Messages in this thread | | | Date | Wed, 25 Nov 2015 13:34:30 +0100 | From | Frederic Weisbecker <> | Subject | Re: [PATCH 3/7] perf: Migrate perf to use new tick dependency mask model |
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On Tue, Nov 24, 2015 at 11:19:33AM -0500, Chris Metcalf wrote: > 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.
Hmm, how did I miss that...
So in the case of perf, either we need locking, in which case we may want to use something like tick_nohz_add_dep() which takes care of counting. But perf would be the only user.
Another possibility is to rather set/clear the tick mask on the task level in event_sched_in/event_sched_out using ctx->nr_freq which is protected by ctx->lock. I think I should rather do that.
Concerning the others:
_ sched: we are under the rq lock, like you noticed, we are fine.
_ posix timers: we are under sighand lock, so we are fine too.
_ sched_clock_stable: that one is more obscure. It seems that set_sched_clock_stable() and clear_sched_clock_stable() can race on static keys if running concurrently, and that would concern tick mask as well.
Thanks.
> > -- > Chris Metcalf, EZChip Semiconductor > http://www.ezchip.com >
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