On Thu, Oct 17, 2019 at 05:27:46PM -0700, Stephane Eranian wrote:
> @@ -2153,6 +2157,7 @@ __perf_remove_from_context(struct perf_event *event,
> void *info)
> {
> unsigned long flags = (unsigned long)info;
> + int was_necessary = ctx->rotate_necessary;
>
> if (ctx->is_active & EVENT_TIME) {
> update_context_time(ctx);
> @@ -2171,6 +2176,37 @@ __perf_remove_from_context(struct perf_event *event,
> cpuctx->task_ctx = NULL;
> }
> }
> +
> + /*
> + * sanity check that event_sched_out() does not and will not
> + * change the state of ctx->rotate_necessary
> + */
> + WARN_ON(was_necessary != event->ctx->rotate_necessary);
It doesn't... why is this important to check?
> + /*
> + * if we remove an event AND we were multiplexing then, that means
> + * we had more events than we have counters for, and thus, at least,
> + * one event was in INACTIVE state. Now, that we removed an event,
> + * we need to resched to give a chance to all events to get scheduled,
> + * otherwise some may get stuck.
> + *
> + * By the time this function is called the event is usually in the OFF
> + * state.
> + * Note that this is not a duplicate of the same code in
> _perf_event_disable()
> + * because the call path are different. Some events may be simply
> disabled
It is the exact same code twice though; IIRC this C language has a
feature to help with that.
> + * others removed. There is a way to get removed and not be disabled
> first.
> + */
> + if (ctx->rotate_necessary && ctx->nr_events) {
> + int type = get_event_type(event);
> + /*
> + * In case we removed a pinned event, then we need to
> + * resched for both pinned and flexible events. The
> + * opposite is not true. A pinned event can never be
> + * inactive due to multiplexing.
> + */
> + if (type & EVENT_PINNED)
> + type |= EVENT_FLEXIBLE;
> + ctx_resched(cpuctx, cpuctx->task_ctx, type);
> + }
What you're relying on is that ->rotate_necessary implies ->is_active
and there's pending events. And if we tighten ->rotate_necessary you can
remove the && ->nr_events.
> @@ -2232,6 +2270,35 @@ static void __perf_event_disable(struct perf_event
> *event,
> event_sched_out(event, cpuctx, ctx);
>
> perf_event_set_state(event, PERF_EVENT_STATE_OFF);
> + /*
> + * sanity check that event_sched_out() does not and will not
> + * change the state of ctx->rotate_necessary
> + */
> + WARN_ON_ONCE(was_necessary != event->ctx->rotate_necessary);
> +
> + /*
> + * if we disable an event AND we were multiplexing then, that means
> + * we had more events than we have counters for, and thus, at least,
> + * one event was in INACTIVE state. Now, that we disabled an event,
> + * we need to resched to give a chance to all events to be scheduled,
> + * otherwise some may get stuck.
> + *
> + * Note that this is not a duplicate of the same code in
> + * __perf_remove_from_context()
> + * because events can be disabled without being removed.
It _IS_ a duplicate, it is the _exact_ same code twice. What you're
trying to say is that we need it in both places, but that's something
else entirely.
> + */
> + if (ctx->rotate_necessary && ctx->nr_events) {
> + int type = get_event_type(event);
> + /*
> + * In case we removed a pinned event, then we need to
> + * resched for both pinned and flexible events. The
> + * opposite is not true. A pinned event can never be
> + * inactive due to multiplexing.
> + */
> + if (type & EVENT_PINNED)
> + type |= EVENT_FLEXIBLE;
> + ctx_resched(cpuctx, cpuctx->task_ctx, type);
> + }
> }
