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

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Date:	Fri, 8 Jan 2016 10:01:24 -0800
From:	Stephane Eranian <eranian@...gle.com>
To:	Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:	Namhyung Kim <namhyung@...il.com>,
	LKML <linux-kernel@...r.kernel.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>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>
Subject: Re: [RFC] perf record: missing buildid for callstack modules

On Thu, Jan 7, 2016 at 3:47 PM, Arnaldo Carvalho de Melo
<acme@...nel.org> wrote:
> Em Fri, Jan 08, 2016 at 07:47:03AM +0900, Namhyung Kim escreveu:
>> On January 8, 2016 7:00:35 AM GMT+09:00, Stephane Eranian <eranian@...gle.com> wrote:
>> >On Thu, Jan 7, 2016 at 1:59 PM, Arnaldo Carvalho de Melo
>> ><acme@...nel.org> wrote:
>> >> Em Thu, Jan 07, 2016 at 01:56:14PM -0800, Stephane Eranian escreveu:
>> >>> 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.
>> >>
>> >> I agree, would consider applying a patch that provides the option but
>> >> does not do this by default.
>> >>
>> >I agree, not the default.
>>
>> Hi Stephane,
>>
>> Please see
>>
>> https://lkml.org/lkml/2015/3/22/249
>
>
> Oops, Stephane, please try this, so that we can finally merge it :-\
>
I will try it today. However, I am a bit worried about the performance
impact. Unless I am missing something in this approach we may end up
looking up N times the same module if it appears in N callstacks. In
Andi's suggested approach, there would be only one pass at the beginning
(or the end of the run). But you could miss some modules if they are gone
by the time you run the pass.