Message-ID: <AANLkTim2uXu+XBWdA-ZJOYFUyMrY+ze84QbMzohcN3Fo@mail.gmail.com>
Date:	Thu, 17 Feb 2011 17:01:49 +0100
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	mingo@...hat.com, hpa@...or.com, linux-kernel@...r.kernel.org,
	tglx@...utronix.de, mingo@...e.hu,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:perf/core] perf: Add cgroup support

On Thu, Feb 17, 2011 at 4:50 PM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> On Thu, 2011-02-17 at 15:45 +0100, Stephane Eranian wrote:
>
>> > CONFIG_PROVE_RCU=y, its a bit of a shiny feature but most of the false
>> > positives are gone these days I think.
>> >
>> I have this one enabled, yet no message.
>
> Hmm, Ingo triggered it, not sure what he did.
>
>
>> >> > @@ -5794,9 +5795,14 @@ static void task_clock_event_read(struct perf_event *event)
>> >> >        u64 time;
>> >> >
>> >> >        if (!in_nmi()) {
>> >> > -               update_context_time(event->ctx);
>> >> > +               struct perf_event_context *ctx = event->ctx;
>> >> > +               unsigned long flags;
>> >> > +
>> >> > +               spin_lock_irqsave(&ctx->lock, flags);
>> >> > +               update_context_time(ctx);
>> >> >                update_cgrp_time_from_event(event);
>> >> > -               time = event->ctx->time;
>> >> > +               time = ctx->time;
>> >> > +               spin_unlock_irqrestore(&ctx->lock, flags);
>> >> >        } else {
>> >> >                u64 now = perf_clock();
>> >> >                u64 delta = now - event->ctx->timestamp;
>> >
>> > I just thought we should probably kill the !in_nmi branch, I'm not quite
>> > sure why that exists..
>>
>> I don't quite understand what this event is supposed to count in system-wide
>> mode. This function adds a time delta. It may be using the wrong time source
>> in cgroup mode.
>>
>> Having said that, it seems to me like we may not even need the call to
>> update_cgrp_time_from_event() there. It is not even used to compute
>> the time delta in that function. Yet, we do get correct timings in cgroup
>> mode. Thus, I suspect the timing is taken care by callers already whenever
>> needed. I looked at the pmu->read() callers, and it seems they do exactly
>> that. In summary, I believe we may be able to drop this call.
>
> ok, nice!
>
>> >> > I then realized that the events themselves pin the cgroup, so its all
>> >> > cosmetic at best, but then I already had the below patch...
>> >> >
>> >> I assume by 'pin the group' you mean the cgroup cannot disappear
>> >> while there is at least one event pointing to it. That's is indeed true
>> >> thanks to refcounting (css_get()).
>> >
>> > Right, that's what I was thinking, but now I think that's not
>> > sufficient, we can have cgroups without events but with tasks in for
>> > which the races are still valid.
>> >
>> But in that case, no perf_event code should be fiddling with cgroups.
>> I think there are guards for that, either is_cgroup_event() or ctx->nr_cgroups.
>>
>> But it seems perf_cgroup_from_event() is the one exception. So maybe
>> we could rewrite it:
>>
>> static inline void update_cgrp_time_from_event(struct perf_event *event)
>> {
>>         struct perf_cgroup *cgrp;
>>
>>         if (!is_cgroup_event(event))
>>                 return;
>>
>>         cgrp = perf_cgroup_from_task(current);
>>         /*
>>          * do not update time when cgroup is not active
>>          */
>>         if (cgrp != event->cgrp)
>>                 return;
>>
>>         __update_cgrp_time(event->cgrp);
>> }
>
> That might indeed work. We'd still need to shut up that RCU warning
> though, we can do that by annotating it away by using
> task_subsys_state(.c=1), and put a comment in explaining things.
>
>> @@ -1613,7 +1614,7 @@ static int __perf_event_enable(void *info)
>>        /*
>>         * set current task's cgroup time reference point
>>         */
>> -       perf_cgroup_set_timestamp(current, perf_clock());
>> +       perf_cgroup_set_timestamp(current, ctx);
>
> That part ended up avoiding a perf_clock() call, we could write that as:
>
>  perf_cgroup_set_timestamp(current, ctx->timestamp);
>
> since ctx->timestamp has just been set to perf_clock().

Ok so this one is just an optimization and not a locking problem, right?

I just realized that perf_cgroup_set_timestamp() is systematically
calling perf_cgroup_from_task(). perf_events is touching cgroup
data without knowing if this is really needed. But according to your
earlier message, the call from __perf_event_enable() should be fine
because we're holding ctx->lock. So  I think we should be fine here.

>
> Could you send a nice set of patches addressing all concerns?
>
Yes, I will take yours and add what we just discussed.
Thanks.
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