Hi Yehuda, The trigger mechanism is asynchroneous. The applications (or the kernel) won't block until the session daemon has processed the actions of the trigger.
As such, it is likely that you will lose events between the moment the trigger is triggered and when the session is started. We have been considering implementing synchroneous start/stop actions, but no one has sponsored that work for the moment. In your case, a work-around you could consider is to: * setup a snapshot session with the same events * set the trigger's actions to record a snapshot on that session, and start your existing session In that setup, you would most likely see the events that led to the "start" (since they are captured in the in-memory buffers of the snapshot session), and you would capture the follow-up events in the on-disk session. I hope that helps, Jérémie -- Jérémie Galarneau EfficiOS Inc. https://www.efficios.com ________________________________ From: lttng-dev <lttng-dev-boun...@lists.lttng.org> on behalf of Yitschak, Yehuda via lttng-dev <lttng-dev@lists.lttng.org> Sent: May 23, 2023 08:17 To: lttng-dev@lists.lttng.org <lttng-dev@lists.lttng.org> Subject: [lttng-dev] delay between trigger and action Hi everyone I am experimenting with enabling trace for a specific iteration of a loop in my application. I created 2 trace points at the start and end of the loop which log the iteration number. the loop itself creates thousands of trace points. It runs for ~5ms. on lttng side I created 2 triggers to start and stop my session in case the loop tracepoint registers a specific iteration number. the triggers seem to work since there is another session entry created under the session folder but there are very few tracepoints. is it possible that the delay between the event happening and the session trace starting causes loss of events ? If yes, is there a way around this ? here is my setup : #lttng-status Recording session session: [inactive] Trace output: /home/user/lttng-traces/session-20230523-113537 === Domain: User space === Buffering scheme: per-user Tracked process attributes Virtual process IDs: all Virtual user IDs: all Virtual group IDs: all Channels: ------------- - user-channel: [enabled] Attributes: Event-loss mode: discard Sub-buffer size: 16777216 bytes Sub-buffer count: 128 Switch timer: inactive Read timer: inactive Monitor timer: 1000000 us Blocking timeout: 0 us Trace file count: 1 per stream Trace file size: unlimited Output mode: mmap Statistics: Discarded events: 0 Recording event rules: trace_events* (type: tracepoint) [enabled] loop* (type: tracepoint) [enabled] #lttng list-triggers - name: iter-start owner uid: 1000 condition: event rule matches rule: loop:iteration (type: user tracepoint, filter: iter==2 && start == 1) errors: none actions: start session `session` errors: none errors: none - name: iter-stop owner uid: 1000 condition: event rule matches rule: loop:iteration (type: user tracepoint, filter: iter==2 && start == 0) errors: none actions: stop session `session` errors: none errors: none Thanks Yehuda
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