linux-kernel - Re: [PATCH 4/5] perf_events: add cgroup support (v7)

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Date:	Mon, 10 Jan 2011 15:30:39 +0100
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, paulus@...ba.org,
	davem@...emloft.net, fweisbec@...il.com,
	perfmon2-devel@...ts.sf.net, eranian@...il.com,
	robert.richter@....com, acme@...hat.com, lizf@...fujitsu.com
Subject: Re: [PATCH 4/5] perf_events: add cgroup support (v7)

Peter,

There is indeed a problem in the exit path. We cannot use TASK_DEAD.
Using task->flags & PF_EXITING would work, but there is one last gotcha.

We need to have the sequence:
        perf_event_exit_task(tsk);
        cgroup_exit(tsk, 1);

Be atomic w.r.t., IPI. I am running into timing problem (time_ena,
time_run) if you
have an IPI to read the counts between perf_event_exit_task() and cgroup_exit().
The issue is that during that time window, tsk->cgrp == current->cgrp, thus the
timing code assumes cgroup monitoring is still on and it updates the cgroup time
(time_enabled).

I am working on a solution that does not involve interrupt masking for those
two calls.



On Thu, Jan 6, 2011 at 11:15 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Wed, 2011-01-05 at 22:39 +0100, Stephane Eranian wrote:
>> Peter,
>>
>> On Wed, Jan 5, 2011 at 2:01 PM, Stephane Eranian <eranian@...gle.com> wrote:
>> > On Wed, Jan 5, 2011 at 12:23 PM, Peter Zijlstra <peterz@...radead.org> wrote:
>> >> On Mon, 2011-01-03 at 18:20 +0200, Stephane Eranian wrote:
>> >>> +void
>> >>> +perf_cgroup_switch(struct task_struct *task, struct task_struct *next)
>> >>> +{
>> >>> +     struct perf_cgroup *cgrp_out = perf_cgroup_from_task(task);
>> >>> +     struct perf_cgroup *cgrp_in = perf_cgroup_from_task(next);
>> >>> +     struct perf_cpu_context *cpuctx;
>> >>> +     struct pmu *pmu;
>> >>> +     /*
>> >>> +      * if task is DEAD, then css_out is irrelevant, it has
>> >>> +      * been changed to init_cgrp in cgroup_exit() from do_exit().
>> >>> +      * Furthermore, perf_cgroup_exit_task(), has scheduled out
>> >>> +      * all css constrained events, only unconstrained events
>> >>> +      * remain. Therefore we need to reschedule based on css_in.
>> >>> +      */
>> >>> +     if (task->state != TASK_DEAD && cgrp_out == cgrp_in)
>> >>> +             return;
>> >>
>> >> I think that check is broken, TASK_DEAD is set way after calling
>> >> cgroup_exit(), so if we get preempted in between there you'll still go
>> >> funny.
>> >>
>>
>> I looked at this part again.
>>
>> The original code checking for TASK_DEAD is correct.
>>
>> The reason is simple, you're looking at perf_cgroup_switch() which is
>> invoked as part of schedule() and NOT perf_event_task_exit() (called
>> prior to cgroup_exit()).
>> Thus, by the time you do the final schedule(), the task state has indeed
>> been switched to TASK_DEAD.
>>
>> I remember testing for this condition during the debug phase.
>
> But, cgroup_exit() detaches the task from the cgroup, after which the
> cgroup can disappear. Furthermore, we can schedule after cgroup_exit()
> and before the explicit schedule() invocation.
>
> Some of the exit functions (say proc_exit_connector) can block and cause
> scheduling, and with PREEMPT=y we can get preempted.
>
> This means you'll be context switching, and thus possibly calling
> perf_cgroup_switch(), on a task who's cgroup is possibly destroyed.
>
> So I'm not at all seeing how this is correct.
>
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