Re: [PATCH 3/7] perf: Migrate perf to use new tick dependency mask model

[Chris Metcalf <cmetcalf@ezchip.com>]

On 11/13/2015 09:22 AM, Frederic Weisbecker wrote:
> Instead of providing asynchronous checks for the nohz subsystem to verify
> perf event tick dependency, migrate perf to the new mask.
>
> Perf needs the tick for two situations:
>
> 1) Freq events. We could set the tick dependency when those are
> installed on a CPU context. But setting a global dependency on top of
> the global freq events accounting is much easier. If people want that
> to be optimized, we can still refine that on the per-CPU tick dependency
> level. This patch dooesn't change the current behaviour anyway.
>
> 2) Throttled events: this is a per-cpu dependency.
>
>
> @@ -3540,8 +3530,10 @@ static void unaccount_event(struct perf_event *event)
>   		atomic_dec(&nr_comm_events);
>   	if (event->attr.task)
>   		atomic_dec(&nr_task_events);
> -	if (event->attr.freq)
> -		atomic_dec(&nr_freq_events);
> +	if (event->attr.freq) {
> +		if (atomic_dec_and_test(&nr_freq_events))
> +			tick_nohz_clear_dep(TICK_PERF_EVENTS_BIT);
> +	}
>   	if (event->attr.context_switch) {
>   		static_key_slow_dec_deferred(&perf_sched_events);
>   		atomic_dec(&nr_switch_events);
>
> @@ -7695,7 +7687,7 @@ static void account_event(struct perf_event *event)
>   		atomic_inc(&nr_task_events);
>   	if (event->attr.freq) {
>   		if (atomic_inc_return(&nr_freq_events) == 1)
> -			tick_nohz_full_kick_all();
> +			tick_nohz_set_dep(TICK_PERF_EVENTS_BIT);
>   	}
>   	if (event->attr.context_switch) {
>   		atomic_inc(&nr_switch_events);

It would be helpful to have a comment explaining why these two
can't race with each other, e.g. this race:

[cpu 1]  atomic_dec_and_test
[cpu 2] atomic_inc_return
[cpu 2] tick_nohz_set_dep()
[cpu 1] tick_nohz_clear_dep()

Or perhaps this is a true race condition possibility?

I think we're OK for the sched cases since they're protected under
the rq lock, I think.  I'm not sure about the POSIX cpu timers.

-- 
Chris Metcalf, EZChip Semiconductor
http://www.ezchip.com