linux-kernel - Re: [GIT PULL 0/8] Annotation weekly ponies delivery

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Date:	Tue, 15 May 2012 13:48:47 +0900
From:	Namhyung Kim <namhyung.kim@....com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	linux-kernel@...r.kernel.org,
	Arjan van de Ven <arjan@...radead.org>,
	David Ahern <dsahern@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mike Galbraith <efault@....de>,
	Paul Mackerras <paulus@...ba.org>,
	Stephane Eranian <eranian@...gle.com>, arnaldo.melo@...il.com,
	Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: Re: [GIT PULL 0/8] Annotation weekly ponies delivery

Hi,

On Tue, 15 May 2012 00:06:25 +0900, Namhyung Kim wrote:
> 2012-05-14 (월), 13:55 +0200, Ingo Molnar:
>> Another detail seems to be that the bug takes per-task-inherited 
>> profiling. Doing:
>> 
>>    perf record -a -e cycles:pp make -j64 bzImage
>> 
>> produces the expected number of events. Without the '-a' the bug 
>> Linus found triggers.
>> 
>> Similarly, 'perf top' does not trigger the bug - because it's 
>> using per CPU, not per task (inherited) profiling.
>> 
>
> perf_evlist__config_attrs() has this:
>
> 	if (evlist->cpus->map[0] < 0)
> 		opts->no_inherit = true;
>
> meaning that per task profiling won't enable event inheritance. I don't
> know why it's needed though.

I found this on commit 5d2cd90922c7 ("perf evsel: Fix use of inherit"):

-               /*
-                * Don't allow mmap() of inherited per-task counters. This
-                * would create a performance issue due to all children writing
-                * to the same buffer.
-                *
-                * FIXME:
-                * Proper fix is not to pass 'inherit' to perf_evsel__open*,
-                * but a 'flags' parameter, with 'group' folded there as well,
-                * then introduce a PERF_O_{MMAP,GROUP,INHERIT} enum, and if
-                * O_MMAP is set, emit a warning if cpu < 0 and O_INHERIT is
-                * set. Lets go for the minimal fix first tho.
-                */
-               evsel->attr.inherit = (cpus->map[cpu] >= 0) && inherit;

Hmm.. still don't know what to do then.

Thanks,
Namhyung
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