[lttng-dev] Misc questions regarding kernel tracing

Sébastien Barthélémy barthelemy at crans.org
Fri Jul 26 12:36:07 EDT 2013

Hello all,

I am investigating a problem on a rt system which misses deadlines under
load. Yet another occasion to use LTTng! But this comes with a bunch of
newcomer questions.

I'd like to reconstruct the state of the scheduler (the control view in
eclipse) with the minimum amount of traces from the kernel. So far I
recorded only the sched* events and it seems ok. But I suspect I should
also enable some lttng_statedump* to garantee a clean state at the very
beginning of the trace. Could you confirm this?

Also, I see threads which do "clone" a lot in the eclipse control view, but
looking at the raw traces (only the sched* events) I do not understand how
eclipse can figure it out. Is this an artefact of trying to use a kernel
trace which misses the syscall events or can I trust these "clone"?

More generally, is there a comprehensive guide somewhere that you would
recommend in order to understand the precise meaning of these sched* events?

Is there a way to trim a trace (remove the beginning and/or the end)
without compromising the scheduler state reconstruction?

I sometimes miss kernel events. Is there a point in using different
channels for kernel and user space traces (besides giving them distinct
sizes)? I suspect they're already effectively different even if they have
the same name. Also, I have increased the subbuffer size a lot, but did not
change the number of subbuffers. Would that help preventing missed events?

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