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

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Date:	Fri, 12 Jun 2015 09:36:43 -0300
From:	Arnaldo Carvalho de Melo <acme@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Adrian Hunter <adrian.hunter@...el.com>,
	Andi Kleen <ak@...ux.intel.com>,
	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

Em Fri, Jun 12, 2015 at 02:09:38PM +0200, Peter Zijlstra escreveu:
> 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.

The fact that there is this duplication is because sample_id_all came
after those events, but this new one being proposed doesn't have to do
it :-)
 
> 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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