doc/man/lttng-add-trigger.1.txt

   1 lttng-add-trigger(1)
   2 =====================
   3 :revdate: 27 May 2020
   4
   5
   6 NAME
   7 ----
   8 lttng-add-trigger - Create LTTng triggers
   9
  10
  11 SYNOPSIS
  12 --------
  13
  14 [verse]
  15 *lttng* ['linkgenoptions:(GENERAL OPTIONS)'] *add-trigger* [--id ID]
  16       [--fire-every N] [--fire-once-after N]
  17       --condition CONDITION-NAME CONDITION-ARGS
  18       --action ACTION-NAME ACTION-ARGS
  19       [--action ACTION-NAME ACTION-ARGS]...
  20
  21
  22 DESCRIPTION
  23 -----------
  24
  25 The `lttng add-trigger` command is used to create triggers.  A
  26 trigger is an association between a *condition* and one or more
  27 *actions*.  When the condition associated to a trigger is met, the
  28 actions associated to that trigger are executed.  The tracing does not
  29 have to be active for the conditions to be met, and triggers are
  30 independent from tracing sessions.
  31
  32 By default, a trigger fires every time its condition is met.  The
  33 '--fire-every' and '--fire-once-after' options can be used to change
  34 this mode.
  35
  36 The syntax by which conditions and actions are specified is described
  37 below.
  38
  39 [[conditions]]
  40 Conditions
  41 ~~~~~~~~~~
  42
  43 Conditions are specified with the `--condition` option, followed by a
  44 condition name, and possibly some more arguments, depending on the
  45 specific condition.  There must be exactly one condition given in the
  46 `lttng add-trigger` invocation.
  47
  48 The available conditions are:
  49
  50 Event rule: `on-event [event rule arguments]`::
  51     This type of condition is met when the tracer encounters an event
  52     matching the given even rule.  The arguments describing the event
  53     rule are the same as those describing the event rules of the
  54     man:lttng-enable-event(1) command, with these exceptions:
  55
  56     - It is not possible to use filter operands that use values from
  57       the context.
  58
  59 +
  60 Fields to capture can be specified with the option:--capture option, followed by
  61 a capture expression. Zero or more captures can be configured. See the
  62 <<capture-expr, Capture expression>> section below for more information.
  63
  64 [[actions]]
  65 Actions
  66 ~~~~~~~
  67
  68 Actions are specified with the `--action` option, followed by an action
  69 name, and possibly some more arguments, depending on the specific
  70 action.  There must be at least one action given in the
  71 `lttng add-trigger` invocation.
  72
  73 The available actions are:
  74
  75 Notify: *notify*::
  76     This action causes the LTTng session daemon to send a notification,
  77     through its notification mechanism (see man:lttng-sessiond(8)).
  78     Some details about the condition evaluation are sent along with the
  79     notification.
  80
  81 Start session: *start-session* session-name::
  82     This action causes the LTTng session daemon to start tracing for the
  83     session with the given name.  If no session with the given name exist
  84     at the time the condition is met, nothing is done.
  85
  86 Stop session: *stop-session* session-name::
  87     This action causes the LTTng session daemon to stop tracing for the
  88     session with the given name.  If no session with the given name exist
  89     at the time the condition is met, nothing is done.
  90
  91 Rotate session: *rotate-session* session-name::
  92     This action causes the LTTng session daemon to rotate the session
  93     with the given name.  See  man:lttng-rotate(1) for more information
  94     about the session rotation concept.  If no session with the given
  95     name exist at the time the condition is met, nothing is done.
  96
  97 Snapshot session: *snapshot-session* session-name::
  98     This action causes the LTTng session daemon to take a snapshot of the
  99     session with the given name.  See man:lttng-snapshot(1) for more
 100     information about the session snapshot concept.  If no session with
 101     the given name exist at the time the condition is met, nothing is
 102     done.
 103
 104
 105 [[capture-expr]]
 106 Capture expression
 107 ~~~~~~~~~~~~~~~~~~
 108
 109 A capture expression can be specified with the option:--capture option when
 110 creating a new on-event condition. If the capture expression corresponds with an
 111 event's field when tracing, the runtime dynamic value corresponding to the
 112 capture expression is captured.
 113
 114 NOTE: Make sure to **single-quote** the capture expression when running
 115 the command from a shell, as capture expressions typically include
 116 characters having a special meaning for most shells.
 117
 118 * Supported field types:
 119  - integer,
 120  - unsigned integer,
 121  - floating point value,
 122  - fixed-size array of integers,
 123  - variable-size array of integers (sequence),
 124  - enumeration,
 125  - text string,
 126  - element of any allowing previous type.
 127
 128 * The dynamic value of an event field is captured by using its name as a C
 129   identifier.
 130 +
 131 The square bracket notation is available, like in the C
 132 language, to access array/sequence field.
 133 Only a constant, positive integer number can be used within square
 134 brackets. If the index is out of bounds, the capture expression
 135 evaluates to `unavailable`.
 136 +
 137 An enumeration field's value is an integer.
 138 +
 139 When the capture's field does not exist, the capture expression
 140 evaluates to `unavailable`.
 141 +
 142 Examples: `my_field`, `target_cpu`, `seq[7]`
 143
 144 * The dynamic value of a statically-known context field is captured by
 145   prefixing its name with `$ctx.`. See man:lttng-add-context(1) to get a list of
 146   available contexts.
 147 +
 148 When the expression's statically-known context field does not exist,
 149 the capture expression evaluates to `unavailable`.
 150 +
 151 Examples: `$ctx.prio`, `$ctx.preemptible`,
 152 `$ctx.perf:cpu:stalled-cycles-frontend`.
 153 +
 154 NOTE: The statically-known context field does NOT need to be added using the
 155 man:lttng-add-context(1) command. The statically-known context fields are
 156 always available in the context of triggers.
 157
 158 * The dynamic value of an application-specific context field is captured by
 159   prefixing its name with `$app.` (follows the format used to add such a context
 160   field with the man:lttng-add-context(1) command).
 161 +
 162 When the expression's application-specific context field does not exist,
 163 the capture expression evaluates to `unavailable`.
 164 +
 165 Example: `$app.server:cur_user`.
 166 +
 167 NOTE: The application-specific context field does NOT need to be added using the
 168 man:lttng-add-context(1) command. The application-specific context fields fields
 169 are always available in the context of triggers.
 170
 171
 172 OPTIONS
 173 -------
 174
 175 option:--condition::
 176     Define the condition for the trigger.  See the
 177     <<conditions,CONDITIONS>> section for more details.
 178
 179 option:--action::
 180     Define an action for the trigger.  See the <<actions,ACTIONS>>
 181     section for more details.
 182
 183 option:--id='ID'::
 184     Set the id of the trigger to 'ID'.  If omitted, an id will
 185     automatically be assigned to the trigger by the session daemon.
 186 +
 187 If a trigger with the specified 'ID' already exists, the trigger
 188 creation will fail.
 189
 190 option:--fire-every 'N'::
 191     Execute the trigger's actions every 'N' times the condition is met.
 192
 193 option:--fire-once-after 'N'::
 194     Execute the trigger's actions once after 'N' times the condition is
 195     met, then never after that.
 196
 197 include::common-cmd-help-options.txt[]
 198
 199
 200 include::common-cmd-footer.txt[]
 201
 202
 203 SEE ALSO
 204 --------
 205 man:lttng-list-triggers(1),
 206 man:lttng-remove-trigger(1),
 207 man:lttng(1)