linux-kernel - Re: [PATCH 00/32] tracing: Inter-event (e.g. latency) support

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Date:   Sat, 1 Jul 2017 00:01:39 -0700
From:   "Joel Fernandes (Google)" <joel.opensrc@...il.com>
To:     Tom Zanussi <tom.zanussi@...ux.intel.com>
Cc:     rostedt@...dmis.org, tglx@...utronix.de, mhiramat@...nel.org,
        namhyung@...nel.org, vedang.patel@...el.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-rt-users@...r.kernel.org
Subject: Re: [PATCH 00/32] tracing: Inter-event (e.g. latency) support

Hi Tom,
Nice series and nice ELC talk as well. Thanks.

On Mon, Jun 26, 2017 at 3:49 PM, Tom Zanussi
<tom.zanussi@...ux.intel.com> wrote:
> This patchset adds support for 'inter-event' quantities to the trace
> event subsystem.  The most important example of inter-event quantities
> are latencies, or the time differences between two events.

I tried your patches out and they are working fine for me. I will test
them out more.

I think for the wakeup latency in your examples, its better / more
accurate to use sched_waking instead of sched_wakeup tracepoint? [1]

Also, one other comment I had is, it would be nice to suppress the
output of individual trace events that are part of the synthetic event
into the trace buffer. Otherwise I feel the value of it is slightly
diminished - because one can simply post-process the individual
non-synthetic trace events themselves to get wake up latencies which
the synthetic events is trying to calculate in the first place.
Inorder to conserve space, if a user doesn't care about individual
events, and just the synthetic events then the individual ones
shouldn't be written to the trace buffer IMO.

-Joel

[1] commit fbd705a0c6184580d0e2fbcbd47a37b6e5822511 (sched: Introduce
the 'trace_sched_waking' tracepoint)