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

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Date:	Tue, 11 Oct 2011 09:47:52 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Eric B Munson <emunson@...bm.net>, 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

On Tue, Oct 11, 2011 at 9:44 AM, Peter Zijlstra <a.p.zijlstra@...llo.nl> 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?
>
Looking at 3.1-rc9, I doubt it's coming from perf_cgroup_switch(). The function
is checking for perf context with at least of cgroup event before
calling perf_pmu_disable().
If there is no active perf context, there is no cgroup event, thus the
function is a nop.
Even if you have a competing perf session, it would have to have at
least one cgroup
event for this code to touch HW. The problem must come from somewhere else.

>
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