lttng-enable-event(1) (v2.10)

lttng-enable-event — Create or enable LTTng event rules

Create or enable Linux kernel event rules:

lttng [GENERAL OPTIONS] enable-event --kernel
      [--probe=SOURCE | --function=SOURCE | --syscall]
      [--filter=EXPR] [--session=SESSION]
      [--channel=CHANNEL] EVENT[,EVENT]…

Create or enable an "all" Linux kernel event rule:

lttng [GENERAL OPTIONS] enable-event --kernel --all [--syscall]
      [--filter=EXPR] [--session=SESSION] [--channel=CHANNEL]

Create or enable application event rules:

lttng [GENERAL OPTIONS] enable-event
      (--userspace | --jul | --log4j | --python)
      [--filter=EXPR] [--exclude=EVENT[,EVENT]…]
      [--loglevel=LOGLEVEL | --loglevel-only=LOGLEVEL]
      [--session=SESSION] [--channel=CHANNEL] (--all | EVENT[,EVENT]…)

The lttng enable-event command can create a new event rule, or enable one or more existing and disabled ones.

An event rule created by lttng enable-event is a set of conditions that must be satisfied in order for an actual event to be emitted by an LTTng tracer when the execution of an application or the Linux kernel reaches an event source (tracepoint, system call, dynamic probe). Event sources can be listed with the lttng-list(1) command.

The lttng-disable-event(1) command can be used to disable existing event rules.

Event rules are always assigned to a channel when they are created. If the --channel option is omitted, a default channel named channel0 is used (and created automatically if it does not exist for the specified domain in the selected tracing session).

If the --session option is omitted, the chosen channel is picked from the current tracing session.

Events can be enabled while tracing is active (use lttng-start(1) to make a tracing session active).

Four types of event sources are available in the Linux kernel tracing domain (--kernel option):

Tracepoint (--tracepoint option; default)

A Linux kernel tracepoint, that is, a static instrumentation point placed in the kernel source code. Standard tracepoints are designed and placed in the source code by developers and record useful payload fields.

Dynamic probe (--probe option)

A Linux kernel kprobe, that is, an instrumentation point placed dynamically in the compiled kernel code. Dynamic probe events do not record any payload field.

Function probe (--function option)

A Linux kernel kretprobe, that is, two instrumentation points placed dynamically where a function is entered and where it returns in the compiled kernel code. Function probe events do not record any payload field.

System call (--syscall option)

A Linux kernel system call. Two instrumentation points are statically placed where a system call function is entered and where it returns in the compiled kernel code. System call event sources record useful payload fields.

The application tracing domains (--userspace, --jul, --log4j, or --python options) only support tracepoints. In the cases of the JUL, Apache log4j, and Python domains, the event names correspond to logger names.

When creating an event rule with lttng enable-event, conditions are specified using options. The logical conjunction (logical AND) of all those conditions must be true when an event source is reached by an application or by the Linux kernel in order for an actual event to be emitted by an LTTng tracer.

Any condition that is not explicitly specified on creation is considered a don’t care.

For example, consider the following commands:

$
$

lttng enable-event --userspace hello:world
lttng enable-event --userspace hello:world --loglevel=TRACE_INFO

Here, two event rules are created. The first one has a single condition: the tracepoint name must match hello:world. The second one has two conditions:

• The tracepoint name must match hello:world, and

• The tracepoint’s defined log level must be at least as severe as the TRACE_INFO level.

In this case, the second event rule is pointless because the first one is more general: it does not care about the tracepoint’s log level. If an event source matching both event rules is reached by the application’s execution, only one event is emitted.

The available conditions for the Linux kernel domain are:

• Tracepoint/system call name (EVENT argument with --tracepoint or --syscall options) or dynamic probe/function name/address (--probe or --function option’s argument) which must match event source’s equivalent.

  You can use * characters at any place in the tracepoint or system call name as wildcards to match zero or more characters. To use a literal * character, use \*.

• Filter expression (--filter option) executed against the dynamic values of event fields at execution time that must evaluate to true. See the Filter expression syntax section below for more information.

The available conditions for the application domains are:

• Tracepoint name (EVENT with --tracepoint option) which must match event source’s equivalent.

  You can use * characters at any place in the tracepoint name as wildcards to match zero or more characters. To use a literal * character, use \*. When you create an event rule with a tracepoint name containing a wildcard, you can exclude specific tracepoint names from the match with the --exclude option.

• Filter expression (--filter option) executed against the dynamic values of event fields at execution time that must evaluate to true. See the Filter expression syntax section below for more information.

• Event’s log level that must be at least as severe as a given log level (--loglevel option) or match exactly a given log level (--loglevel-only option).

When using lttng enable-event with a set of conditions that does not currently exist for the chosen tracing session, domain, and channel, a new event rule is created. Otherwise, the existing event rule is enabled if it is currently disabled (see lttng-disable-event(1)).

The --all option can be used alongside the --tracepoint or --syscall options. When this option is used, no EVENT argument must be specified. This option defines a single event rule matching all the possible events of a given tracing domain for the chosen channel and tracing session. It is the equivalent of an EVENT argument named * (wildcard).

A filter expression can be specified with the --filter option when creating a new event rule. If the filter expression evaluates to true when executed against the dynamic values of an event’s fields when tracing, the filtering condition passes.

The filter expression syntax is very similar to C language conditional expressions (expressions that can be evaluated by an if statement).

The following logical operators are supported:

The following comparison operators/relational operators are supported:

The arithmetic and bitwise operators are not supported.

The precedence table of the operators above is the same as the one of the C language. Parentheses are supported to bypass this.

The dynamic value of an event field is read by using its name as a C identifier.

The dynamic value of a statically-known context field is read by prefixing its name with $ctx.. Statically-known context fields are context fields added to channels without the $app. prefix using the lttng-add-context(1) command. $ctx.cpu_id is also available as the ID of the CPU which emits the event.

The dynamic value of an application-specific context field is read by prefixing its name with $app. (follows the format used to add such a context field with the lttng-add-context(1) command).

When a comparison includes a non existent event field, the whole filter expression evaluates to false (the event is discarded).

C integer and floating point number constants are supported, as well as literal strings between double quotes ("). You can use * characters at any place in a literal string as wildcards to match zero or more characters. To use a literal * character, use \*.

LTTng-UST enumeration fields can be compared to integer values (fields or constants).

Examples:

msg_id == 23 && size >= 2048

$ctx.procname == "lttng*" && (!flag || poel < 34)

$app.my_provider:my_context == 17.34e9 || some_enum >= 14

$ctx.cpu_id == 2 && filename != "*.log"

Tracepoints and log statements in applications have an attached log level. Application event rules can contain a log level condition.

With the --loglevel option, the event source’s log level must be at least as severe as the option’s argument. With the --loglevel-only option, the event source’s log level must match the option’s argument.

The available log levels are:

User space domain (--userspace option)

Shortcuts such as system are allowed.

• TRACE_EMERG (0)

• TRACE_ALERT (1)

• TRACE_CRIT (2)

• TRACE_ERR (3)

• TRACE_WARNING (4)

• TRACE_NOTICE (5)

• TRACE_INFO (6)

• TRACE_DEBUG_SYSTEM (7)

• TRACE_DEBUG_PROGRAM (8)

• TRACE_DEBUG_PROCESS (9)

• TRACE_DEBUG_MODULE (10)

• TRACE_DEBUG_UNIT (11)

• TRACE_DEBUG_FUNCTION (12)

• TRACE_DEBUG_LINE (13)

• TRACE_DEBUG (14)

java.util.logging domain (--jul option)

Shortcuts such as severe are allowed.

• JUL_OFF (INT32_MAX)

• JUL_SEVERE (1000)

• JUL_WARNING (900)

• JUL_INFO (800)

• JUL_CONFIG (700)

• JUL_FINE (500)

• JUL_FINER (400)

• JUL_FINEST (300)

• JUL_ALL (INT32_MIN)

Apache log4j domain (--log4j option)

Shortcuts such as severe are allowed.

• LOG4J_OFF (INT32_MAX)

• LOG4J_FATAL (50000)

• LOG4J_ERROR (40000)

• LOG4J_WARN (30000)

• LOG4J_INFO (20000)

• LOG4J_DEBUG (10000)

• LOG4J_TRACE (5000)

• LOG4J_ALL (INT32_MIN)

Python domain (--python option)

Shortcuts such as critical are allowed.

• PYTHON_CRITICAL (50)

• PYTHON_ERROR (40)

• PYTHON_WARNING (30)

• PYTHON_INFO (20)

• PYTHON_DEBUG (10)

• PYTHON_NOTSET (0)

General options are described in lttng(1).

One of:

-j, --jul

Create or enable event rules in the java.util.logging (JUL) domain.

-k, --kernel

Create or enable event rules in the Linux kernel domain.

-l, --log4j

Create or enable event rules in the Apache log4j domain.

-p, --python

Create or enable event rules in the Python domain.

-u, --userspace

Create or enable event rules in the user space domain.

-c CHANNEL, --channel=CHANNEL

Create or enable event rules in the channel named CHANNEL instead of the default channel name channel0.

-s SESSION, --session=SESSION

Create or enable event rules in the tracing session named SESSION instead of the current tracing session.

One of:

--function=SOURCE

Linux kernel kretprobe. Only available with the --kernel domain option. SOURCE is one of:

• Function address (0x prefix supported)

• Function symbol

• Function symbol and offset (SYMBOL+OFFSET format)

--probe=SOURCE

Linux kernel kprobe. Only available with the --kernel domain option. SOURCE is one of:

• Address (0x prefix supported)

• Symbol

• Symbol and offset (SYMBOL+OFFSET format)

--syscall

Linux kernel system call. Only available with the --kernel domain option.

--tracepoint

Linux kernel or application tracepoint (default).

One of:

--loglevel=LOGLEVEL

Add log level condition to the event rule: the event source’s defined log level must be at least as severe as LOGLEVEL. See the Log levels section above for the available log levels. Only available with application domains.

--loglevel-only=LOGLEVEL

Add log level condition to the event rule: the event source’s defined log level must match LOGLEVEL. See the Log levels section above for the available log levels. Only available with application domains.

-x EVENT[,EVENT]…, --exclude=EVENT[,EVENT]…

Exclude events named EVENT from the event rule. This option can be used when the command’s EVENT argument contains at least one wildcard star (*) to exclude specific names. EVENT can also contain wildcard stars. To use a literal , character, use \,. Only available with the --userspace domain.

-f EXPR, --filter=EXPR

Add filter expression condition to the event rule. Expression EXPR must evaluate to true when executed against the dynamic values of event fields. See the Filter expression syntax section above for more information.

-a, --all

Equivalent to an EVENT argument named * (wildcard) when also using the --tracepoint (default) or --syscall option.

-h, --help

Show command help.

This option, like lttng-help(1), attempts to launch /usr/bin/man to view the command’s man page. The path to the man pager can be overridden by the LTTNG_MAN_BIN_PATH environment variable.

--list-options

List available command options.

LTTNG_ABORT_ON_ERROR

Set to 1 to abort the process after the first error is encountered.

LTTNG_HOME

Overrides the $HOME environment variable. Useful when the user running the commands has a non-writable home directory.

LTTNG_MAN_BIN_PATH

Absolute path to the man pager to use for viewing help information about LTTng commands (using lttng-help(1) or lttng COMMAND --help).

LTTNG_SESSION_CONFIG_XSD_PATH

Path in which the session.xsd session configuration XML schema may be found.

LTTNG_SESSIOND_PATH

Full session daemon binary path.

The --sessiond-path option has precedence over this environment variable.

Note that the lttng-create(1) command can spawn an LTTng session daemon automatically if none is running. See lttng-sessiond(8) for the environment variables influencing the execution of the session daemon.

$LTTNG_HOME/.lttngrc

User LTTng runtime configuration.

This is where the per-user current tracing session is stored between executions of lttng(1). The current tracing session can be set with lttng-set-session(1). See lttng-create(1) for more information about tracing sessions.

$LTTNG_HOME/lttng-traces

Default output directory of LTTng traces. This can be overridden with the --output option of the lttng-create(1) command.

$LTTNG_HOME/.lttng

User LTTng runtime and configuration directory.

$LTTNG_HOME/.lttng/sessions

Default location of saved user tracing sessions (see lttng-save(1) and lttng-load(1)).

/etc/lttng/sessions

System-wide location of saved tracing sessions (see lttng-save(1) and lttng-load(1)).

0

Success

1

Command error

2

Undefined command

3

Fatal error

4

Command warning (something went wrong during the command)

If you encounter any issue or usability problem, please report it on the LTTng bug tracker.

• LTTng project website

• LTTng documentation

• Git repositories

• GitHub organization

• Continuous integration

• Mailing list for support and development: lttng-dev@lists.lttng.org

• IRC channel: #lttng on irc.oftc.net

This program is part of the LTTng-tools project.

LTTng-tools is distributed under the GNU General Public License version 2. See the LICENSE file for details.

Special thanks to Michel Dagenais and the DORSAL laboratory at École Polytechnique de Montréal for the LTTng journey.

Also thanks to the Ericsson teams working on tracing which helped us greatly with detailed bug reports and unusual test cases.

LTTng-tools was originally written by Mathieu Desnoyers, Julien Desfossez, and David Goulet. More people have since contributed to it.

LTTng-tools is currently maintained by Jérémie Galarneau.

lttng-disable-event(1), lttng(1)