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    SubjectRe: Oprofile Regression Caused by commit e5d1367f17ba6a6fed5fd8b74e4d5720923e0c25 on PPC
    On Tue, 11 Oct 2011, Stephane Eranian wrote:

    > On Tue, Oct 11, 2011 at 9:44 AM, Peter Zijlstra <a.p.zijlstra@chello.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.
    >
    > >
    >

    Does activating any cgroup setup a cgroup event?

    Eric
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