linux-kernel - Re: [RFC] perf record: missing buildid for callstack modules

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Message-ID: <CABPqkBTOAmfCckW0L7eTyoLVtdJpiwW8PdXJ5K+Ny+mQNtyUnQ@mail.gmail.com>
Date:	Thu, 7 Jan 2016 14:00:11 -0800
From:	Stephane Eranian <eranian@...gle.com>
To:	Andi Kleen <ak@...ux.intel.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Jiri Olsa <jolsa@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Adrian Hunter <adrian.hunter@...el.com>
Subject: Re: [RFC] perf record: missing buildid for callstack modules

On Thu, Jan 7, 2016 at 1:57 PM, Andi Kleen <ak@...ux.intel.com> wrote:
> On Thu, Jan 07, 2016 at 01:56:14PM -0800, Stephane Eranian wrote:
>> Hi,
>>
>> Whenever you do:
>>
>>     $ perf record -g -a sleep 10
>>
>> Perf will collect the callstack for each sample. At the end of the
>> run, perf record
>> adds the buildid for all dso with at least one sample. But when it does this, it
>> only looks at the sampled IP and ignore the modules traversed by the callstack.
>> That means that, it is not possible to uniquely identify the modules executed,
>> unless they had at least one IP sample captured. But this is not
>> always the case.
>>
>> How about providing an option to perf record to force collecting
>> buildid for all IPs
>> captured in the callstack? I understand that would cost more at the end of the
>> collection, but this would be beneficial to several monitoring scenarios.
>
> If you do that I would rather collect buildid for everything that is synthesized
> (everything in /proc)
>
But that would be much more than whatever would be captured with hits in
callstacks.

A valid scenario here is sampling on tracepoints (which are kernel only events)
but you want to know where the syscall originated from or which user code was
interrupted.