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

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Message-ID: <1318319095.14400.54.camel@laptop>
Date:	Tue, 11 Oct 2011 09:44:55 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Eric B Munson <emunson@...bm.net>
Cc:	eranian@...gle.com, 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 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?

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