On Thu, Apr 04, 2019 at 01:09:09PM +0200, Peter Zijlstra wrote: > That is not entirely the scenario I talked about, but *groan*. > > So what I meant was: > > CPU-0 CPU-n > > __schedule() > local_irq_disable() > > ... > deactivate_task(prev); > > > try_to_wake_up(@p) > ... > > smp_cond_load_acquire(&p->on_cpu, !VAL); > > <PMI> > .. > perf_event_disable_inatomic() > event->pending_disable = 1; > irq_work_queue() /* self-IPI */ > </PMI> > > context_switch() > prepare_task_switch() > perf_event_task_sched_out() > // the above chain that clears pending_disable > > finish_task_switch() > finish_task() > smp_store_release(prev->on_cpu, 0); > /* > finally.... */ > // take woken > // > context_switch to @p > finish_lock_switch() > raw_spin_unlock_irq() > /* w00t, IRQs enabled, self-IPI time */ > <self-IPI> > perf_pending_event() > // event->pending_disable == 0 > </self-IPI> > > > What you're suggesting, is that the time between: > > smp_store_release(prev->on_cpu, 0); > > and > > <self-IPI> > > on CPU-0 is sufficient for CPU-n to context switch to the task, enable > the event there, trigger a PMI that calls perf_event_disable_inatomic() > _again_ (this would mean irq_work_queue() failing, which we don't check) > (and schedule out again, although that's not required). > > This being virt that might actually be possible if (v)CPU-0 takes a nap > I suppose. > > Let me think about this a little more...
Does the below cure things? It's not exactly pretty, but it could just do the trick. --- diff --git a/kernel/events/core.c b/kernel/events/core.c index dfc4bab0b02b..d496e6911442 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -2009,8 +2009,8 @@ event_sched_out(struct perf_event *event, event->pmu->del(event, 0); event->oncpu = -1; - if (event->pending_disable) { - event->pending_disable = 0; + if (event->pending_disable == smp_processor_id()) { + event->pending_disable = -1; state = PERF_EVENT_STATE_OFF; } perf_event_set_state(event, state); @@ -2198,7 +2198,7 @@ EXPORT_SYMBOL_GPL(perf_event_disable); void perf_event_disable_inatomic(struct perf_event *event) { - event->pending_disable = 1; + event->pending_disable = smp_processor_id(); irq_work_queue(&event->pending); } @@ -5822,8 +5822,8 @@ static void perf_pending_event(struct irq_work *entry) * and we won't recurse 'further'. */ - if (event->pending_disable) { - event->pending_disable = 0; + if (event->pending_disable == smp_processor_id()) { + event->pending_disable = -1; perf_event_disable_local(event); } @@ -10236,6 +10236,7 @@ perf_event_alloc(struct perf_event_attr *attr, int cpu, init_waitqueue_head(&event->waitq); + event->pending_disable = -1; init_irq_work(&event->pending, perf_pending_event); mutex_init(&event->mmap_mutex);