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

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Date:	Tue, 11 Oct 2011 09:32:13 -0400
From:	Eric B Munson <emunson@...bm.net>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
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 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 tests first showed in Beta testing the new RHEL and SLES kernels, but the
perf cgroup code looks relatively unchanged since.  I have asked that the tests
be re-run with Linus' HEAD.

Eric

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