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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
