size)
        ctf_sequence_text(char, half_my_str_arg_field, my_str_arg,
                          size_t, strlen(my_str_arg) / 2)
    )
)

Refer to this tracepoint definition with the tracepoint() macro in the source code of your application like this:

Application source file.

#define TRACEPOINT_DEFINE
#include "tp.h"

int main(void)
{
    struct stat s;

    stat("/etc/fstab", &s);
    tracepoint(my_provider, my_tracepoint, 23, "Hello, World!", &s);

    return 0;
}

If you look at the event record that LTTng writes when tracing this program, assuming the file size of /etc/fstab is 301 bytes, it should look like this:

Event record fields

Field name Field value

my_constant_field

40

my_int_arg_field

23

my_int_arg_field2

529

sum4_field

389

my_str_arg_field

Hello, World!

size_field

0x12d

size_dbl_field

301.0

half_my_str_arg_field

Hello,

Sometimes, the arguments you pass to tracepoint() are expensive to compute—they use the call stack, for example. To avoid this computation when the tracepoint is disabled, use the tracepoint_enabled() and do_tracepoint() macros.

The syntax of the tracepoint_enabled() and do_tracepoint() macros is:

tracepoint_enabled() and do_tracepoint() macros syntax.

tracepoint_enabled(provider_name, tracepoint_name)
do_tracepoint(provider_name, tracepoint_name, ...)

Replace:

  • provider_name with the tracepoint provider name.

  • tracepoint_name with the tracepoint name.

tracepoint_enabled() returns a non-zero value if the tracepoint named tracepoint_name from the provider named provider_name is enabled at run time.

do_tracepoint() is like tracepoint(), except that it doesn’t check if the tracepoint is enabled. Using tracepoint() with tracepoint_enabled() is dangerous since tracepoint() also contains the tracepoint_enabled() check, thus a race condition is possible in this situation:

Possible race condition when using tracepoint_enabled() with tracepoint().

if (tracepoint_enabled(my_provider, my_tracepoint)) {
    stuff = prepare_stuff();
}

tracepoint(my_provider, my_tracepoint, stuff);

If the tracepoint is enabled after the condition, then stuff isn’t prepared: the emitted event will either contain wrong data, or the whole application could crash (segmentation fault, for example).

Note:Neither tracepoint_enabled() nor do_tracepoint() have an STAP_PROBEV() call. If you need it, you must emit this call yourself.

Build and link a tracepoint provider package and an application

Once you have one or more tracepoint provider header files and a tracepoint provider package source file, create the tracepoint provider package by compiling its source file. From here, multiple build and run scenarios are possible. The following table shows common application and library configurations along with the required command lines to achieve them.

In the following diagrams, we use the following file names:

app

Executable application.

app.o

Application object file.

tpp.o

Tracepoint provider package object file.

tpp.a

Tracepoint provider package archive file.

libtpp.so

Tracepoint provider package shared object file.

emon.o

User library object file.

libemon.so

User library shared object file.

We use the following symbols in the diagrams of table below:

Symbols used in the build scenario diagrams.

We assume that . is part of the LD_LIBRARY_PATH environment variable in the following instructions.

Common tracepoint provider package scenarios.

Scenario Instructions

The instrumented application is statically linked with the tracepoint provider package object.

To build the tracepoint provider package object file:

To build the instrumented application:

  1. In app.c, before including tpp.h, add the following line:

    #define TRACEPOINT_DEFINE
    
  2. Compile the application source file:

    $
    gcc -c app.c
  3. Build the application:

    $
    gcc -o app app.o tpp.o -llttng-ust -ldl

To run the instrumented application:

  • Start the application:

    $
    ./app

The instrumented application is statically linked with the tracepoint provider package archive file.

To create the tracepoint provider package archive file:

  1. Compile the tracepoint provider package source file:

    $
    gcc -I. -c tpp.c
  2. Create the tracepoint provider package archive file:

    $
    ar rcs tpp.a tpp.o

To build the instrumented application:

  1. In app.c, before including tpp.h, add the following line:

    #define TRACEPOINT_DEFINE
    
  2. Compile the application source file:

    $
    gcc -c app.c
  3. Build the application:

    $
    gcc -o app app.o tpp.a -llttng-ust -ldl

To run the instrumented application:

  • Start the application:

    $
    ./app

The instrumented application is linked with the tracepoint provider package shared object.

To build the tracepoint provider package shared object:

  1. Compile the tracepoint provider package source file:

    $
    gcc -I. -fpic -c tpp.c
  2. Build the tracepoint provider package shared object:

    $
    gcc -shared -o libtpp.so tpp.o -llttng-ust -ldl

To build the instrumented application:

  1. In app.c, before including tpp.h, add the following line:

    #define TRACEPOINT_DEFINE
    
  2. Compile the application source file:

    $
    gcc -c app.c
  3. Build the application:

    $
    gcc -o app app.o -ldl -L. -ltpp

To run the instrumented application:

  • Start the application:

    $
    ./app

The tracepoint provider package shared object is preloaded before the instrumented application starts.

To build the tracepoint provider package shared object:

  1. Compile the tracepoint provider package source file:

    $
    gcc -I. -fpic -c tpp.c
  2. Build the tracepoint provider package shared object:

    $
    gcc -shared -o libtpp.so tpp.o -llttng-ust -ldl

To build the instrumented application:

  1. In app.c, before including tpp.h, add the following lines:

    #define TRACEPOINT_DEFINE
    #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
    
  2. Compile the application source file:

    $
    gcc -c app.c
  3. Build the application:

    $
    gcc -o app app.o -ldl

To run the instrumented application with tracing support:

  • Preload the tracepoint provider package shared object and start the application:

    $
    LD_PRELOAD=./libtpp.so ./app

To run the instrumented application without tracing support:

  • Start the application:

    $
    ./app

The instrumented application dynamically loads the tracepoint provider package shared object.

To build the tracepoint provider package shared object:

  1. Compile the tracepoint provider package source file:

    $
    gcc -I. -fpic -c tpp.c
  2. Build the tracepoint provider package shared object:

    $
    gcc -shared -o libtpp.so tpp.o -llttng-ust -ldl

To build the instrumented application:

  1. In app.c, before including tpp.h, add the following lines:

    #define TRACEPOINT_DEFINE
    #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
    
  2. Compile the application source file:

    $
    gcc -c app.c
  3. Build the application:

    $
    gcc -o app app.o -ldl

To run the instrumented application:

  • Start the application:

    $
    ./app

The application is linked with the instrumented user library.

The instrumented user library is statically linked with the tracepoint provider package object file.

To build the tracepoint provider package object file:

To build the instrumented user library:

  1. In emon.c, before including tpp.h, add the following line:

    #define TRACEPOINT_DEFINE
    
  2. Compile the user library source file:

    $
    gcc -I. -fpic -c emon.c
  3. Build the user library shared object:

    $
    gcc -shared -o libemon.so emon.o tpp.o -llttng-ust -ldl

To build the application:

  1. Compile the application source file:

    $
    gcc -c app.c
  2. Build the application:

    $
    gcc -o app app.o -L. -lemon

To run the application:

  • Start the application:

    $
    ./app

The application is linked with the instrumented user library.

The instrumented user library is linked with the tracepoint provider package shared object.

To build the tracepoint provider package shared object:

  1. Compile the tracepoint provider package source file:

    $
    gcc -I. -fpic -c tpp.c
  2. Build the tracepoint provider package shared object:

    $
    gcc -shared -o libtpp.so tpp.o -llttng-ust -ldl

To build the instrumented user library:

  1. In emon.c, before including tpp.h, add the following line:

    #define TRACEPOINT_DEFINE
    
  2. Compile the user library source file:

    $
    gcc -I. -fpic -c emon.c
  3. Build the user library shared object:

    $
    gcc -shared -o libemon.so emon.o -ldl -L. -ltpp

To build the application:

  1. Compile the application source file:

    $
    gcc -c app.c
  2. Build the application:

    $
    gcc -o app app.o -L. -lemon

To run the application:

  • Start the application:

    $
    ./app

The tracepoint provider package shared object is preloaded before the application starts.

The application is linked with the instrumented user library.

To build the tracepoint provider package shared object:

  1. Compile the tracepoint provider package source file:

    $
    gcc -I. -fpic -c tpp.c
  2. Build the tracepoint provider package shared object:

    $
    gcc -shared -o libtpp.so tpp.o -llttng-ust -ldl

To build the instrumented user library:

  1. In emon.c, before including tpp.h, add the following lines:

    #define TRACEPOINT_DEFINE
    #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
    
  2. Compile the user library source file:

    $
    gcc -I. -fpic -c emon.c
  3. Build the user library shared object:

    $
    gcc -shared -o libemon.so emon.o -ldl

To build the application:

  1. Compile the application source file:

    $
    gcc -c app.c
  2. Build the application:

    $
    gcc -o app app.o -L. -lemon

To run the application with tracing support:

  • Preload the tracepoint provider package shared object and start the application:

    $
    LD_PRELOAD=./libtpp.so ./app

To run the application without tracing support:

  • Start the application:

    $
    ./app

The application is linked with the instrumented user library.

The instrumented user library dynamically loads the tracepoint provider package shared object.

To build the tracepoint provider package shared object:

  1. Compile the tracepoint provider package source file:

    $
    gcc -I. -fpic -c tpp.c
  2. Build the tracepoint provider package shared object:

    $
    gcc -shared -o libtpp.so tpp.o -llttng-ust -ldl

To build the instrumented user library:

  1. In emon.c, before including tpp.h, add the following lines:

    #define TRACEPOINT_DEFINE
    #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
    
  2. Compile the user library source file:

    $
    gcc -I. -fpic -c emon.c
  3. Build the user library shared object:

    $
    gcc -shared -o libemon.so emon.o -ldl

To build the application:

  1. Compile the application source file:

    $
    gcc -c app.c
  2. Build the application:

    $
    gcc -o app app.o -L. -lemon

To run the application:

  • Start the application:

    $
    ./app

The application dynamically loads the instrumented user library.

The instrumented user library is linked with the tracepoint provider package shared object.

To build the tracepoint provider package shared object:

  1. Compile the tracepoint provider package source file:

    $
    gcc -I. -fpic -c tpp.c
  2. Build the tracepoint provider package shared object:

    $
    gcc -shared -o libtpp.so tpp.o -llttng-ust -ldl

To build the instrumented user library:

  1. In emon.c, before including tpp.h, add the following line:

    #define TRACEPOINT_DEFINE
    
  2. Compile the user library source file:

    $
    gcc -I. -fpic -c emon.c
  3. Build the user library shared object:

    $
    gcc -shared -o libemon.so emon.o -ldl -L. -ltpp

To build the application:

  1. Compile the application source file:

    $
    gcc -c app.c
  2. Build the application:

    $
    gcc -o app app.o -ldl -L. -lemon

To run the application:

  • Start the application:

    $
    ./app

The application dynamically loads the instrumented user library.

The instrumented user library dynamically loads the tracepoint provider package shared object.

To build the tracepoint provider package shared object:

  1. Compile the tracepoint provider package source file:

    $
    gcc -I. -fpic -c tpp.c
  2. Build the tracepoint provider package shared object:

    $
    gcc -shared -o libtpp.so tpp.o -llttng-ust -ldl

To build the instrumented user library:

  1. In emon.c, before including tpp.h, add the following lines:

    #define TRACEPOINT_DEFINE
    #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
    
  2. Compile the user library source file:

    $
    gcc -I. -fpic -c emon.c
  3. Build the user library shared object:

    $
    gcc -shared -o libemon.so emon.o -ldl

To build the application:

  1. Compile the application source file:

    $
    gcc -c app.c
  2. Build the application:

    $
    gcc -o app app.o -ldl -L. -lemon

To run the application:

  • Start the application:

    $
    ./app

The tracepoint provider package shared object is preloaded before the application starts.

The application dynamically loads the instrumented user library.

To build the tracepoint provider package shared object:

  1. Compile the tracepoint provider package source file:

    $
    gcc -I. -fpic -c tpp.c
  2. Build the tracepoint provider package shared object:

    $
    gcc -shared -o libtpp.so tpp.o -llttng-ust -ldl

To build the instrumented user library:

  1. In emon.c, before including tpp.h, add the following lines:

    #define TRACEPOINT_DEFINE
    #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
    
  2. Compile the user library source file:

    $
    gcc -I. -fpic -c emon.c
  3. Build the user library shared object:

    $
    gcc -shared -o libemon.so emon.o -ldl

To build the application:

  1. Compile the application source file:

    $
    gcc -c app.c
  2. Build the application:

    $
    gcc -o app app.o -L. -lemon

To run the application with tracing support:

  • Preload the tracepoint provider package shared object and start the application:

    $
    LD_PRELOAD=./libtpp.so ./app

To run the application without tracing support:

  • Start the application:

    $
    ./app

The application is statically linked with the tracepoint provider package object file.

The application is linked with the instrumented user library.

To build the tracepoint provider package object file:

To build the instrumented user library:

  1. In emon.c, before including tpp.h, add the following line:

    #define TRACEPOINT_DEFINE
    
  2. Compile the user library source file:

    $
    gcc -I. -fpic -c emon.c
  3. Build the user library shared object:

    $
    gcc -shared -o libemon.so emon.o

To build the application:

  1. Compile the application source file:

    $
    gcc -c app.c
  2. Build the application:

    $
    gcc -o app app.o tpp.o -llttng-ust -ldl -L. -lemon

To run the instrumented application:

  • Start the application:

    $
    ./app

The application is statically linked with the tracepoint provider package object file.

The application dynamically loads the instrumented user library.

To build the tracepoint provider package object file:

To build the application:

  1. In app.c, before including tpp.h, add the following line:

    #define TRACEPOINT_DEFINE
    
  2. Compile the application source file:

    $
    gcc -c app.c
  3. Build the application:

    $
     
    gcc -Wl,--export-dynamic -o app app.o tpp.o \
      -llttng-ust -ldl

    The --export-dynamic option passed to the linker is necessary for the dynamically loaded library to “see” the tracepoint symbols defined in the application.

To build the instrumented user library:

  1. Compile the user library source file:

    $
    gcc -I. -fpic -c emon.c
  2. Build the user library shared object:

    $
    gcc -shared -o libemon.so emon.o

To run the application:

  • Start the application:

    $
    ./app
Use LTTng-UST with daemons

If your instrumented application calls fork(2), clone(2), or BSD’s rfork(2), without a following exec(3)-family system call, you must preload the liblttng-ust-fork.so shared object when you start the application.

$
LD_PRELOAD=liblttng-ust-fork.so ./my-app

If your tracepoint provider package is a shared library which you also preload, you must put both shared objects in LD_PRELOAD:

$
LD_PRELOAD=liblttng-ust-fork.so:/path/to/tp.so ./my-app
Use LTTng-UST with applications which close file descriptors that don’t belong to them
Since 2.9

If your instrumented application closes one or more file descriptors which it did not open itself, you must preload the liblttng-ust-fd.so shared object when you start the application:

$
LD_PRELOAD=liblttng-ust-fd.so ./my-app

Typical use cases include closing all the file descriptors after fork(2) or rfork(2) and buggy applications doing “double closes”.

Use pkg-config

On some distributions, LTTng-UST ships with a pkg-config metadata file. If this is your case, then use pkg-config to build an application on the command line:

$
gcc -o my-app my-app.o tp.o $(pkg-config --cflags --libs lttng-ust)
Build a 32-bit instrumented application for a 64-bit target system

In order to trace a 32-bit application running on a 64-bit system, LTTng must use a dedicated 32-bit consumer daemon.

The following steps show how to build and install a 32-bit consumer daemon, which is not part of the default 64-bit LTTng build, how to build and install the 32-bit LTTng-UST libraries, and how to build and link an instrumented 32-bit application in that context.

To build a 32-bit instrumented application for a 64-bit target system, assuming you have a fresh target system with no installed Userspace RCU or LTTng packages:

  1. Download, build, and install a 32-bit version of Userspace RCU:

    $
     
     
     
     
     
     
     
    cd $(mktemp -d) &&
    wget http://lttng.org/files/urcu/userspace-rcu-latest-0.9.tar.bz2 &&
    tar -xf userspace-rcu-latest-0.9.tar.bz2 &&
    cd userspace-rcu-0.9.* &&
    ./configure --libdir=/usr/local/lib32 CFLAGS=-m32 &&
    make &&
    sudo make install &&
    sudo ldconfig
  2. Using the package manager of your distribution, or from source, install the following 32-bit versions of the following dependencies of LTTng-tools and LTTng-UST:

  3. Download, build, and install a 32-bit version of the latest LTTng-UST 2.12:

    $
     
     
     
     
     
     
     
     
     
    cd $(mktemp -d) &&
    wget http://lttng.org/files/lttng-ust/lttng-ust-latest-2.12.tar.bz2 &&
    tar -xf lttng-ust-latest-2.12.tar.bz2 &&
    cd lttng-ust-2.12.* &&
    ./configure --libdir=/usr/local/lib32 \
              CFLAGS=-m32 CXXFLAGS=-m32 \
              LDFLAGS='-L/usr/local/lib32 -L/usr/lib32' &&
    make &&
    sudo make install &&
    sudo ldconfig

    Note:Depending on your distribution, 32-bit libraries could be installed at a different location than /usr/lib32. For example, Debian is known to install some 32-bit libraries in /usr/lib/i386-linux-gnu.

    In this case, make sure to set LDFLAGS to all the relevant 32-bit library paths, for example:

    $
    LDFLAGS='-L/usr/lib/i386-linux-gnu -L/usr/lib32'
  4. Download the latest LTTng-tools 2.12, build, and install the 32-bit consumer daemon:

    $
     
     
     
     
     
     
     
     
     
     
     
    cd $(mktemp -d) &&
    wget http://lttng.org/files/lttng-tools/lttng-tools-latest-2.12.tar.bz2 &&
    tar -xf lttng-tools-latest-2.12.tar.bz2 &&
    cd lttng-tools-2.12.* &&
    ./configure --libdir=/usr/local/lib32 CFLAGS=-m32 CXXFLAGS=-m32 \
              LDFLAGS='-L/usr/local/lib32 -L/usr/lib32' \
              --disable-bin-lttng --disable-bin-lttng-crash \
              --disable-bin-lttng-relayd --disable-bin-lttng-sessiond &&
    make &&
    cd src/bin/lttng-consumerd &&
    sudo make install &&
    sudo ldconfig
  5. From your distribution or from source, install the 64-bit versions of LTTng-UST and Userspace RCU.

  6. Download, build, and install the 64-bit version of the latest LTTng-tools 2.12:

    $
     
     
     
     
     
     
     
     
    cd $(mktemp -d) &&
    wget http://lttng.org/files/lttng-tools/lttng-tools-latest-2.12.tar.bz2 &&
    tar -xf lttng-tools-latest-2.12.tar.bz2 &&
    cd lttng-tools-2.12.* &&
    ./configure --with-consumerd32-libdir=/usr/local/lib32 \
              --with-consumerd32-bin=/usr/local/lib32/lttng/libexec/lttng-consumerd &&
    make &&
    sudo make install &&
    sudo ldconfig
  7. Pass the following options to gcc(1), g++(1), or clang(1) when linking your 32-bit application:

    -m32 -L/usr/lib32 -L/usr/local/lib32 \
    -Wl,-rpath,/usr/lib32,-rpath,/usr/local/lib32

    For example, let’s rebuild the quick start example in Trace a user application as an instrumented 32-bit application:

    $
    $
    $
     
     
     
    gcc -m32 -c -I. hello-tp.c
    gcc -m32 -c hello.c
    gcc -m32 -o hello hello.o hello-tp.o \
      -L/usr/lib32 -L/usr/local/lib32 \
      -Wl,-rpath,/usr/lib32,-rpath,/usr/local/lib32 \
      -llttng-ust -ldl

No special action is required to execute the 32-bit application and to trace it: use the command-line lttng(1) tool as usual.

Use tracef()

Since 2.5

tracef(3) is a small LTTng-UST API designed for quick, printf(3)-like instrumentation without the burden of creating and building a tracepoint provider package.

To use tracef() in your application:

  1. In the C or C++ source files where you need to use tracef(), include <lttng/tracef.h>:

    #include <lttng/tracef.h>
    
  2. In the source code of the application, use tracef() like you would use printf(3):

        /* ... */
    
        tracef("my message: %d (%s)", my_integer, my_string);
    
        /* ... */
    
  3. Link your application with liblttng-ust:

    $
    gcc -o app app.c -llttng-ust

To trace the events that tracef() calls emit:

  • Create an event rule which matches the lttng_ust_tracef:* event name:

    $
    lttng enable-event --userspace 'lttng_ust_tracef:*'

Limitations of tracef()

Important:The tracef() utility function was developed to make user space tracing super simple, albeit with notable disadvantages compared to user-defined tracepoints:

  • All the emitted events have the same tracepoint provider and tracepoint names, respectively lttng_ust_tracef and event.

  • There is no static type checking.

  • The only event record field you actually get, named msg, is a string potentially containing the values you passed to tracef() using your own format string. This also means that you can’t filter events with a custom expression at run time because there are no isolated fields.

  • Since tracef() uses the vasprintf(3) function of the C standard library behind the scenes to format the strings at run time, its expected performance is lower than with user-defined tracepoints, which don’t require a conversion to a string.

Taking this into consideration, tracef() is useful for some quick prototyping and debugging, but you shouldn’t consider it for any permanent and serious applicative instrumentation.

Use tracelog()

Since 2.7

The tracelog(3) API is very similar to tracef(), with the difference that it accepts an additional log level parameter.

The goal of tracelog() is to ease the migration from logging to tracing.

To use tracelog() in your application:

  1. In the C or C++ source files where you need to use tracelog(), include <lttng/tracelog.h>:

    #include <lttng/tracelog.h>
    
  2. In the source code of the application, use tracelog() like you would use printf(3), except for the first parameter which is the log level:

        /* ... */
    
        tracelog(TRACE_WARNING, "my message: %d (%s)",
                 my_integer, my_string);
    
        /* ... */
    

    See lttng-ust(3) for a list of available log level names.

  3. Link your application with liblttng-ust:

    $
    gcc -o app app.c -llttng-ust

To trace the events that tracelog() calls emit with a log level as severe as a specific log level:

  • Create an event rule which matches the lttng_ust_tracelog:* event name and a minimum level of severity:

    $
     
    lttng enable-event --userspace 'lttng_ust_tracelog:*'
                     --loglevel=TRACE_WARNING

To trace the events that tracelog() calls emit with a specific log level:

  • Create an event rule which matches the lttng_ust_tracelog:* event name and a specific log level:

    $
     
    lttng enable-event --userspace 'lttng_ust_tracelog:*'
                     --loglevel-only=TRACE_INFO

Prebuilt user space tracing helpers

The LTTng-UST package provides a few helpers in the form of preloadable shared objects which automatically instrument system functions and calls.

The helper shared objects are normally found in /usr/lib. If you built LTTng-UST from source, they are probably located in /usr/local/lib.

The installed user space tracing helpers in LTTng-UST 2.12 are:

liblttng-ust-libc-wrapper.so
liblttng-ust-pthread-wrapper.so

C standard library memory and POSIX threads function tracing.

liblttng-ust-cyg-profile.so
liblttng-ust-cyg-profile-fast.so

Function entry and exit tracing.

liblttng-ust-dl.so

Dynamic linker tracing.

To use a user space tracing helper with any user application:

  • Preload the helper shared object when you start the application:

    $
    LD_PRELOAD=liblttng-ust-libc-wrapper.so my-app

    You can preload more than one helper:

    $
    LD_PRELOAD=liblttng-ust-libc-wrapper.so:liblttng-ust-dl.so my-app

Instrument C standard library memory and POSIX threads functions

Since 2.3

The liblttng-ust-libc-wrapper.so and liblttng-ust-pthread-wrapper.so helpers add instrumentation to some C standard library and POSIX threads functions.

Functions instrumented by preloading liblttng-ust-libc-wrapper.so.

TP provider name TP name Instrumented function

lttng_ust_libc

malloc

malloc(3)

calloc

calloc(3)

realloc

realloc(3)

free

free(3)

memalign

memalign(3)

posix_memalign

posix_memalign(3)

Functions instrumented by preloading liblttng-ust-pthread-wrapper.so.

TP provider name TP name Instrumented function

lttng_ust_pthread

pthread_mutex_lock_req

pthread_mutex_lock(3p) (request time)

pthread_mutex_lock_acq

pthread_mutex_lock(3p) (acquire time)

pthread_mutex_trylock

pthread_mutex_trylock(3p)

pthread_mutex_unlock

pthread_mutex_unlock(3p)

When you preload the shared object, it replaces the functions listed in the previous tables by wrappers which contain tracepoints and call the replaced functions.

Instrument function entry and exit

The liblttng-ust-cyg-profile*.so helpers can add instrumentation to the entry and exit points of functions.

gcc(1) and clang(1) have an option named -finstrument-functions which generates instrumentation calls for entry and exit to functions. The LTTng-UST function tracing helpers, liblttng-ust-cyg-profile.so and liblttng-ust-cyg-profile-fast.so, take advantage of this feature to add tracepoints to the two generated functions (which contain cyg_profile in their names, hence the name of the helper).

To use the LTTng-UST function tracing helper, the source files to instrument must be built using the -finstrument-functions compiler flag.

There are two versions of the LTTng-UST function tracing helper:

  • liblttng-ust-cyg-profile-fast.so is a lightweight variant that you should only use when it can be guaranteed that the complete event stream is recorded without any lost event record. Any kind of duplicate information is left out.

    Assuming no event record is lost, having only the function addresses on entry is enough to create a call graph, since an event record always contains the ID of the CPU that generated it.

    Use a tool like addr2line(1) to convert function addresses back to source file names and line numbers.

  • liblttng-ust-cyg-profile.so is a more robust variant which also works in use cases where event records might get discarded or not recorded from application startup. In these cases, the trace analyzer needs more information to be able to reconstruct the program flow.

See lttng-ust-cyg-profile(3) to learn more about the instrumentation points of this helper.

All the tracepoints that this helper provides have the log level TRACE_DEBUG_FUNCTION (see lttng-ust(3)).

Tip:It’s sometimes a good idea to limit the number of source files that you compile with the -finstrument-functions option to prevent LTTng from writing an excessive amount of trace data at run time. When using gcc(1), use the -finstrument-functions-exclude-function-list option to avoid instrument entries and exits of specific function names.

Instrument the dynamic linker

Since 2.4

The liblttng-ust-dl.so helper adds instrumentation to the dlopen(3) and dlclose(3) function calls.

See lttng-ust-dl(3) to learn more about the instrumentation points of this helper.

User space Java agent

Since 2.4

You can instrument any Java application which uses one of the following logging frameworks:

LTTng-UST Java agent imported by a Java application.

Note that the methods described below are new in LTTng 2.8. Previous LTTng versions use another technique.

Note:We use OpenJDK 8 for development and continuous integration, thus this version is directly supported. However, the LTTng-UST Java agent is also tested with OpenJDK 7.

Use the LTTng-UST Java agent for java.util.logging

Since 2.8

To use the LTTng-UST Java agent in a Java application which uses java.util.logging (JUL):

  1. In the source code of the Java application, import the LTTng-UST log handler package for java.util.logging:

    import org.lttng.ust.agent.jul.LttngLogHandler;
    
  2. Create an LTTng-UST JUL log handler:

    Handler lttngUstLogHandler = new LttngLogHandler();
    
  3. Add this handler to the JUL loggers which should emit LTTng events:

    Logger myLogger = Logger.getLogger("some-logger");
    
    myLogger.addHandler(lttngUstLogHandler);
    
  4. Use java.util.logging log statements and configuration as usual. The loggers with an attached LTTng-UST log handler can emit LTTng events.

  5. Before exiting the application, remove the LTTng-UST log handler from the loggers attached to it and call its close() method:

    myLogger.removeHandler(lttngUstLogHandler);
    lttngUstLogHandler.close();
    

    This isn’t strictly necessary, but it is recommended for a clean disposal of the resources of the handler.

  6. Include the common and JUL-specific JAR files of the LTTng-UST Java agent, lttng-ust-agent-common.jar and lttng-ust-agent-jul.jar, in the class path when you build the Java application.

    The JAR files are typically located in /usr/share/java.

    Important:The LTTng-UST Java agent must be installed for the logging framework your application uses.

Example:Use the LTTng-UST Java agent for java.util.logging.

Test.java

import java.io.IOException;
import java.util.logging.Handler;
import java.util.logging.Logger;
import org.lttng.ust.agent.jul.LttngLogHandler;

public class Test
{
    private static final int answer = 42;

    public static void main(String[] argv) throws Exception
    {
        // Create a logger
        Logger logger = Logger.getLogger("jello");

        // Create an LTTng-UST log handler
        Handler lttngUstLogHandler = new LttngLogHandler();

        // Add the LTTng-UST log handler to our logger
        logger.addHandler(lttngUstLogHandler);

        // Log at will!
        logger.info("some info");
        logger.warning("some warning");
        Thread.sleep(500);
        logger.finer("finer information; the answer is " + answer);
        Thread.sleep(123);
        logger.severe("error!");

        // Not mandatory, but cleaner
        logger.removeHandler(lttngUstLogHandler);
        lttngUstLogHandler.close();
    }
}

Build this example:

$
javac -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar Test.java

Create a tracing session, create an event rule matching the jello JUL logger, and start tracing:

$
$
$
lttng create
lttng enable-event --jul jello
lttng start

Run the compiled class:

$
java -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar:. Test

Stop tracing and inspect the recorded events:

$
$
lttng stop
lttng view

In the resulting trace, an event record generated by a Java application using java.util.logging is named lttng_jul:event and has the following fields:

msg

Log record message.

logger_name

Logger name.

class_name

Name of the class in which the log statement was executed.

method_name

Name of the method in which the log statement was executed.

long_millis

Logging time (timestamp in milliseconds).

int_loglevel

Log level integer value.

int_threadid

ID of the thread in which the log statement was executed.

Use the --loglevel or --loglevel-only option of the lttng-enable-event(1) command to target a range of JUL log levels or a specific JUL log level.

Use the LTTng-UST Java agent for Apache log4j

Since 2.8

To use the LTTng-UST Java agent in a Java application which uses Apache log4j 1.2:

  1. In the source code of the Java application, import the LTTng-UST log appender package for Apache log4j:

    import org.lttng.ust.agent.log4j.LttngLogAppender;
    
  2. Create an LTTng-UST log4j log appender:

    Appender lttngUstLogAppender = new LttngLogAppender();
    
  3. Add this appender to the log4j loggers which should emit LTTng events:

    Logger myLogger = Logger.getLogger("some-logger");
    
    myLogger.addAppender(lttngUstLogAppender);
    
  4. Use Apache log4j log statements and configuration as usual. The loggers with an attached LTTng-UST log appender can emit LTTng events.

  5. Before exiting the application, remove the LTTng-UST log appender from the loggers attached to it and call its close() method:

    myLogger.removeAppender(lttngUstLogAppender);
    lttngUstLogAppender.close();
    

    This isn’t strictly necessary, but it is recommended for a clean disposal of the resources of the appender.

  6. Include the common and log4j-specific JAR files of the LTTng-UST Java agent, lttng-ust-agent-common.jar and lttng-ust-agent-log4j.jar, in the class path when you build the Java application.

    The JAR files are typically located in /usr/share/java.

    Important:The LTTng-UST Java agent must be installed for the logging framework your application uses.

Example:Use the LTTng-UST Java agent for Apache log4j.

Test.java

import org.apache.log4j.Appender;
import org.apache.log4j.Logger;
import org.lttng.ust.agent.log4j.LttngLogAppender;

public class Test
{
    private static final int answer = 42;

    public static void main(String[] argv) throws Exception
    {
        // Create a logger
        Logger logger = Logger.getLogger("jello");

        // Create an LTTng-UST log appender
        Appender lttngUstLogAppender = new LttngLogAppender();

        // Add the LTTng-UST log appender to our logger
        logger.addAppender(lttngUstLogAppender);

        // Log at will!
        logger.info("some info");
        logger.warn("some warning");
        Thread.sleep(500);
        logger.debug("debug information; the answer is " + answer);
        Thread.sleep(123);
        logger.fatal("error!");

        // Not mandatory, but cleaner
        logger.removeAppender(lttngUstLogAppender);
        lttngUstLogAppender.close();
    }
}

Build this example ($LOG4JPATH is the path to the Apache log4j JAR file):

$
javac -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-log4j.jar:$LOG4JPATH Test.java

Create a tracing session, create an event rule matching the jello log4j logger, and start tracing:

$
$
$
lttng create
lttng enable-event --log4j jello
lttng start

Run the compiled class:

$
java -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-log4j.jar:$LOG4JPATH:. Test

Stop tracing and inspect the recorded events:

$
$
lttng stop
lttng view

In the resulting trace, an event record generated by a Java application using log4j is named lttng_log4j:event and has the following fields:

msg

Log record message.

logger_name

Logger name.

class_name

Name of the class in which the log statement was executed.

method_name

Name of the method in which the log statement was executed.

filename

Name of the file in which the executed log statement is located.

line_number

Line number at which the log statement was executed.

timestamp

Logging timestamp.

int_loglevel

Log level integer value.

thread_name

Name of the Java thread in which the log statement was executed.

Use the --loglevel or --loglevel-only option of the lttng-enable-event(1) command to target a range of Apache log4j log levels or a specific log4j log level.

Provide application-specific context fields in a Java application

Since 2.8

A Java application-specific context field is a piece of state provided by the application which you can add, using the lttng-add-context(1) command, to each event record produced by the log statements of this application.

For example, a given object might have a current request ID variable. You can create a context information retriever for this object and assign a name to this current request ID. You can then, using the lttng-add-context(1) command, add this context field by name to the JUL or log4j channel.

To provide application-specific context fields in a Java application:

  1. In the source code of the Java application, import the LTTng-UST Java agent context classes and interfaces:

    import org.lttng.ust.agent.context.ContextInfoManager;
    import org.lttng.ust.agent.context.IContextInfoRetriever;
    
  2. Create a context information retriever class, that is, a class which implements the IContextInfoRetriever interface:

    class MyContextInfoRetriever implements IContextInfoRetriever
    {
        @Override
        public Object retrieveContextInfo(String key)
        {
            if (key.equals("intCtx")) {
                return (short) 17;
            } else if (key.equals("strContext")) {
                return "context value!";
            } else {
                return null;
            }
        }
    }
    

    This retrieveContextInfo() method is the only member of the IContextInfoRetriever interface. Its role is to return the current value of a state by name to create a context field. The names of the context fields and which state variables they return depends on your specific scenario.

    All primitive types and objects are supported as context fields. When retrieveContextInfo() returns an object, the context field serializer calls its toString() method to add a string field to event records. The method can also return null, which means that no context field is available for the required name.

  3. Register an instance of your context information retriever class to the context information manager singleton:

    IContextInfoRetriever cir = new MyContextInfoRetriever();
    ContextInfoManager cim = ContextInfoManager.getInstance();
    cim.registerContextInfoRetriever("retrieverName", cir);
    
  4. Before exiting the application, remove your context information retriever from the context information manager singleton:

    ContextInfoManager cim = ContextInfoManager.getInstance();
    cim.unregisterContextInfoRetriever("retrieverName");
    

    This isn’t strictly necessary, but it is recommended for a clean disposal of some resources of the manager.

  5. Build your Java application with LTTng-UST Java agent support as usual, following the procedure for either the JUL or Apache log4j framework.

Example:Provide application-specific context fields in a Java application.

Test.java

import java.util.logging.Handler;
import java.util.logging.Logger;
import org.lttng.ust.agent.jul.LttngLogHandler;
import org.lttng.ust.agent.context.ContextInfoManager;
import org.lttng.ust.agent.context.IContextInfoRetriever;

public class Test
{
    // Our context information retriever class
    private static class MyContextInfoRetriever
    implements IContextInfoRetriever
    {
        @Override
        public Object retrieveContextInfo(String key) {
            if (key.equals("intCtx")) {
                return (short) 17;
            } else if (key.equals("strContext")) {
                return "context value!";
            } else {
                return null;
            }
        }
    }

    private static final int answer = 42;

    public static void main(String args[]) throws Exception
    {
        // Get the context information manager instance
        ContextInfoManager cim = ContextInfoManager.getInstance();

        // Create and register our context information retriever
        IContextInfoRetriever cir = new MyContextInfoRetriever();
        cim.registerContextInfoRetriever("myRetriever", cir);

        // Create a logger
        Logger logger = Logger.getLogger("jello");

        // Create an LTTng-UST log handler
        Handler lttngUstLogHandler = new LttngLogHandler();

        // Add the LTTng-UST log handler to our logger
        logger.addHandler(lttngUstLogHandler);

        // Log at will!
        logger.info("some info");
        logger.warning("some warning");
        Thread.sleep(500);
        logger.finer("finer information; the answer is " + answer);
        Thread.sleep(123);
        logger.severe("error!");

        // Not mandatory, but cleaner
        logger.removeHandler(lttngUstLogHandler);
        lttngUstLogHandler.close();
        cim.unregisterContextInfoRetriever("myRetriever");
    }
}

Build this example:

$
javac -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar Test.java

Create a tracing session and create an event rule matching the jello JUL logger:

$
$
lttng create
lttng enable-event --jul jello

Add the application-specific context fields to the JUL channel:

$
$
lttng add-context --jul --type='$app.myRetriever:intCtx'
lttng add-context --jul --type='$app.myRetriever:strContext'

Start tracing:

$
lttng start

Run the compiled class:

$
java -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar:. Test

Stop tracing and inspect the recorded events:

$
$
lttng stop
lttng view

User space Python agent

Since 2.7

You can instrument a Python 2 or Python 3 application which uses the standard logging package.

Each log statement emits an LTTng event once the application module imports the LTTng-UST Python agent package.

A Python application importing the LTTng-UST Python agent.

To use the LTTng-UST Python agent:

  1. In the source code of the Python application, import the LTTng-UST Python agent:

    import lttngust
    

    The LTTng-UST Python agent automatically adds its logging handler to the root logger at import time.

    Any log statement that the application executes before this import does not emit an LTTng event.

    Important:The LTTng-UST Python agent must be installed.

  2. Use log statements and logging configuration as usual. Since the LTTng-UST Python agent adds a handler to the root logger, you can trace any log statement from any logger.

Example:Use the LTTng-UST Python agent.

test.py

import lttngust
import logging
import time


def example():
    logging.basicConfig()
    logger = logging.getLogger('my-logger')

    while True:
        logger.debug('debug message')
        logger.info('info message')
        logger.warn('warn message')
        logger.error('error message')
        logger.critical('critical message')
        time.sleep(1)


if __name__ == '__main__':
    example()

Note:logging.basicConfig(), which adds to the root logger a basic logging handler which prints to the standard error stream, isn’t strictly required for LTTng-UST tracing to work, but in versions of Python preceding 3.2, you could see a warning message which indicates that no handler exists for the logger my-logger.

Create a tracing session, create an event rule matching the my-logger Python logger, and start tracing:

$
$
$
lttng create
lttng enable-event --python my-logger
lttng start

Run the Python script:

$
python test.py

Stop tracing and inspect the recorded events:

$
$
lttng stop
lttng view

In the resulting trace, an event record generated by a Python application is named lttng_python:event and has the following fields:

asctime

Logging time (string).

msg

Log record message.

logger_name

Logger name.

funcName

Name of the function in which the log statement was executed.

lineno

Line number at which the log statement was executed.

int_loglevel

Log level integer value.

thread

ID of the Python thread in which the log statement was executed.

threadName

Name of the Python thread in which the log statement was executed.

Use the --loglevel or --loglevel-only option of the lttng-enable-event(1) command to target a range of Python log levels or a specific Python log level.

When an application imports the LTTng-UST Python agent, the agent tries to register to a session daemon. Note that you must start the session daemon before you run the Python application. If a session daemon is found, the agent tries to register to it during five seconds, after which the application continues without LTTng tracing support. Override this timeout value with the LTTNG_UST_PYTHON_REGISTER_TIMEOUT environment variable (milliseconds).

If the session daemon stops while a Python application with an imported LTTng-UST Python agent runs, the agent retries to connect and to register to a session daemon every three seconds. Override this delay with the LTTNG_UST_PYTHON_REGISTER_RETRY_DELAY environment variable.

LTTng logger

Since 2.5

The lttng-tracer Linux kernel module, part of LTTng-modules, creates the special LTTng logger files /proc/lttng-logger and /dev/lttng-logger (since LTTng 2.11) when it’s loaded. Any application can write text data to any of those files to emit an LTTng event.

An application writes to the LTTng logger file to emit an LTTng event.

The LTTng logger is the quickest method—not the most efficient, however—to add instrumentation to an application. It is designed mostly to instrument shell scripts:

$
echo "Some message, some $variable" > /dev/lttng-logger

Any event that the LTTng logger emits is named lttng_logger and belongs to the Linux kernel tracing domain. However, unlike other instrumentation points in the kernel tracing domain, any Unix user can create an event rule which matches its event name, not only the root user or users in the tracing group.

To use the LTTng logger:

  • From any application, write text data to the /dev/lttng-logger file.

The msg field of lttng_logger event records contains the recorded message.

Note:The maximum message length of an LTTng logger event is 1024 bytes. Writing more than this makes the LTTng logger emit more than one event to contain the remaining data.

You shouldn’t use the LTTng logger to trace a user application which can be instrumented in a more efficient way, namely:

Example:Use the LTTng logger.

test.bash

echo 'Hello, World!' > /dev/lttng-logger
sleep 2
df --human-readable --print-type / > /dev/lttng-logger

Create a tracing session, create an event rule matching the lttng_logger Linux kernel tracepoint, and start tracing:

$
$
$
lttng create
lttng enable-event --kernel lttng_logger
lttng start

Run the Bash script:

$
bash test.bash

Stop tracing and inspect the recorded events:

$
$
lttng stop
lttng view

LTTng kernel tracepoints

Note:This section shows how to add instrumentation points to the Linux kernel. The subsystems of the kernel are already thoroughly instrumented at strategic places for LTTng when you install the LTTng-modules package.

Add an LTTng layer to an existing ftrace tracepoint

This section shows how to add an LTTng layer to existing ftrace instrumentation using the TRACE_EVENT() API.

This section doesn’t document the TRACE_EVENT() macro. Read the following articles to learn more about this API:

The following procedure assumes that your ftrace tracepoints are correctly defined in their own header and that they are created in one source file using the CREATE_TRACE_POINTS definition.

To add an LTTng layer over an existing ftrace tracepoint:

  1. Make sure the following kernel configuration options are enabled:

    • CONFIG_MODULES

    • CONFIG_KALLSYMS

    • CONFIG_HIGH_RES_TIMERS

    • CONFIG_TRACEPOINTS

  2. Build the Linux source tree with your custom ftrace tracepoints.

  3. Boot the resulting Linux image on your target system.

    Confirm that the tracepoints exist by looking for their names in the /sys/kernel/debug/tracing/events/subsys directory, where subsys is your subsystem name.

  4. Get a copy of the latest LTTng-modules 2.12:

    $
     
     
     
    cd $(mktemp -d) &&
    wget http://lttng.org/files/lttng-modules/lttng-modules-latest-2.12.tar.bz2 &&
    tar -xf lttng-modules-latest-2.12.tar.bz2 &&
    cd lttng-modules-2.12.*
  5. In instrumentation/events/lttng-module, relative to the root of the LTTng-modules source tree, create a header file named subsys.h for your custom subsystem subsys and write your LTTng-modules tracepoint definitions using the LTTng-modules macros in it.

    Start with this template:

    instrumentation/events/lttng-module/my_subsys.h

    #undef TRACE_SYSTEM
    #define TRACE_SYSTEM my_subsys
    
    #if !defined(_LTTNG_MY_SUBSYS_H) || defined(TRACE_HEADER_MULTI_READ)
    #define _LTTNG_MY_SUBSYS_H
    
    #include "../../../probes/lttng-tracepoint-event.h"
    #include <linux/tracepoint.h>
    
    LTTNG_TRACEPOINT_EVENT(
        /*
         * Format is identical to the TRACE_EVENT() version for the three
         * following macro parameters:
         */
        my_subsys_my_event,
        TP_PROTO(int my_int, const char *my_string),
        TP_ARGS(my_int, my_string),
    
        /* LTTng-modules specific macros */
        TP_FIELDS(
            ctf_integer(int, my_int_field, my_int)
            ctf_string(my_bar_field, my_bar)
        )
    )
    
    #endif /* !defined(_LTTNG_MY_SUBSYS_H) || defined(TRACE_HEADER_MULTI_READ) */
    
    #include "../../../probes/define_trace.h"
    

    The entries in the TP_FIELDS() section are the list of fields for the LTTng tracepoint. This is similar to the TP_STRUCT__entry() part of the TRACE_EVENT() ftrace macro.

    See Tracepoint fields macros for a complete description of the available ctf_*() macros.

  6. Create the kernel module C source file of the LTTng-modules probe, probes/lttng-probe-subsys.c, where subsys is your subsystem name:

    probes/lttng-probe-my-subsys.c

    #include <linux/module.h>
    #include "../lttng-tracer.h"
    
    /*
     * Build-time verification of mismatch between mainline
     * TRACE_EVENT() arguments and the LTTng-modules adaptation
     * layer LTTNG_TRACEPOINT_EVENT() arguments.
     */
    #include <trace/events/my_subsys.h>
    
    /* Create LTTng tracepoint probes */
    #define LTTNG_PACKAGE_BUILD
    #define CREATE_TRACE_POINTS
    #define TRACE_INCLUDE_PATH ../instrumentation/events/lttng-module
    
    #include "../instrumentation/events/lttng-module/my_subsys.h"
    
    MODULE_LICENSE("GPL and additional rights");
    MODULE_AUTHOR("Your name <your-email>");
    MODULE_DESCRIPTION("LTTng my_subsys probes");
    MODULE_VERSION(__stringify(LTTNG_MODULES_MAJOR_VERSION) "."
        __stringify(LTTNG_MODULES_MINOR_VERSION) "."
        __stringify(LTTNG_MODULES_PATCHLEVEL_VERSION)
        LTTNG_MODULES_EXTRAVERSION);
    
  7. Edit probes/KBuild and add your new kernel module object next to the existing ones:

    probes/KBuild

    # ...
    
    obj-m += lttng-probe-module.o
    obj-m += lttng-probe-power.o
    
    obj-m += lttng-probe-my-subsys.o
    
    # ...
    
  8. Build and install the LTTng kernel modules:

    $
    #
    make KERNELDIR=/path/to/linux
    make modules_install && depmod -a

    Replace /path/to/linux with the path to the Linux source tree where you defined and used tracepoints with the TRACE_EVENT() ftrace macro.

Note that you can also use the LTTNG_TRACEPOINT_EVENT_CODE() macro instead of LTTNG_TRACEPOINT_EVENT() to use custom local variables and C code that need to be executed before the event fields are recorded.

The best way to learn how to use the previous LTTng-modules macros is to inspect the existing LTTng-modules tracepoint definitions in the instrumentation/events/lttng-module header files. Compare them with the Linux kernel mainline versions in the include/trace/events directory of the Linux source tree.

Use custom C code to access the data for tracepoint fields
Since 2.7

Although we recommended to always use the LTTNG_TRACEPOINT_EVENT() macro to describe the arguments and fields of an LTTng-modules tracepoint when possible, sometimes you need a more complex process to access the data that the tracer records as event record fields. In other words, you need local variables and multiple C statements instead of simple argument-based expressions that you pass to the ctf_*() macros of TP_FIELDS().

Use the LTTNG_TRACEPOINT_EVENT_CODE() macro instead of LTTNG_TRACEPOINT_EVENT() to declare custom local variables and define a block of C code to be executed before LTTng records the fields. The structure of this macro is:

LTTNG_TRACEPOINT_EVENT_CODE() macro syntax.

LTTNG_TRACEPOINT_EVENT_CODE(
    /*
     * Format identical to the LTTNG_TRACEPOINT_EVENT()
     * version for the following three macro parameters:
     */
    my_subsys_my_event,
    TP_PROTO(int my_int, const char *my_string),
    TP_ARGS(my_int, my_string),

    /* Declarations of custom local variables */
    TP_locvar(
        int a = 0;
        unsigned long b = 0;
        const char *name = "(undefined)";
        struct my_struct *my_struct;
    ),

    /*
     * Custom code which uses both tracepoint arguments
     * (in TP_ARGS()) and local variables (in TP_locvar()).
     *
     * Local variables are actually members of a structure pointed
     * to by the special variable tp_locvar.
     */
    TP_code(
        if (my_int) {
            tp_locvar->a = my_int + 17;
            tp_locvar->my_struct = get_my_struct_at(tp_locvar->a);
            tp_locvar->b = my_struct_compute_b(tp_locvar->my_struct);
            tp_locvar->name = my_struct_get_name(tp_locvar->my_struct);
            put_my_struct(tp_locvar->my_struct);

            if (tp_locvar->b) {
                tp_locvar->a = 1;
            }
        }
    ),

    /*
     * Format identical to the LTTNG_TRACEPOINT_EVENT()
     * version for this, except that tp_locvar members can be
     * used in the argument expression parameters of
     * the ctf_*() macros.
     */
    TP_FIELDS(
        ctf_integer(unsigned long, my_struct_b, tp_locvar->b)
        ctf_integer(int, my_struct_a, tp_locvar->a)
        ctf_string(my_string_field, my_string)
        ctf_string(my_struct_name, tp_locvar->name)
    )
)

Important:The C code defined in TP_code() must not have any side effects when executed. In particular, the code must not allocate memory or get resources without deallocating this memory or putting those resources afterwards.

Load and unload a custom probe kernel module

You must load a created LTTng-modules probe kernel module in the kernel before it can emit LTTng events.

To load the default probe kernel modules and a custom probe kernel module:

  • Use the --extra-kmod-probes option to give extra probe modules to load when starting a root session daemon:

    Example:Load the my_subsys, usb, and the default probe modules.

    #
    lttng-sessiond --extra-kmod-probes=my_subsys,usb

    You only need to pass the subsystem name, not the whole kernel module name.

To load only a given custom probe kernel module:

  • Use the --kmod-probes option to give the probe modules to load when starting a root session daemon:

    Example:Load only the my_subsys and usb probe modules.

    #
    lttng-sessiond --kmod-probes=my_subsys,usb

To confirm that a probe module is loaded:

  • Use lsmod(8):

    $
    lsmod | grep lttng_probe_usb

To unload the loaded probe modules:

  • Kill the session daemon with SIGTERM:

    #
    pkill lttng-sessiond

    You can also use the modprobe(8) --remove option if the session daemon terminates abnormally.

Tracing control

Once an application or a Linux kernel is instrumented for LTTng tracing, you can trace it.

This section is divided in topics on how to use the various components of LTTng, in particular the lttng command-line tool, to control the LTTng daemons and tracers.

Note:In the following subsections, we refer to an lttng(1) command using its man page name. For example, instead of Run the create command to…, we use Run the lttng-create(1) command to….

Start a session daemon

In some situations, you need to run a session daemon (lttng-sessiond(8)) before you can use the lttng(1) command-line tool.

You will see the following error when you run a command while no session daemon is running:

Error: No session daemon is available

The only command that automatically runs a session daemon is lttng-create(1), which you use to create a tracing session. While this is most of the time the first operation that you do, sometimes it’s not. Some examples are:

Each Unix user must have its own running session daemon to trace user applications. The session daemon that the root user starts is the only one allowed to control the LTTng kernel tracer. Users that are part of the tracing group can control the root session daemon. The default tracing group name is tracing; set it to something else with the --group option when you start the root session daemon.

To start a user session daemon:

To start the root session daemon:

In both cases, remove the --daemonize option to start the session daemon in foreground.

To stop a session daemon, use kill(1) on its process ID (standard TERM signal).

Note that some Linux distributions could manage the LTTng session daemon as a service. In this case, you should use the service manager to start, restart, and stop session daemons.

Create and destroy a tracing session

Almost all the LTTng control operations happen in the scope of a tracing session, which is the dialogue between the session daemon and you.

To create a tracing session with a generated name:

The name of the created tracing session is auto followed by the creation date.

To create a tracing session with a specific name:

  • Use the optional argument of the lttng-create(1) command:

    $
    lttng create my-session

    Replace my-session with the specific tracing session name.

LTTng appends the creation date to the name of the created tracing session.

LTTng writes the traces of a tracing session in $LTTNG_HOME/lttng-trace/name by default, where name is the name of the tracing session. Note that the LTTNG_HOME environment variable defaults to $HOME if not set.

To output LTTng traces to a non-default location:

You may create as many tracing sessions as you wish.

To list all the existing tracing sessions for your Unix user:

When you create a tracing session, it is set as the current tracing session. The following lttng(1) commands operate on the current tracing session when you don’t specify one:

To change the current tracing session:

  • Use the lttng-set-session(1) command:

    $
    lttng set-session new-session

    Replace new-session by the name of the new current tracing session.

When you’re done tracing in a given tracing session, destroy it. This operation frees the resources taken by the tracing session to destroy; it doesn’t destroy the trace data that LTTng wrote for this tracing session (see Clear a tracing session for one way to do this).

To destroy the current tracing session:

The lttng-destroy(1) command also runs the lttng-stop(1) command implicitly (see Start and stop a tracing session). You need to stop tracing to make LTTng flush the remaining trace data and make the trace readable.

List the available instrumentation points

The session daemon can query the running instrumented user applications and the Linux kernel to get a list of available instrumentation points. For the Linux kernel tracing domain, they are tracepoints and system calls. For the user space tracing domain, they are tracepoints. For the other tracing domains, they are logger names.

To list the available instrumentation points:

  • Use the lttng-list(1) command with the option of the requested tracing domain amongst:

    --kernel

    Linux kernel tracepoints (your Unix user must be a root user, or it must be a member of the tracing group).

    --kernel with --syscall

    Linux kernel system calls (your Unix user must be a root user, or it must be a member of the tracing group).

    --userspace

    User space tracepoints.

    --jul

    java.util.logging loggers.

    --log4j

    Apache log4j loggers.

    --python

    Python loggers.

Example:List the available user space tracepoints.

$
lttng list --userspace

Example:List the available Linux kernel system call tracepoints.

$
lttng list --kernel --syscall

Create and enable an event rule

Once you create a tracing session, you can create event rules with the lttng-enable-event(1) command.

You specify each condition with a command-line option. The available condition arguments are shown in the following table.

Condition command-line arguments for the lttng-enable-event(1) command.

Argument Description Applicable tracing domains

One of:

  1. --syscall

  2. --probe=ADDR

  3. --function=ADDR

  4. --userspace-probe=PATH:SYMBOL

  5. --userspace-probe=sdt:PATH:PROVIDER:NAME

Instead of using the default tracepoint instrumentation type, use:

  1. A Linux system call (entry and exit).

  2. A Linux kprobe (symbol or address).

  3. The entry and return points of a Linux function (symbol or address).

  4. The entry point of a user application or library function (path to application/library and symbol).

  5. A SystemTap Statically Defined Tracing (USDT) probe (path to application/library, provider and probe names).

Linux kernel.

First positional argument.

Tracepoint or system call name.

With the --probe, --function, and --userspace-probe options, this is a custom name given to the event rule. With the JUL, log4j, and Python domains, this is a logger name.

With a tracepoint, logger, or system call name, use the special * globbing character to match anything (for example, sched_*, my_comp*:*msg_*).

All.

One of:

  1. --loglevel=LEVEL

  2. --loglevel-only=LEVEL

  1. Match only tracepoints or log statements with a logging level at least as severe as LEVEL.

  2. Match only tracepoints or log statements with a logging level equal to LEVEL.

See lttng-enable-event(1) for the list of available logging level names.

User space, JUL, log4j, and Python.

--exclude=EXCLUSIONS

When you use a * character at the end of the tracepoint or logger name (first positional argument), exclude the specific names in the comma-delimited list EXCLUSIONS.

User space, JUL, log4j, and Python.

--filter=EXPR

Match only events which satisfy the expression EXPR.

See lttng-enable-event(1) to learn more about the syntax of a filter expression.

All.

You attach an event rule to a channel on creation. If you do not specify the channel with the --channel option, and if the event rule to create is the first in its tracing domain for a given tracing session, then LTTng creates a default channel for you. This default channel is reused in subsequent invocations of the lttng-enable-event(1) command for the same tracing domain.

An event rule is always enabled at creation time.

The following examples show how to combine the previous command-line options to create simple to more complex event rules.

Example:Create an event rule targetting a Linux kernel tracepoint (default channel).

$
lttng enable-event --kernel sched_switch

Example:Create an event rule matching four Linux kernel system calls (default channel).

$
lttng enable-event --kernel --syscall open,write,read,close

Example:Create event rules matching tracepoints with filter expressions (default channel).

$
lttng enable-event --kernel sched_switch --filter='prev_comm == "bash"'
$
 
lttng enable-event --kernel --all \
                 --filter='$ctx.tid == 1988 || $ctx.tid == 1534'
$
 
lttng enable-event --jul my_logger \
                 --filter='$app.retriever:cur_msg_id > 3'

Important:Make sure to always quote the filter string when you use lttng(1) from a shell.

See also Track process attributes which offers another, more efficient filtering mechanism for process ID, user ID, and group ID attributes.

Example:Create an event rule matching any user space tracepoint of a given tracepoint provider with a log level range (default channel).

$
lttng enable-event --userspace my_app:'*' --loglevel=TRACE_INFO

Important:Make sure to always quote the wildcard character when you use lttng(1) from a shell.

Example:Create an event rule matching multiple Python loggers with a wildcard and with exclusions (default channel).

$
 
lttng enable-event --python my-app.'*' \
                 --exclude='my-app.module,my-app.hello'

Example:Create an event rule matching any Apache log4j logger with a specific log level (default channel).

$
lttng enable-event --log4j --all --loglevel-only=LOG4J_WARN

Example:Create an event rule attached to a specific channel matching a specific user space tracepoint provider and tracepoint.

$
lttng enable-event --userspace my_app:my_tracepoint --channel=my-channel

Example:Create an event rule matching the malloc function entry in /usr/lib/libc.so.6:

$
 
lttng enable-event --kernel --userspace-probe=/usr/lib/libc.so.6:malloc \
                 libc_malloc

Example:Create an event rule matching the server/accept_request USDT probe in /usr/bin/serv:

$
 
lttng enable-event --kernel --userspace-probe=sdt:serv:server:accept_request \
                server_accept_request

The event rules of a given channel form a whitelist: as soon as an emitted event passes one of them, LTTng can record the event. For example, an event named my_app:my_tracepoint emitted from a user space tracepoint with a TRACE_ERROR log level passes both of the following rules:

$
$
 
lttng enable-event --userspace my_app:my_tracepoint
lttng enable-event --userspace my_app:my_tracepoint \
                 --loglevel=TRACE_INFO

The second event rule is redundant: the first one includes the second one.

Disable an event rule

To disable an event rule that you created previously, use the lttng-disable-event(1) command. This command disables all the event rules (of a given tracing domain and channel) which match an instrumentation point. The other conditions aren’t supported as of LTTng 2.12.

The LTTng tracer doesn’t record an emitted event which passes a disabled event rule.

Example:Disable an event rule matching a Python logger (default channel).

$
lttng disable-event --python my-logger

Example:Disable an event rule matching all java.util.logging loggers (default channel).

$
lttng disable-event --jul '*'

Example:Disable all the event rules of the default channel.

The --all-events option isn’t, like the --all option of lttng-enable-event(1), the equivalent of the event name * (wildcard): it disables all the event rules of a given channel.

$
lttng disable-event --jul --all-events

Note:You can’t delete an event rule once you create it.

Get the status of a tracing session

To get the status of the current tracing session, that is, its parameters, its channels, event rules, and their attributes:

To get the status of any tracing session:

  • Use the lttng-list(1) command with the name of the tracing session:

    $
    lttng list my-session

    Replace my-session with the desired tracing session name.

Start and stop a tracing session

Once you create a tracing session and create one or more event rules, you can start and stop the tracers for this tracing session.

To start tracing in the current tracing session:

LTTng is very flexible: you can launch user applications before or after the you start the tracers. The tracers only record the events if they pass enabled event rules and if they occur while the tracers are started.

To stop tracing in the current tracing session:

Important:You need to stop tracing to make LTTng flush the remaining trace data and make the trace readable. Note that the lttng-destroy(1) command (see Create and destroy a tracing session) also runs the lttng-stop(1) command implicitly.

Clear a tracing session

Since 2.12

You might need to remove all the current tracing data of one or more tracing sessions between multiple attempts to reproduce a problem without interrupting the LTTng tracing activity.

To clear the tracing data of the current tracing session:

To clear the tracing data of all the tracing sessions:

  • Use the lttng clear command with the --all option:

    $
    lttng clear --all

Create a channel

Once you create a tracing session, you can create a channel with the lttng-enable-channel(1) command.

Note that LTTng automatically creates a default channel when, for a given tracing domain, no channels exist and you create the first event rule. This default channel is named channel0 and its attributes are set to reasonable values. Therefore, you only need to create a channel when you need non-default attributes.

You specify each non-default channel attribute with a command-line option when you use the lttng-enable-channel(1) command. The available command-line options are:

Command-line options for the lttng-enable-channel(1) command.

Option Description

--overwrite

Use the overwrite event record loss mode instead of the default discard mode.

--buffers-pid (user space tracing domain only)

Use the per-process buffering scheme instead of the default per-user buffering scheme.

--subbuf-size=SIZE

Allocate sub-buffers of SIZE bytes (power of two), for each CPU, either for each Unix user (default), or for each instrumented process.

See Sub-buffer count and size.

--num-subbuf=COUNT

Allocate COUNT sub-buffers (power of two), for each CPU, either for each Unix user (default), or for each instrumented process.

See Sub-buffer count and size.

--tracefile-size=SIZE

Set the maximum size of each trace file that this channel writes within a stream to SIZE bytes instead of no maximum.

See Trace file count and size.

--tracefile-count=COUNT

Limit the number of trace files that this channel creates to COUNT channels instead of no limit.

See Trace file count and size.

--switch-timer=PERIODUS

Set the switch timer period to PERIODUS µs.

--read-timer=PERIODUS

Set the read timer period to PERIODUS µs.

--blocking-timeout=TIMEOUTUS

Set the timeout of user space applications which load LTTng-UST in blocking mode to TIMEOUTUS:

0 (default)

Never block (non-blocking mode).

inf

Block forever until space is available in a sub-buffer to record the event.

n, a positive value

Wait for at most n µs when trying to write into a sub-buffer.

Note that, for this option to have any effect on an instrumented user space application, you need to run the application with a set LTTNG_UST_ALLOW_BLOCKING environment variable.

--output=TYPE (Linux kernel tracing domain only)

Set the output type of the channel to TYPE, either mmap or splice.

You can only create a channel in the Linux kernel and user space tracing domains: other tracing domains have their own channel created on the fly when creating event rules.

Important:Because of a current LTTng limitation, you must create all channels before you start tracing in a given tracing session, that is, before the first time you run lttng-start(1).

Since LTTng automatically creates a default channel when you use the lttng-enable-event(1) command with a specific tracing domain, you can’t, for example, create a Linux kernel event rule, start tracing, and then create a user space event rule, because no user space channel exists yet and it’s too late to create one.

For this reason, make sure to configure your channels properly before starting the tracers for the first time!

The following examples show how to combine the previous command-line options to create simple to more complex channels.

Example:Create a Linux kernel channel with default attributes.

$
lttng enable-channel --kernel my-channel

Example:Create a user space channel with four sub-buffers or 1 MiB each, per CPU, per instrumented process.

$
 
lttng enable-channel --userspace --num-subbuf=4 --subbuf-size=1M \
                   --buffers-pid my-channel

Example:Create a default user space channel with an infinite blocking timeout.

Create a tracing-session, create the channel, create an event rule, and start tracing:

$
$
$
$
lttng create
lttng enable-channel --userspace --blocking-timeout=inf blocking-channel
lttng enable-event --userspace --channel=blocking-channel --all
lttng start

Run an application instrumented with LTTng-UST and allow it to block:

$
LTTNG_UST_ALLOW_BLOCKING=1 my-app

Example:Create a Linux kernel channel which rotates eight trace files of 4 MiB each for each stream

$
 
lttng enable-channel --kernel --tracefile-count=8 \
                   --tracefile-size=4194304 my-channel

Example:Create a user space channel in overwrite (or flight recorder) mode.

$
lttng enable-channel --userspace --overwrite my-channel

Create the same event rule in two different channels:

$
$
lttng enable-event --userspace --channel=my-channel app:tp
lttng enable-event --userspace --channel=other-channel app:tp

If both channels are enabled, when a tracepoint named app:tp is reached, LTTng records two events, one for each channel.

Disable a channel

To disable a specific channel that you created previously, use the lttng-disable-channel(1) command.

Example:Disable a specific Linux kernel channel.

$
lttng disable-channel --kernel my-channel

The state of a channel precedes the individual states of event rules attached to it: event rules which belong to a disabled channel, even if they are enabled, are also considered disabled.

Add context fields to a channel

Event record fields in trace files provide important information about events that occured previously, but sometimes some external context may help you solve a problem faster.

Examples of context fields are:

  • The process ID, thread ID, process name, and process priority of the thread in which the event occurs.

  • The hostname of the system on which the event occurs.

  • The Linux kernel and user call stacks (since LTTng 2.11).

  • The current values of many possible performance counters using perf, for example:

    • CPU cycles, stalled cycles, idle cycles, and the other cycle types.

    • Cache misses.

    • Branch instructions, misses, and loads.

    • CPU faults.

  • Any context defined at the application level (supported for the JUL and log4j tracing domains).

To get the full list of available context fields, see lttng add-context --list. Some context fields are reserved for a specific tracing domain (Linux kernel or user space).

You add context fields to channels. All the events that a channel with added context fields records contain those fields.

To add context fields to one or all the channels of a given tracing session:

Example:Add context fields to all the channels of the current tracing session.

The following command line adds the virtual process identifier and the per-thread CPU cycles count fields to all the user space channels of the current tracing session.

$
lttng add-context --userspace --type=vpid --type=perf:thread:cpu-cycles

Example:Add performance counter context fields by raw ID

See lttng-add-context(1) for the exact format of the context field type, which is partly compatible with the format used in perf-record(1).

$
$
lttng add-context --userspace --type=perf:thread:raw:r0110:test
lttng add-context --kernel --type=perf:cpu:raw:r0013c:x86unhalted

Example:Add context fields to a specific channel.

The following command line adds the thread identifier and user call stack context fields to the Linux kernel channel named my-channel in the current tracing session.

$
 
lttng add-context --kernel --channel=my-channel \
   --type=tid --type=callstack-user

Example:Add an application-specific context field to a specific channel.

The following command line adds the cur_msg_id context field of the retriever context retriever for all the instrumented Java applications recording event records in the channel named my-channel:

$
 
lttng add-context --kernel --channel=my-channel \
                --type='$app:retriever:cur_msg_id'

Important:Make sure to always quote the $ character when you use lttng-add-context(1) from a shell.

Note:You can’t remove context fields from a channel once you add it.

Track process attributes

Since 2.7

It’s often useful to only allow processes with specific attributes to emit events. For example, you may wish to record all the system calls which a given process makes (à la strace).

The lttng-track(1) and lttng-untrack(1) commands serve this purpose. Both commands operate on inclusion sets of process attribute values. The available process attribute types are:

Linux kernel tracing domain only
  • Process ID (PID).

  • Virtual process ID (VPID).

    This is the PID as seen by the application.

  • Unix user ID (UID) (since LTTng 2.12).

  • Virtual Unix user ID (VUID) (since LTTng 2.12).

    This is the UID as seen by the application.

  • Unix group ID (GID) (since LTTng 2.12).

  • Virtual Unix group ID (VGID) (since LTTng 2.12).

    This is the GID as seen by the application.

User space tracing domain
  • VPID.

  • VUID (since LTTng 2.12).

  • VGID (since LTTng 2.12).

Each tracing domain has one inclusion set per process attribute type: the Linux kernel tracing domain has six while the user space tracing domain has three.

For a given event which passes an enabled event rule to be recorded, all the attributes of its executing process must be part of the inclusion sets of the tracing domain of the event rule.

Add entries to an inclusion set with the lttng-track(1) command and remove entries with the lttng-untrack(1) command. A process attribute is tracked when it’s part of an inclusion set and untracked otherwise.

Note:The process attribute values are numeric.

Should a process with a given tracked process ID, for example, exit, and then a new process be given this ID, then the latter would also be allowed to emit events.

With the lttng track command, you can add Unix user and group names to the user and group inclusion sets: the session daemon finds the corresponding UID, VUID, GID, or VGID once on addition to the inclusion set. This means that if you rename the user or group after you run lttng track, its user/group ID remains tracked.

Example:Track and untrack virtual process IDs.

For the sake of the following example, assume the target system has 16 possible VPIDs.

When you create a tracing session, the user space VPID inclusion set contains all the possible VPIDs:

All VPIDs are tracked.

When the inclusion set is full and you use the lttng-track(1) command to specify some VPIDs to track, LTTng first clears the inclusion set, and then it adds the specific VPIDs to track. After:

$
lttng track --userspace --vpid=3,4,7,10,13

the VPID inclusion set is:

VPIDs 3, 4, 7, 10, and 13 are tracked.

Add more VPIDs to the inclusion set afterwards:

$
lttng track --userspace --vpid=1,15,16

The result is:

VPIDs 1, 15, and 16 are added to the inclusion set.

The lttng-untrack(1) command removes entries from process attribute inclusion sets. Given the previous example, the following command:

$
lttng untrack --userspace --vpid=3,7,10,13

leads to this VPID inclusion set:

VPIDs 3, 7, 10, and 13 are removed from the inclusion set.

LTTng can track all the possible VPIDs again using the --all option:

$
lttng track --userspace --vpid --all

The result is, again:

All VPIDs are tracked.

Example:Track only specific process attributes.

A typical use case with process attribute tracking is to start with an empty inclusion set, then start the tracers, and then add entries manually while the tracers are active.

Use the --all option of the lttng-untrack(1) command to clear the inclusion set after you create a tracing session, for example (with UIDs):

$
lttng untrack --kernel --uid --all

gives:

No UIDs are tracked.

If you trace with this inclusion set configuration, the LTTng kernel tracer records no events within the current tracing session because it doesn’t track any UID. Use the lttng-track(1) command as usual to track specific UIDs when you need to, for example:

$
lttng track --kernel --uid=http,11

Result:

UIDs 6 (http) and 11 are tracked.

Save and load tracing session configurations

Since 2.5

Configuring a tracing session can be long. Some of the tasks involved are:

If you use LTTng to solve real world problems, chances are you have to record events using the same tracing session setup over and over, modifying a few variables each time in your instrumented program or environment. To avoid constant tracing session reconfiguration, the lttng(1) command-line tool can save and load tracing session configurations to/from XML files.

To save a given tracing session configuration:

  • Use the lttng-save(1) command:

    $
    lttng save my-session

    Replace my-session with the name of the tracing session to save.

LTTng saves tracing session configurations to $LTTNG_HOME/.lttng/sessions by default. Note that the LTTNG_HOME environment variable defaults to $HOME if not set. Use the --output-path option to change this destination directory.

LTTng saves all configuration parameters, for example:

  • The tracing session name.

  • The trace data output path.

  • The channels with their state and all their attributes.

  • The context fields you added to channels.

  • The event rules with their state, log level and filter conditions.

To load a tracing session:

  • Use the lttng-load(1) command:

    $
    lttng load my-session

    Replace my-session with the name of the tracing session to load.

When LTTng loads a configuration, it restores your saved tracing session as if you just configured it manually.

See lttng-load(1) for the complete list of command-line options. You can also save and load many sessions at a time, and decide in which directory to output the XML files.

Send trace data over the network

LTTng can send the recorded trace data to a remote system over the network instead of writing it to the local file system.

To send the trace data over the network:

  1. On the remote system (which can also be the target system), start an LTTng relay daemon (lttng-relayd(8)):

    $
    lttng-relayd
  2. On the target system, create a tracing session configured to send trace data over the network:

    $
    lttng create my-session --set-url=net://remote-system

    Replace remote-system by the host name or IP address of the remote system. See lttng-create(1) for the exact URL format.

  3. On the target system, use the lttng(1) command-line tool as usual. When tracing is active, the consumer daemon of the target sends sub-buffers to the relay daemon running on the remote system instead of flushing them to the local file system. The relay daemon writes the received packets to the local file system.

The relay daemon writes trace files to $LTTNG_HOME/lttng-traces/hostname/session by default, where hostname is the host name of the target system and session is the tracing session name. Note that the LTTNG_HOME environment variable defaults to $HOME if not set. Use the --output option of lttng-relayd(8) to write trace files to another base directory.

View events as LTTng emits them (LTTng live)

Since 2.4

LTTng live is a network protocol implemented by the relay daemon (lttng-relayd(8)) to allow compatible trace viewers to display events as LTTng emits them on the target system while tracing is active.

The relay daemon creates a tee: it forwards the trace data to both the local file system and to connected live viewers:

The relay daemon creates a tee, forwarding the trace data to both trace files and a connected live viewer.

To use LTTng live:

  1. On the target system, create a tracing session in live mode:

    $
    lttng create my-session --live

    This spawns a local relay daemon.

  2. Start the live viewer and configure it to connect to the relay daemon. For example, with babeltrace2:

    $
    babeltrace2 net://localhost/host/hostname/my-session

    Replace:

    • hostname with the host name of the target system.

    • my-session with the name of the tracing session to view.

  3. Configure the tracing session as usual with the lttng(1) command-line tool, and start tracing.

List the available live tracing sessions with Babeltrace 2:

$
babeltrace2 net://localhost

You can start the relay daemon on another system. In this case, you need to specify the URL of the relay daemon when you create the tracing session with the --set-url option. You also need to replace localhost in the procedure above with the host name of the system on which the relay daemon is running.

See lttng-create(1) and lttng-relayd(8) for the complete list of command-line options.

Take a snapshot of the current sub-buffers of a tracing session

Since 2.3

The normal behavior of LTTng is to append full sub-buffers to growing trace data files. This is ideal to keep a full history of the events that occurred on the target system, but it can represent too much data in some situations. For example, you may wish to trace your application continuously until some critical situation happens, in which case you only need the latest few recorded events to perform the desired analysis, not multi-gigabyte trace files.

With the lttng-snapshot(1) command, you can take a snapshot of the current sub-buffers of a given tracing session. LTTng can write the snapshot to the local file system or send it over the network.

A snapshot is a copy of the current sub-buffers, which aren’t cleared after the operation.

If you wish to create unmanaged, self-contained, non-overlapping trace chunk archives instead of a simple copy of the current sub-buffers, see the tracing session rotation feature (available since LTTng 2.11).

To take a snapshot:

  1. Create a tracing session in snapshot mode:

    $
    lttng create my-session --snapshot

    The event record loss mode of channels created in this mode is automatically set to overwrite (flight recorder mode).

  2. Configure the tracing session as usual with the lttng(1) command-line tool, and start tracing.

  3. Optional: When you need to take a snapshot, stop tracing.

    You can take a snapshot when the tracers are active, but if you stop them first, you’re sure that the data in the sub-buffers doesn’t change before you actually take the snapshot.

  4. Take a snapshot:

    $
    lttng snapshot record --name=my-first-snapshot

    LTTng writes the current sub-buffers of all the channels of the current tracing session to trace files on the local file system. Those trace files have my-first-snapshot in their name.

There is no difference between the format of a normal trace file and the format of a snapshot: viewers of LTTng traces also support LTTng snapshots.

By default, LTTng writes snapshot files to the path shown by lttng snapshot list-output. You can change this path or decide to send snapshots over the network using either:

  1. An output path or URL that you specify when you create the tracing session.

  2. A snapshot output path or URL that you add using lttng snapshot add-output.

  3. An output path or URL that you provide directly to the lttng snapshot record command.

Method 3 overrides method 2, which overrides method 1. When you specify a URL, a relay daemon must listen on a remote system (see Send trace data over the network).

Archive the current trace chunk (rotate a tracing session)

Since 2.11

The snapshot user guide shows how to dump the current sub-buffers of a tracing session to the file system or send them over the network. When you take a snapshot, LTTng doesn’t clear the ring buffers of the tracing session: if you take another snapshot immediately after, both snapshots could contain overlapping trace data.

Inspired by log rotation, tracing session rotation is a feature which appends the content of the ring buffers to what’s already on the file system or sent over the network since the creation of the tracing session or since the last rotation, and then clears those ring buffers to avoid trace data overlaps.

What LTTng is about to write when performing a tracing session rotation is called the current trace chunk. When this current trace chunk is written to the file system or sent over the network, it becomes a trace chunk archive. Therefore, a tracing session rotation archives the current trace chunk.

A tracing session rotation operation archives the current trace chunk.

A trace chunk archive is a self-contained LTTng trace which LTTng doesn’t manage anymore: you can read it, modify it, move it, or remove it.

There are two methods to perform a tracing session rotation: immediately or with a rotation schedule.

To perform an immediate tracing session rotation:

  1. Create a tracing session in normal mode or network streaming mode (only those two creation modes support tracing session rotation):

    $
    lttng create my-session
  2. Create one or more event rules and start tracing:

    $
    $
    lttng enable-event --kernel sched_'*'
    lttng start
  3. When needed, immediately rotate the current tracing session:

    $
    lttng rotate

    The lttng-rotate command prints the path to the created trace chunk archive. See lttng-rotate(1) to learn about the format of trace chunk archive directory names.

    Perform other immediate rotations while the tracing session is active. It is guaranteed that all the trace chunk archives don’t contain overlapping trace data. You can also perform an immediate rotation once you have stopped the tracing session.

  4. When you’re done tracing, destroy the current tracing session:

    $
    lttng destroy

    The tracing session destruction operation creates one last trace chunk archive from the current trace chunk.

A tracing session rotation schedule is a planned rotation which LTTng performs automatically based on one of the following conditions:

  • A timer with a configured period times out.

  • The total size of the flushed part of the current trace chunk becomes greater than or equal to a configured value.

To schedule a tracing session rotation, set a rotation schedule:

  1. Create a tracing session in normal mode or network streaming mode (only those two creation modes support tracing session rotation):

    $
    lttng create my-session
  2. Create one or more event rules:

    $
    lttng enable-event --kernel sched_'*'
  3. Set a tracing session rotation schedule:

    $
    lttng enable-rotation --timer=10s

    In this example, we set a rotation schedule so that LTTng performs a tracing session rotation every ten seconds.

    See lttng-enable-rotation(1) to learn more about other ways to set a rotation schedule.

  4. Start tracing:

    $
    lttng start

    LTTng performs tracing session rotations automatically while the tracing session is active thanks to the rotation schedule.

  5. When you’re done tracing, destroy the current tracing session:

    $
    lttng destroy

    The tracing session destruction operation creates one last trace chunk archive from the current trace chunk.

Use lttng-disable-rotation(1) to unset a tracing session rotation schedule.

Note:lttng-rotate(1) and lttng-enable-rotation(1) list limitations regarding those two commands.

Use the machine interface

Since 2.6

With any command of the lttng(1) command-line tool, set the --mi option to xml (before the command name) to get an XML machine interface output, for example:

$
lttng --mi=xml enable-event --kernel --syscall open

A schema definition (XSD) is available to ease the integration with external tools as much as possible.

Regenerate the metadata of an LTTng trace

Since 2.8

An LTTng trace, which is a CTF trace, has both data stream files and a metadata file. This metadata file contains, amongst other things, information about the offset of the clock sources used to timestamp event records when tracing.

If, once a tracing session is started, a major NTP correction happens, the clock offset of the trace also needs to be updated. Use the metadata item of the lttng-regenerate(1) command to do so.

The main use case of this command is to allow a system to boot with an incorrect wall time and trace it with LTTng before its wall time is corrected. Once the system is known to be in a state where its wall time is correct, it can run lttng regenerate metadata.

To regenerate the metadata of an LTTng trace:

Important:lttng regenerate metadata has the following limitations:

Regenerate the state dump of a tracing session

Since 2.9

The LTTng kernel and user space tracers generate state dump event records when the application starts or when you start a tracing session. An analysis can use the state dump event records to set an initial state before it builds the rest of the state from the following event records. Trace Compass is a notable example of an application which uses the state dump of an LTTng trace.

When you take a snapshot, it’s possible that the state dump event records aren’t included in the snapshot because they were recorded to a sub-buffer that has been consumed or overwritten already.

Use the lttng regenerate statedump command to emit the state dump event records again.

To regenerate the state dump of the current tracing session, provided create it in snapshot mode, before you take a snapshot:

  1. Use the statedump item of the lttng-regenerate(1) command:

    $
    lttng regenerate statedump
  2. Stop the tracing session:

    $
    lttng stop
  3. Take a snapshot:

    $
    lttng snapshot record --name=my-snapshot

Depending on the event throughput, you should run steps 1 and 2 as closely as possible.

Note:To record the state dump events, you need to create event rules which enable them. LTTng-UST state dump tracepoints start with lttng_ust_statedump:. LTTng-modules state dump tracepoints start with lttng_statedump_.

Record trace data on persistent memory file systems

Since 2.7

Non-volatile random-access memory (NVRAM) is random-access memory that retains its information when power is turned off (non-volatile). Systems with such memory can store data structures in RAM and retrieve them after a reboot, without flushing to typical storage.

Linux supports NVRAM file systems thanks to either DAX + pmem (requires Linux 4.1+) or PRAMFS (requires Linux < 4).

This section doesn’t describe how to operate such file systems; we assume that you have a working persistent memory file system.

When you create a tracing session, you can specify the path of the shared memory holding the sub-buffers. If you specify a location on an NVRAM file system, then you can retrieve the latest recorded trace data when the system reboots after a crash.

To record trace data on a persistent memory file system and retrieve the trace data after a system crash:

  1. Create a tracing session with a sub-buffer shared memory path located on an NVRAM file system:

    $
    lttng create my-session --shm-path=/path/to/shm
  2. Configure the tracing session as usual with the lttng(1) command-line tool, and start tracing.

  3. After a system crash, use the lttng-crash(1) command-line tool to view the trace data recorded on the NVRAM file system:

    $
    lttng-crash /path/to/shm

The binary layout of the ring buffer files isn’t exactly the same as the trace files layout. This is why you need to use lttng-crash(1) instead of your preferred trace viewer directly.

To convert the ring buffer files to LTTng trace files:

Get notified when the buffer usage of a channel is too high or too low

Since 2.10

With the C/C++ notification and trigger API of LTTng, your user application can get notified when the buffer usage of one or more channels becomes too low or too high. Use this API and enable or disable event rules during tracing to avoid discarded event records.

Example:Have a user application get notified when the buffer usage of an LTTng channel is too high.

In this example, we create and build an application which gets notified when the buffer usage of a specific LTTng channel is higher than 75 %. We only print that it is the case in the example, but we could as well use the API of liblttng-ctl to disable event rules when this happens.

  1. Create the C source file of application:

    notif-app.c

    #include <stdio.h>
    #include <assert.h>
    #include <lttng/domain.h>
    #include <lttng/action/action.h>
    #include <lttng/action/notify.h>
    #include <lttng/condition/condition.h>
    #include <lttng/condition/buffer-usage.h>
    #include <lttng/condition/evaluation.h>
    #include <lttng/notification/channel.h>
    #include <lttng/notification/notification.h>
    #include <lttng/trigger/trigger.h>
    #include <lttng/endpoint.h>
    
    int main(int argc, char *argv[])
    {
        int exit_status = 0;
        struct lttng_notification_channel *notification_channel;
        struct lttng_condition *condition;
        struct lttng_action *action;
        struct lttng_trigger *trigger;
        const char *tracing_session_name;
        const char *channel_name;
    
        assert(argc >= 3);
        tracing_session_name = argv[1];
        channel_name = argv[2];
    
        /*
         * Create a notification channel. A notification channel
         * connects the user application to the LTTng session daemon.
         * This notification channel can be used to listen to various
         * types of notifications.
         */
        notification_channel = lttng_notification_channel_create(
            lttng_session_daemon_notification_endpoint);
    
        /*
         * Create a "high buffer usage" condition. In this case, the
         * condition is reached when the buffer usage is greater than or
         * equal to 75 %. We create the condition for a specific tracing
         * session name, channel name, and for the user space tracing
         * domain.
         *
         * The "low buffer usage" condition type also exists.
         */
        condition = lttng_condition_buffer_usage_high_create();
        lttng_condition_buffer_usage_set_threshold_ratio(condition, .75);
        lttng_condition_buffer_usage_set_session_name(
            condition, tracing_session_name);
        lttng_condition_buffer_usage_set_channel_name(condition,
            channel_name);
        lttng_condition_buffer_usage_set_domain_type(condition,
            LTTNG_DOMAIN_UST);
    
        /*
         * Create an action (get a notification) to take when the
         * condition created above is reached.
         */
        action = lttng_action_notify_create();
    
        /*
         * Create a trigger. A trigger associates a condition to an
         * action: the action is executed when the condition is reached.
         */
        trigger = lttng_trigger_create(condition, action);
    
        /* Register the trigger to LTTng. */
        lttng_register_trigger(trigger);
    
        /*
         * Now that we have registered a trigger, a notification will be
         * emitted everytime its condition is met. To receive this
         * notification, we must subscribe to notifications that match
         * the same condition.
         */
        lttng_notification_channel_subscribe(notification_channel,
            condition);
    
        /*
         * Notification loop. Put this in a dedicated thread to avoid
         * blocking the main thread.
         */
        for (;;) {
            struct lttng_notification *notification;
            enum lttng_notification_channel_status status;
            const struct lttng_evaluation *notification_evaluation;
            const struct lttng_condition *notification_condition;
            double buffer_usage;
    
            /* Receive the next notification. */
            status = lttng_notification_channel_get_next_notification(
                notification_channel, &notification);
    
            switch (status) {
            case LTTNG_NOTIFICATION_CHANNEL_STATUS_OK:
                break;
            case LTTNG_NOTIFICATION_CHANNEL_STATUS_NOTIFICATIONS_DROPPED:
                /*
                 * The session daemon can drop notifications if a monitoring
                 * application isn't consuming the notifications fast
                 * enough.
                 */
                continue;
            case LTTNG_NOTIFICATION_CHANNEL_STATUS_CLOSED:
                /*
                 * The notification channel has been closed by the
                 * session daemon. This is typically caused by a session
                 * daemon shutting down.
                 */
                goto end;
            default:
                /* Unhandled conditions or errors. */
                exit_status = 1;
                goto end;
            }
    
            /*
             * A notification provides, amongst other things:
             *
             * * The condition that caused this notification to be
             *   emitted.
             * * The condition evaluation, which provides more
             *   specific information on the evaluation of the
             *   condition.
             *
             * The condition evaluation provides the buffer usage
             * value at the moment the condition was reached.
             */
            notification_condition = lttng_notification_get_condition(
                notification);
            notification_evaluation = lttng_notification_get_evaluation(
                notification);
    
            /* We're subscribed to only one condition. */
            assert(lttng_condition_get_type(notification_condition) ==
                LTTNG_CONDITION_TYPE_BUFFER_USAGE_HIGH);
    
            /*
             * Get the exact sampled buffer usage from the
             * condition evaluation.
             */
            lttng_evaluation_buffer_usage_get_usage_ratio(
                notification_evaluation, &buffer_usage);
    
            /*
             * At this point, instead of printing a message, we
             * could do something to reduce the buffer usage of the channel,
             * like disable specific events.
             */
            printf("Buffer usage is %f %% in tracing session \"%s\", "
                "user space channel \"%s\".\n", buffer_usage * 100,
                tracing_session_name, channel_name);
            lttng_notification_destroy(notification);
        }
    
    end:
        lttng_action_destroy(action);
        lttng_condition_destroy(condition);
        lttng_trigger_destroy(trigger);
        lttng_notification_channel_destroy(notification_channel);
        return exit_status;
    }
    
  2. Build the notif-app application, linking it to liblttng-ctl:

    $
    gcc -o notif-app notif-app.c -llttng-ctl
  3. Create a tracing session, create an event rule matching all the user space tracepoints, and start tracing:

    $
    $
    $
    lttng create my-session
    lttng enable-event --userspace --all
    lttng start

    If you create the channel manually with the lttng-enable-channel(1) command, control how frequently LTTng samples the current values of the channel properties to evaluate user conditions with the --monitor-timer option.

  4. Run the notif-app application. This program accepts the tracing session name and the user space channel name as its two first arguments. The channel which LTTng automatically creates with the lttng-enable-event(1) command above is named channel0:

    $
    ./notif-app my-session channel0
  5. In another terminal, run an application with a very high event throughput so that the 75 % buffer usage condition is reached.

    In the first terminal, the application should print lines like this:

    Buffer usage is 81.45197 % in tracing session "my-session", user space
    channel "channel0".

    If you don’t see anything, try modifying the condition in notif-app.c to a lower value (0.1, for example), rebuilding it (step 2) and running it again (step 4).

Reference

LTTng-modules

LTTNG_TRACEPOINT_ENUM() usage

Since 2.9

Use the LTTNG_TRACEPOINT_ENUM() macro to define an enumeration:

LTTNG_TRACEPOINT_ENUM(name, TP_ENUM_VALUES(entries))

Replace:

  • name with the name of the enumeration (C identifier, unique amongst all the defined enumerations).

  • entries with a list of enumeration entries.

The available enumeration entry macros are:

ctf_enum_value(name, value)

Entry named name mapped to the integral value value.

ctf_enum_range(name, begin, end)

Entry named name mapped to the range of integral values between begin (included) and end (included).

ctf_enum_auto(name)

Entry named name mapped to the integral value following the last mapping value.

The last value of a ctf_enum_value() entry is its value parameter.

The last value of a ctf_enum_range() entry is its end parameter.

If ctf_enum_auto() is the first entry in the list, its integral value is 0.

Use the ctf_enum() field definition macro to use a defined enumeration as a tracepoint field.

Example:Define an enumeration with LTTNG_TRACEPOINT_ENUM().

LTTNG_TRACEPOINT_ENUM(
    my_enum,
    TP_ENUM_VALUES(
        ctf_enum_auto("AUTO: EXPECT 0")
        ctf_enum_value("VALUE: 23", 23)
        ctf_enum_value("VALUE: 27", 27)
        ctf_enum_auto("AUTO: EXPECT 28")
        ctf_enum_range("RANGE: 101 TO 303", 101, 303)
        ctf_enum_auto("AUTO: EXPECT 304")
    )
)

Tracepoint fields macros (for TP_FIELDS())

Since 2.7

The available macros to define tracepoint fields, which must be listed within TP_FIELDS() in LTTNG_TRACEPOINT_EVENT(), are:

Available macros to define LTTng-modules tracepoint fields

Macro Description and parameters

ctf_integer(t, n, e)

ctf_integer_nowrite(t, n, e)

ctf_user_integer(t, n, e)

ctf_user_integer_nowrite(t, n, e)

Standard integer, displayed in base 10.

t

Integer C type (int, long, size_t, …).

n

Field name.

e

Argument expression.

ctf_integer_hex(t, n, e)

ctf_user_integer_hex(t, n, e)

Standard integer, displayed in base 16.

t

Integer C type.

n

Field name.

e

Argument expression.

ctf_integer_oct(t, n, e)

Standard integer, displayed in base 8.

t

Integer C type.

n

Field name.

e

Argument expression.

ctf_integer_network(t, n, e)

ctf_user_integer_network(t, n, e)

Integer in network byte order (big-endian), displayed in base 10.

t

Integer C type.

n

Field name.

e

Argument expression.

ctf_integer_network_hex(t, n, e)

ctf_user_integer_network_hex(t, n, e)

Integer in network byte order, displayed in base 16.

t

Integer C type.

n

Field name.

e

Argument expression.

ctf_enum(N, t, n, e)

ctf_enum_nowrite(N, t, n, e)

ctf_user_enum(N, t, n, e)

ctf_user_enum_nowrite(N, t, n, e)

Enumeration.

N

Name of a previously defined enumeration.

t

Integer C type (int, long, size_t, …).

n

Field name.

e

Argument expression.

ctf_string(n, e)

ctf_string_nowrite(n, e)

ctf_user_string(n, e)

ctf_user_string_nowrite(n, e)

Null-terminated string; undefined behavior if e is NULL.

n

Field name.

e

Argument expression.

ctf_array(t, n, e, s)

ctf_array_nowrite(t, n, e, s)

ctf_user_array(t, n, e, s)

ctf_user_array_nowrite(t, n, e, s)

Statically-sized array of integers.

t

Array element C type.

n

Field name.

e

Argument expression.

s

Number of elements.

ctf_array_bitfield(t, n, e, s)

ctf_array_bitfield_nowrite(t, n, e, s)

ctf_user_array_bitfield(t, n, e, s)

ctf_user_array_bitfield_nowrite(t, n, e, s)

Statically-sized array of bits.

The type of e must be an integer type. s is the number of elements of such type in e, not the number of bits.

t

Array element C type.

n

Field name.

e

Argument expression.

s

Number of elements.

ctf_array_text(t, n, e, s)

ctf_array_text_nowrite(t, n, e, s)

ctf_user_array_text(t, n, e, s)

ctf_user_array_text_nowrite(t, n, e, s)

Statically-sized array, printed as text.

The string doesn’t need to be null-terminated.

t

Array element C type (always char).

n

Field name.

e

Argument expression.

s

Number of elements.

ctf_sequence(t, n, e, T, E)

ctf_sequence_nowrite(t, n, e, T, E)

ctf_user_sequence(t, n, e, T, E)

ctf_user_sequence_nowrite(t, n, e, T, E)

Dynamically-sized array of integers.

The type of E must be unsigned.

t

Array element C type.

n

Field name.

e

Argument expression.

T

Length expression C type.

E

Length expression.

ctf_sequence_hex(t, n, e, T, E)

ctf_user_sequence_hex(t, n, e, T, E)

Dynamically-sized array of integers, displayed in base 16.

The type of E must be unsigned.

t

Array element C type.

n

Field name.

e

Argument expression.

T

Length expression C type.

E

Length expression.

ctf_sequence_network(t, n, e, T, E)

Dynamically-sized array of integers in network byte order (big-endian), displayed in base 10.

The type of E must be unsigned.

t

Array element C type.

n

Field name.

e

Argument expression.

T

Length expression C type.

E

Length expression.

ctf_sequence_bitfield(t, n, e, T, E)

ctf_sequence_bitfield_nowrite(t, n, e, T, E)

ctf_user_sequence_bitfield(t, n, e, T, E)

ctf_user_sequence_bitfield_nowrite(t, n, e, T, E)

Dynamically-sized array of bits.

The type of e must be an integer type. s is the number of elements of such type in e, not the number of bits.

The type of E must be unsigned.

t

Array element C type.

n

Field name.

e

Argument expression.

T

Length expression C type.

E

Length expression.

ctf_sequence_text(t, n, e, T, E)

ctf_sequence_text_nowrite(t, n, e, T, E)

ctf_user_sequence_text(t, n, e, T, E)

ctf_user_sequence_text_nowrite(t, n, e, T, E)

Dynamically-sized array, displayed as text.

The string doesn’t need to be null-terminated.

The type of E must be unsigned.

The behaviour is undefined if e is NULL.

t

Sequence element C type (always char).

n

Field name.

e

Argument expression.

T

Length expression C type.

E

Length expression.

Use the _user versions when the argument expression, e, is a user space address. In the cases of ctf_user_integer*() and ctf_user_float*(), &e must be a user space address, thus e must be addressable.

The _nowrite versions omit themselves from the session trace, but are otherwise identical. This means the _nowrite fields won’t be written in the recorded trace. Their primary purpose is to make some of the event context available to the event filters without having to commit the data to sub-buffers.

Glossary

Terms related to LTTng and to tracing in general:

Babeltrace

The Babeltrace project, which includes:

buffering scheme

A layout of sub-buffers applied to a given channel.

channel

An entity which is responsible for a set of ring buffers.

Event rules are always attached to a specific channel.

clock

A source of time for a tracer.

consumer daemon

A process which is responsible for consuming the full sub-buffers and write them to a file system or send them over the network.

current trace chunk

A trace chunk which includes the current content of all the sub-buffers of the tracing session and the stream files produced since the latest event amongst:

  • The creation of the tracing session.

  • The last tracing session rotation, if any.

discard mode

The event record loss mode in which the tracer discards new event records when there’s no sub-buffer space left to store them.

event

The consequence of the execution of an instrumentation point, like a tracepoint that you manually place in some source code, or a Linux kernel kprobe.

An event is said to occur at a specific time. LTTng can take various actions upon the occurrence of an event, like record its payload to a sub-buffer.

event name

The name of an event, which is also the name of the event record.

This is also called the instrumentation point name.

event record

A record, in a trace, of the payload of an event which occured.

event record loss mode

The mechanism by which event records of a given channel are lost (not recorded) when there is no sub-buffer space left to store them.

event rule

Set of conditions which must be satisfied for one or more occuring events to be recorded.

inclusion set

In the process attribute tracking context: a set of process attributes of a given type.

instrumentation

The use of LTTng probes to make a piece of software traceable.

instrumentation point

A point in the execution path of a piece of software that, when reached by this execution, can emit an event.

instrumentation point name

See event name.

java.util.logging

The core logging facilities of the Java platform.

log4j

A logging library for Java developed by the Apache Software Foundation.

log level

Level of severity of a log statement or user space instrumentation point.

LTTng

The Linux Trace Toolkit: next generation project.

lttng

A command-line tool provided by the LTTng-tools project which you can use to send and receive control messages to and from a session daemon.

lttng-consumerd

The name of the consumer daemon program.

lttng-crash

A utility provided by the LTTng-tools project which can convert ring buffer files (usually saved on a persistent memory file system) to trace files.

See lttng-crash(1).

LTTng Documentation

This document.

LTTng live

A communication protocol between the relay daemon and live viewers which makes it possible to see event records “live”, as they are received by the relay daemon.

LTTng-modules

The LTTng-modules project, which contains the Linux kernel modules to make the Linux kernel instrumentation points available for LTTng tracing.

lttng-relayd

The name of the relay daemon program.

lttng-sessiond

The name of the session daemon program.

LTTng-tools

The LTTng-tools project, which contains the various programs and libraries used to control tracing.

LTTng-UST

The LTTng-UST project, which contains libraries to instrument user applications.

LTTng-UST Java agent

A Java package provided by the LTTng-UST project to allow the LTTng instrumentation of java.util.logging and Apache log4j 1.2 logging statements.

LTTng-UST Python agent

A Python package provided by the LTTng-UST project to allow the LTTng instrumentation of Python logging statements.

overwrite mode

The event record loss mode in which new event records overwrite older event records when there’s no sub-buffer space left to store them.

per-process buffering

A buffering scheme in which each instrumented process has its own sub-buffers for a given user space channel.

per-user buffering

A buffering scheme in which all the processes of a Unix user share the same sub-buffers for a given user space channel.

process attribute

In the process attribute tracking context:

  • A process ID.

  • A virtual process ID.

  • A Unix user ID.

  • A virtual Unix user ID.

  • A Unix group ID.

  • A virtual Unix group ID.

relay daemon

A process which is responsible for receiving the trace data which a distant consumer daemon sends.

ring buffer

A set of sub-buffers.

rotation

See tracing session rotation.

session daemon

A process which receives control commands from you and orchestrates the tracers and various LTTng daemons.

snapshot

A copy of the current data of all the sub-buffers of a given tracing session, saved as trace files.

sub-buffer

One part of an LTTng ring buffer which contains event records.

timestamp

The time information attached to an event when it is emitted.

trace (noun)

A set of:

  • One CTF metadata stream file.

  • One or more CTF data stream files which are the concatenations of one or more flushed sub-buffers.

trace (verb)

The action of recording the events emitted by an application or by a system, or to initiate such recording by controlling a tracer.

trace chunk

A self-contained trace which is part of a tracing session. Each tracing session rotation produces a trace chunk archive.

trace chunk archive

The result of a tracing session rotation.

LTTng doesn’t manage any trace chunk archive, even if its containing tracing session is still active: you are free to read it, modify it, move it, or remove it.

Trace Compass

The Trace Compass project and application.

tracepoint

An instrumentation point using the tracepoint mechanism of the Linux kernel or of LTTng-UST.

tracepoint definition

The definition of a single tracepoint.

tracepoint name

The name of a tracepoint.

tracepoint provider

A set of functions providing tracepoints to an instrumented user application.

Not to be confused with a tracepoint provider package: many tracepoint providers can exist within a tracepoint provider package.

tracepoint provider package

One or more tracepoint providers compiled as an object file or as a shared library.

tracer

A software which records emitted events.

tracing domain

A namespace for event sources.

tracing group

The Unix group in which a Unix user can be to be allowed to trace the Linux kernel.

tracing session

A stateful dialogue between you and a session daemon.

tracing session rotation

The action of archiving the current trace chunk of a tracing session.

tracked process attribute

A process attribute which is part of an inclusion set.

untracked process attribute

A process attribute which isn’t part of an inclusion set.

user application

An application running in user space, as opposed to a Linux kernel module, for example.