linux-kernel - Re: [RFC] perf/perf_events: misleading number of samples due to mmap()

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Message-ID: <AANLkTilWIn9VOsIvxQB4BamD7RguNiirtNVTt60hW7XQ@mail.gmail.com>
Date:	Wed, 16 Jun 2010 19:15:37 +0200
From:	stephane eranian <eranian@...glemail.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Stephane Eranian <eranian@...gle.com>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, paulus@...ba.org,
	davem@...emloft.net, fweisbec@...il.com,
	perfmon2-devel@...ts.sf.net
Subject: Re: [RFC] perf/perf_events: misleading number of samples due to 
	mmap()

On Wed, Jun 16, 2010 at 6:41 PM, stephane eranian
<eranian@...glemail.com> wrote:
> On Wed, Jun 16, 2010 at 4:52 PM, Peter Zijlstra <peterz@...radead.org> wrote:
>> On Wed, 2010-06-16 at 16:40 +0200, Stephane Eranian wrote:
>>> This leads me to another point. For per-thread sampling, why
>>> do we need to record mmap() events happening *outside* of
>>> the process? I can understand the exception of kernel modules.
>>
>> How does that happen? The per-thread events should be on the per-task
>> context, so another task's mmap() events should never end up there.
>>
>
> I don't see the test that says the vma does not belong to the current task.
> I also don't see anything in perf_event_mmap_match().
>
> It does seem to work as you said in recent kernels, though. So I am certainly
> missing something here.
>
I think I got it now. It's all based on the current->perf_event_ctxp.
So you only
look at the events (and thus sampling buffers) attached to current which issued
the mmap().

I found out that my problem is coming from perf creating synthetic
mmap() entries
for everything it finds in /proc/PID.
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