Messages in this thread | | | Date | Tue, 11 Oct 2011 09:47:52 +0200 | Subject | Re: Oprofile Regression Caused by commit e5d1367f17ba6a6fed5fd8b74e4d5720923e0c25 on PPC | From | Stephane Eranian <> |
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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.
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