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

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Message-ID: <AANLkTinGaHThAxO+0ObCtv+28W8bKULBEtV1KQ4Nax8H@mail.gmail.com>
Date:	Wed, 5 Jan 2011 22:39:02 +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,

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.

I will resubmit with the other smaller cleanups.
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