linux-kernel - Re: [RFC PATCH] perf: Add PERF_RECORD_SWITCH to indicate context switches

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Date:	Fri, 12 Jun 2015 14:09:38 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Adrian Hunter <adrian.hunter@...el.com>
Cc:	Andi Kleen <ak@...ux.intel.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	Jiri Olsa <jolsa@...hat.com>,
	Stephane Eranian <eranian@...gle.com>,
	mathieu.poirier@...aro.org, Pawel Moll <pawel.moll@....com>
Subject: Re: [RFC PATCH] perf: Add PERF_RECORD_SWITCH to indicate context
 switches

On Fri, Jun 12, 2015 at 02:12:11PM +0300, Adrian Hunter wrote:
> On 11/06/15 17:15, Peter Zijlstra wrote:

> > Right, so the one wee problem I have is that this only provides sched_in
> > data, I imagine people might be interested in sched_out as well.
> 
> That is not a problem although it would be interesting to know the use-case.
> To me it seemed unreasonable to expect to analyze scheduler behaviour
> without admin-level privileges since it is inherently an administrative
> activity.

I was more thinking about it being used to track event duration inside a
task. Say you want measure the time between event A and event B but got
scheduled out in between.

	---- A ----] .... [---- B -----

If you do not have the sched_out time, you cannot correct for that.

> > all 3 are already part of sample_id.
> 
> You have to decide whether you expect to be able to use an event without
> sample_id. MMAP and MMAP2 both have pid, tid which are in sample_id, LOST
> has id, EXIT and FORK have time, all of the THROTTLE/UNTHROTTLE members are
> in sample_id etc. So it currently looks like we expect to be able to use an
> event without requiring sample_id.

I think we recently had this discussion:

  lkml.kernel.org/r/1430940834-8964-8-git-send-email-kan.liang@...el.com

The patch we ended up merging:

  f38b0dbb491a ("perf/x86/intel: Introduce PERF_RECORD_LOST_SAMPLES")

Does indeed require sample_id.
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