Hi,
On Mon, Aug 29, 2016 at 6:02 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Mon, Aug 29, 2016 at 12:03:09PM +0200, Peter Zijlstra wrote:
> > @@ -1802,8 +1802,18 @@ event_sched_out(struct perf_event *event,
> >
> > event->tstamp_stopped = tstamp;
> > event->pmu->del(event, 0);
> > - event->oncpu = -1;
> > - event->state = PERF_EVENT_STATE_INACTIVE;
> > +
> > + WRITE_ONCE(event->state, PERF_EVENT_STATE_INACTIVE);
> > + /*
> > + * pmu::del() will have updated the event count. Now mark it inactive,
> > + * but take care to clear ->oncpu after the INACTIVE store, such that
> > + * while ->state == ACTIVE, ->oncpu must be valid.
> > + *
> > + * See event_sched_in(), perf_event_restart() and perf_event_read().
> > + */
> > + smp_wmb();
> > + WRITE_ONCE(event->oncpu, -1);
> > +
> > if (event->pending_disable) {
> > event->pending_disable = 0;
> > event->state = PERF_EVENT_STATE_OFF;
> > @@ -2015,8 +2025,10 @@ event_sched_in(struct perf_event *event,
> >
> > WRITE_ONCE(event->oncpu, smp_processor_id());
> > /*
> > - * Order event::oncpu write to happen before the ACTIVE state
> > - * is visible.
> > + * Order event::oncpu write to happen before the ACTIVE state is
> > + * visible, such that when we observe ACTIVE, oncpu must be correct.
> > + *
> > + * Matches the smp_rmb() in perf_event_restart().
> > */
> > smp_wmb();
> > WRITE_ONCE(event->state, PERF_EVENT_STATE_ACTIVE);
>
> Urgh.. that cannot work either, because now perf_event_read() can race
> against event_sched_in(). Since that's no longer crossed.
>
> > @@ -3561,28 +3576,36 @@ u64 perf_event_read_local(struct perf_event *event)
> >
> > static int perf_event_read(struct perf_event *event, bool group)
> > {
> > - int ret = 0, cpu_to_read, local_cpu;
> > + int ret = 0, cpu_to_read, local_cpu, state;
> > +
> > + local_cpu = get_cpu(); /* disable preemption to hold off hotplut */
> > + cpu_to_read = READ_ONCE(event->oncpu);
> > + /*
> > + * Matches smp_wmb() from event_sched_out(), ->oncpu must be valid
> > + * IFF we observe ACTIVE.
> > + */
> > + smp_rmb();
> > + state = READ_ONCE(event->state);
>
> The best I can come up with is something like:
>
>
> do {
> state = READ_ONCE(event->state);
> if (state != ACTIVE)
> break;
> smp_rmb();
> cpu = READ_ONCE(event->cpu);
> smp_rmb();
> } while (READ_ONCE(event->state) != state);
>
>
> And I suppose perf_event_restart() should do the same thing... Let me
> ponder this a wee bit more.
I am trying to understand this better. There is a race between
oncpu/active and the smp_call.
By the time you actually do the smp_call the oncpu may be wrong and
smp_call now returns
an error given David's change. I suspect the race was always there. It
boils down to what is
the guarantee of the API in terms of the "freshness" of the value
returned on read().
I am guessing that if you thought you had to do the smp_call, it is
because the event was still
active and oncpu != -1. If it is no longer active, it happened very
recently and, in that case, one can
use the saved count in the perf_event struct as a valid value because
it was necessarily updated
when the event was scheduled out.
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