lttng-enable-event man page

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

Synopsis

Create or enable Linux kernel event rules:

lttng [GENERAL OPTIONS] enable-event --kernel
      [--probe=SOURCE | --function=SOURCE | --syscall |
       --userspace-probe=SOURCE]
      [--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/library 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]...)

Description

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 a library 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).

Event source types

Five 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 kernel 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.

Dynamic user space probe (--userspace-probe option)

A Linux kernel uprobe, that is, an instrumentation point placed dynamically in the compiled user space application/library through the kernel. Dynamic user space probe events do not record any payload field.

See the Dynamic user space probes section for more information.

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.

Understanding event rule conditions

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, --userspace-probe, and --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 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 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).

Filter expression

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.

Note

Make sure to single-quote the filter expression when running the command from a shell, as filter expressions typically include characters having a special meaning for most shells.

The filter expression syntax is similar to C language conditional expressions (expressions that can be evaluated by an if statement), albeit with a few differences:

  • 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 \*.

    Examples: 32, -0x17, 0755, 12.34, "a \"literal string\"", "src/*/*.h".

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

    The dot and square bracket notations are available, like in the C language, to access nested structure and array/sequence fields. Only a constant, positive integer number can be used within square brackets. If the index is out of bounds, the whole filter expression evaluates to false (the event is discarded).

    An enumeration field’s value is an integer.

    When the expression’s field does not exist, the whole filter expression evaluates to false.

    Examples: my_field, target_cpu, seq[7], msg.user[1].data[2][17].

  • 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.

    When the expression’s statically-known context field does not exist, the whole filter expression evaluates to false.

    Examples: $ctx.prio, $ctx.preemptible, $ctx.perf:cpu:stalled-cycles-frontend.

  • 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 the expression’s application-specific context field does not exist, the whole filter expression evaluates to false.

    Example: $app.server:cur_user.

The following precedence table shows the operators which are supported in a filter expression. In this table, the highest precedence is 1. Parentheses are supported to bypass the default order.

Important

Unlike the C language, the lttng enable-event filter expression syntax’s bitwise AND and OR operators (& and |) take precedence over relational operators (<, <=, >, >=, ==, and !=). This means the filter expression 2 & 2 == 2 is true while the equivalent C expression is false.

PrecedenceOperatorDescriptionAssociativity
1-Unary minusRight-to-left
1+Unary plusRight-to-left
1!Logical NOTRight-to-left
1~Bitwise NOTRight-to-left
2<<Bitwise left shiftLeft-to-right
2>>Bitwise right shiftLeft-to-right
3&Bitwise ANDLeft-to-right
4^Bitwise XORLeft-to-right
5|Bitwise ORLeft-to-right
6<Less thanLeft-to-right
6<=Less than or equal toLeft-to-right
6>Greater thanLeft-to-right
6>=Greater than or equal toLeft-to-right
7==Equal toLeft-to-right
7!=Not equal toLeft-to-right
8&&Logical ANDLeft-to-right
9||Logical ORLeft-to-right

The arithmetic operators are NOT supported.

All integer constants and fields are first casted to signed 64-bit integers. The representation of negative integers is two’s complement. This means that, for example, the signed 8-bit integer field 0xff (-1) becomes 0xffffffffffffffff (still -1) once casted.

Before a bitwise operator is applied, all its operands are casted to unsigned 64-bit integers, and the result is casted back to a signed 64-bit integer. For the bitwise NOT operator, it is the equivalent of this C expression:

(int64_t) ~((uint64_t) val)

For the binary bitwise operators, it is the equivalent of those C expressions:

(int64_t) ((uint64_t) lhs >> (uint64_t) rhs)
(int64_t) ((uint64_t) lhs << (uint64_t) rhs)
(int64_t) ((uint64_t) lhs & (uint64_t) rhs)
(int64_t) ((uint64_t) lhs ^ (uint64_t) rhs)
(int64_t) ((uint64_t) lhs | (uint64_t) rhs)

If the right-hand side of a bitwise shift operator (<< and >>) is not in the [0, 63] range, the whole filter expression evaluates to false.

Note

Although it is possible to filter the process ID of an event when the pid context has been added to its channel using, for example, $ctx.pid == 2832, it is recommended to use the PID tracker instead, which is much more efficient (see lttng-track(1)).

Filter expression 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"
eax_reg & 0xff7 == 0x240 && x[4] >> 12 <= 0x1234

Log levels

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)

Dynamic user space probes

With the --userspace-probe option, you can instrument function entries of any user space binary (application or library) using either an available symbol name or a SystemTap User-level Statically Defined Tracing (USDT, a DTrace-style marker) probe’s provider and probe names. As of this version, only USDT probes that are NOT surrounded by a reference counter (semaphore) are supported.

The --userspace-probe option must be specified with the --kernel option because it uses Linux’s uprobe feature to dynamically instrument a user space application or library.

As of this version, dynamic probe events do not record any payload field.

Options

General options are described in lttng(1).

Domain

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.

Target

-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.

Event source type

One of:

--function=SOURCE

Dynamic kernel return probe (kretprobe). Only available with the --kernel domain option. SOURCE is one of:

  • Function address (0x prefix supported)
  • Function symbol name
  • Function symbol name and offset (SYMBOL+OFFSET format)
--probe=SOURCE

Dynamic kernel probe (kprobe). Only available with the --kernel domain option. SOURCE is one of:

  • Address (0x prefix supported)
  • Symbol nane
  • Symbol name and offset (SYMBOL+OFFSET format)
--userspace-probe=SOURCE

Dynamic user space probe (uprobe). Only available with the --kernel domain option. See the Dynamic user space probes section.

SOURCE is one of:

[elf:]PATH:SYMBOL

Dynamically instrument an available symbol within a user space application or library.

PATH

Application or library path.

This can be:

  • An absolute path.
  • A relative path.
  • An application’s name as found in the directories listed in the PATH environment variable.
SYMBOL

Symbol name of the function of which to instrument the entry.

This can be any defined code symbol listed by the nm(1) command (including with its --dynamic option which lists dynamic symbols).

As of this version, not specifying elf: is equivalent to specifying it.

Examples:

sdt:PATH:PROVIDER:NAME

Dynamically instrument a USDT probe within a user space application or library.

PATH

Application or library path.

This can be:

  • An absolute path.
  • A relative path.
  • An application’s name as found in the directories listed in the PATH environment variable.
PROVIDER:NAME

USDT provider and probe names.

For example, with the following USDT probe:

DTRACE_PROBE2("server", "accept_request",
              request_id, ip_addr);

The provider/probe name pair is server:accept_request.

Example:

--syscall

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

--tracepoint

Linux kernel or application tracepoint (default).

Log level

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.

Filtering and exclusion

-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 section above for more information.

Shortcuts

-a, --all

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

Program information

-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.

Environment Variables

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.

Files

$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)).

/usr/local/etc/lttng/sessions

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

Note

$LTTNG_HOME defaults to $HOME when not explicitly set.

Exit Status

0

Success

1

Command error

2

Undefined command

3

Fatal error

4

Command warning (something went wrong during the command)

Bugs

If you encounter any issue or usability problem, please report it on the LTTng bug tracker <https://bugs.lttng.org/projects/lttng-tools>.

Resources

Copyrights

This program is part of the LTTng-tools project.

LTTng-tools is distributed under the GNU General Public License version 2 <http://www.gnu.org/licenses/old-licenses/gpl-2.0.en.html>. See the LICENSE <https://github.com/lttng/lttng-tools/blob/master/LICENSE> file for details.

Thanks

Special thanks to Michel Dagenais and the DORSAL laboratory <http://www.dorsal.polymtl.ca/> 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.

See Also

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

Referenced By

lttng(1), lttng-disable-event(1), lttng-enable-channel(1), lttng-track(1), lttng-ust(3), tracef(3), tracelog(3).

10/17/2019 LTTng 2.11.0 LTTng Manual