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

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Message-ID: <557ADB6E.8080604@intel.com>
Date:	Fri, 12 Jun 2015 16:15:26 +0300
From:	Adrian Hunter <adrian.hunter@...el.com>
To:	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
CC:	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

On 12/06/15 15:36, Arnaldo Carvalho de Melo wrote:
> 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.

Thanks for the example.

>>
>>>> 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 :-)

Thanks, that's clear then.  There will just need to be a flag to indicate
whether it is scheduling in or out.

>  
>> 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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