lttng-add-trigger(1): clarify the `--rate-policy` documentation

[lttng-tools.git] / doc / man / lttng-add-trigger.1.txt

lttng-add-trigger(1)
====================
:revdate: 23 April 2021


NAME
----
lttng-add-trigger - Add an LTTng trigger


SYNOPSIS
--------
[verse]
*lttng* ['linkgenoptions:(GENERAL OPTIONS)'] *add-trigger* [option:--name='NAME'] [option:--owner-id='UID']
      option:--condition='CONDTYPE' ['CONDARGS']
      option:--action='ACTTYPE' ['ACTARGS'] [option:--action='ACTTYPE' ['ACTARGS']]...

DESCRIPTION
-----------
The `lttng add-trigger` command creates and adds an LTTng _trigger_ to
the session daemon (see man:lttng-sessiond(8)).

A trigger is an association between a _condition_ and one or more
_actions_. When the condition of a trigger{nbsp}__T__ is satisfied,
LTTng requests to execute the actions of{nbsp}__T__. Depending on the
rate policy of an action, an execution request can become an actual
execution.

A trigger doesn't belong to a specific tracing session: it's global to
the session daemon. Within the session daemon, and for a given Unix
user, a trigger has a unique name. By default, the `add-trigger` command
automatically assigns a name to the added trigger. Use the option:--name
option to assign a specific name instead.

The `add-trigger` command adds a trigger for your Unix user. If your
Unix user is `root`, you can add the trigger as another user with the
option:--owner-id option.

Specify the condition of the trigger to add with a <<cond-spec,condition
specifier>> and its actions with one or more <<action-spec,action
specifiers>>. The order of the action specifiers is significant: LTTng
attempts to execute the actions of a trigger in order.

List the available triggers for your Unix user (or for all users if your
Unix user is `root`) with the man:lttng-list-triggers(1) command.

Remove an existing trigger with the man:lttng-remove-trigger(1) command.


[[cond-spec]]
Condition specifier
~~~~~~~~~~~~~~~~~~~
Synopsis:

[verse]
option:--condition='CONDTYPE' ['CONDARGS']

A condition specifier is the option:--condition option, which specifies
the type of condition 'CONDTYPE', followed, depending on 'CONDTYPE',
with zero or more arguments 'CONDARGS'.

The available condition types are:

[[er-matches-cond-spec]]`event-rule-matches`::
    Synopsis:
+
[verse]
option:--condition=event-rule-matches [nloption:--capture='CDESCR']... 'ERSPEC'
{nbsp}
+
An `event-rule-matches` condition is considered satisfied when the event
rule specified with 'ERSPEC' matches an event.
+
See man:lttng-event-rule(7) to learn how to specify an event rule
('ERSPEC' part).
+
Capture event record and context fields with one or more
nloption:--capture options (see the <<capture-descr,Capture descriptor>>
section below to learn more). When an `event-rule-matches` condition
with capture descriptors is satisfied, the captured field values are
available in the evaluation object of the condition using the
liblttng-ctl C{nbsp}API.
+
IMPORTANT: Make sure to **single-quote** 'CDESCR' when you run the
`add-trigger` command from a shell, as capture descriptors can include
characters having a special meaning for most shells.


[[capture-descr]]
Capture descriptor
~~~~~~~~~~~~~~~~~~
A capture descriptor is a textual expression which describes how to read
an event record or context field.

The argument of a nloption:--capture option, when using an
<<er-matches-cond-spec,``event rule matches'' condition specifier>>
(`event-rule-matches`), is a capture descriptor.

A capture descriptor expression is one of:

'NAME'::
    An event record field named 'NAME'.
+
The supported event record field types are:
+
--
* Integer
* Enumeration (integral value)
* Floating point number
* Static array of integers
* Dynamic array (``sequence'') of integers
* Text string
--
+
Examples: `my_field`, `target_cpu`, `ip`.

++$ctx.++__NAME__::
    A statically-known context field named 'NAME'.
+
List the available statically-known context field names with
man:lttng-add-context(1).
+
Examples: `$ctx.prio`, `$ctx.preemptible`,
`$ctx.perf:cpu:stalled-cycles-frontend`.

++$app.++__PROVIDER__++.++__NAME__::
    An application-specific context field named 'NAME' from the
    provider 'PROVIDER'.
+
See man:lttng-add-context(1) to learn more about application-specific
context fields.
+
Example: `$app.server:cur_user`.

__EXPR__++[++__INDEX__++]++::
    The element at index 'INDEX' of the array field (static or dynamic)
    identified by the expression 'EXPR'.
+
'INDEX' must be a constant, positive integral value.
+
Examples: `ip[3]`, `user_ids[15]`.

If, when an event rule matches, a given capture descriptor doesn't
identify an existing event or context field, then the captured value is
reported as being unavailable. This applies to:

* A nonexistent event record field name.
* A nonexistent statically-known context field name.
* A nonexistent application-specific context field name.
* An out-of-bounds array field index.


[[action-spec]]
Action specifier
~~~~~~~~~~~~~~~~
Synopsis:

[verse]
option:--action='ACTTYPE' ['ACTARGS']

An action specifier is the option:--action option, which specifies
the type of action 'ACTTYPE', followed, depending on 'ACTTYPE', with zero
or more arguments 'ACTARGS'.

The available action types are:

Notify::
    Synopsis:
+
[verse]
option:--action=notify [nloption:--rate-policy='POLICY']
{nbsp}
+
Sends a notification through the notification
mechanism of the session daemon (see man:lttng-session(8)).
+
The session daemon sends details about the condition evaluation along
with the notification.
+
As of LTTng{nbsp}{lttng_version}, you can write a C/pass:[C++] program
to receive LTTng notifications (see the liblttng-ctl C{nbsp}headers).
+
See below for the nloption:--rate-policy option.

Start a tracing session::
    Synopsis:
+
[verse]
option:--action=start-session 'SESSION' [nloption:--rate-policy='POLICY']
{nbsp}
+
Starts the tracing session named 'SESSION' like man:lttng-start(1)
would.
+
If no tracing session has the name 'SESSION' when LTTng is ready to
execute the action, LTTng does nothing.
+
See below for the nloption:--rate-policy option.

Stop a tracing session::
    Synopsis:
+
[verse]
option:--action=stop-session 'SESSION' [nloption:--rate-policy='POLICY']
{nbsp}
+
Stops the tracing session named 'SESSION' like man:lttng-stop(1) would.
+
If no tracing session has the name 'SESSION' when LTTng is ready to
execute the action, LTTng does nothing.
+
See below for the nloption:--rate-policy option.

Rotate a tracing session::
    Synopsis:
+
[verse]
option:--action=rotate-session 'SESSION' [nloption:--rate-policy='POLICY']
{nbsp}
+
Archives the current trace chunk of the tracing session named 'SESSION'
like man:lttng-rotate(1) would.
+
If no tracing session has the name 'SESSION' when LTTng is ready to
execute the action, LTTng does nothing.
+
See below for the nloption:--rate-policy option.

Take a tracing session snapshot::
    Synopsis:
+
[verse]
option:--action=snapshot-session 'SESSION' [nloption:--rate-policy='POLICY']
{nbsp}
+
Takes a snapshot of the tracing session named 'SESSION' like
man:lttng-snapshot(1) would.
+
When the condition of the trigger is satisfied, the tracing session
named 'SESSION', if any, must be a snapshot-mode tracing session
(see man:lttng-create(1)).
+
If no tracing session has the name 'SESSION' when LTTng is ready to
execute the action, LTTng does nothing.
+
See below for the nloption:--rate-policy option.

Common action options (as of LTTng{nbsp}{lttng_version}):

nloption:--rate-policy='POLICY'::
    Set the rate policy of the action to 'POLICY' instead of
    `every:1` (always execute).
+
A trigger which ``fires'' (its condition is satisfied) leads to an
execution request for each of its actions, in order. An execution
request of a given action{nbsp}__A__ first increments the execution
request count{nbsp}__C__ of{nbsp}__A__. An execution request can then
become an actual execution when{nbsp}__C__ satisfies the rate policy
of{nbsp}__A__.
+
'POLICY' is one of:
+
--
++once-after:++__COUNT__::
    Only execute{nbsp}__A__ when{nbsp}__C__ is equal to 'COUNT'.
+
In other words, execute{nbsp}__A__ a single time after 'COUNT' execution
requests.

++every:++__COUNT__::
    Only execute{nbsp}__A__ when{nbsp}__C__ is a multiple of 'COUNT'.
+
In other words, execute{nbsp}__A__ every 'COUNT' execution requests.
--
+
'COUNT' must be an integer greater than{nbsp}0.
+
As of LTTng{nbsp}{lttng_version}, you can use this option with any
action type, but new action types in the future may not support it.


OPTIONS
-------
Identification
~~~~~~~~~~~~~~
option:--name='NAME'::
    Set the unique name of the trigger to add to 'NAME' instead of the
    `add-trigger` command automatically assigning one.

option:--owner-id='UID'::
    Add the trigger as the Unix user having the user ID 'UID'.
+
You may only use this option if your Unix user is `root`.


Specifier
~~~~~~~~~
option:--condition='CONDTYPE'::
    Introductory option for a condition specifier of type 'CONDTYPE'.
+
See the <<cond-spec,Condition specifier>> section above to learn more.

option:--action='ACTTYPE'::
    Introductory option for an action specifier of type 'ACTTYPE'.
+
See the <<action-spec,Action specifier>> section above to learn more.


include::common-cmd-help-options.txt[]


include::common-cmd-footer.txt[]


SEE ALSO
--------
man:lttng(1),
man:lttng-list-triggers(1),
man:lttng-remove-trigger(1)