linux-kernel - Re: Oprofile Regression Caused by commit e5d1367f17ba6a6fed5fd8b74e4d5720923e0c25 on PPC

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Message-ID: <CABPqkBRHkiOYEW+5Yo9p43c_2Dy+aAdJRXj3tah5J1QsV4AXrw@mail.gmail.com>
Date:	Wed, 12 Oct 2011 22:37:36 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Eric B Munson <emunson@...bm.net>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>, mingo@...e.hu,
	anton@...ba.org, linux-kernel@...r.kernel.org, paulus@...ba.org,
	hbabu@...ibm.com
Subject: Re: Oprofile Regression Caused by commit e5d1367f17ba6a6fed5fd8b74e4d5720923e0c25
 on PPC

Could be:
   a8d757e perf events: Fix slow and broken cgroup context switch code


On Wed, Oct 12, 2011 at 6:51 PM, Eric B Munson <emunson@...bm.net> wrote:
> On Tue, 11 Oct 2011, Peter Zijlstra wrote:
>
>> On Mon, 2011-10-10 at 19:38 -0400, Eric B Munson wrote:
>> > On Fri, 07 Oct 2011, Eric B Munson wrote:
>> >
>> > > This commit seems to have caused a regression with oprofile.  It is fairly easy
>> > > to trigger, simply run oprofile monitoring an event that will fire (something
>> > > frequent like CPU cycles) causes oprofile to fail saying that the PMU is in use.
>> > > If I disable CONFIG_CGROUP_PERF, everything goes back to working.  I suspect the
>> > > problem is that the PMU is being initialized without being reserved for perf.  I
>> > > am not yet sure of the right fix yet so if you have any suggestions I would
>> > > appreciate them.
>> > >
>> > > Eric
>> >
>> > This isn't the best description of the behavior we see, what happens is at some
>> > point in the profiling session the MMCR register is clobbered by
>> > perf_cgroup_switch() which calls perf_pmu_enable() without reserving the PMC
>> > hardware.  When this happens oprofile stops counting.  It doesn't happen each
>> > time so some runs show event counts that are reasonable, but it can also lead to
>> > event counts that are smaller than expected, or completely missing.
>>
>> What kernel are you testing?
>>
>
> The problem seems to be fixed in 3.1-rc9 so I just need to identify the
> patch(es) that fixed it.
>
> Eric
>
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