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

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Message-ID: <CABPqkBRxqC5c1JvvpDawkK9PizAfBGNCksR8bpyJ-rWB4LKmrQ@mail.gmail.com>
Date:	Tue, 18 Oct 2011 16:50:42 +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

Hi,

I suspect it's because of this chunk:
        rcu_read_lock();

        list_for_each_entry_rcu(pmu, &pmus, entry) {
-
                cpuctx = this_cpu_ptr(pmu->pmu_cpu_context);

-               perf_pmu_disable(cpuctx->ctx.pmu);
-
                /*
                 * perf_cgroup_events says at least one
                 * context on this CPU has cgroup events.
@@ -353,6 +366,8 @@ void perf_cgroup_switch(struct task_struct *task, int mode)
                 * events for a context.
                 */
                if (cpuctx->ctx.nr_cgroups > 0) {
+                       perf_ctx_lock(cpuctx, cpuctx->task_ctx);
+                       perf_pmu_disable(cpuctx->ctx.pmu);


In other words, you don't call perf_pmu_disable() unless you know
you have cgroup events.

Without that, I think you will touch the PMU on cgroup switch and
that night conflict with another subsystem using the PMU, e.g. OProfile.


On Tue, Oct 18, 2011 at 4:43 PM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> On Tue, 2011-10-18 at 09:53 -0400, Eric B Munson wrote:
>> On Wed, 12 Oct 2011, Stephane Eranian wrote:
>>
>> > Could be:
>> >    a8d757e perf events: Fix slow and broken cgroup context switch code
>> >
>>
>> Thanks for the pointer, but the fix was in:
>>     facc4307 perf: Optimize event scheduling locking
>>
>> This might be a candidate for stable given that oprofile is broken without it.
>
> I might feel much more confident about recommending that if someone
> could explain why that patches fixes what exact problem.
>
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