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().
Could you send a nice set of patches addressing all concerns?
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