Messages in this thread | | | From | Vincent Guittot <> | Date | Thu, 8 Feb 2018 16:05:58 +0100 | Subject | Re: [PATCH v2 1/3] sched: Stop nohz stats when decayed |
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On 8 February 2018 at 15:00, Peter Zijlstra <peterz@infradead.org> wrote: > On Tue, Feb 06, 2018 at 08:23:05PM +0100, Vincent Guittot wrote: > >> @@ -9207,13 +9231,15 @@ void nohz_balance_enter_idle(int cpu) >> if (!housekeeping_cpu(cpu, HK_FLAG_SCHED)) >> return; >> >> + rq->has_blocked_load = 1; >> + >> if (rq->nohz_tick_stopped) >> - return; > > this case is difficult... needs thinking
The use case happens when a CPU wakes up and goes back to idle before the tick fires and clears nohz_tick_stopped.
> >> + goto out; >> >> /* >> * If we're a completely isolated CPU, we don't play. >> */ >> - if (on_null_domain(cpu_rq(cpu))) >> + if (on_null_domain(rq)) >> return; >> >> rq->nohz_tick_stopped = 1; >> @@ -9222,6 +9248,13 @@ void nohz_balance_enter_idle(int cpu) >> atomic_inc(&nohz.nr_cpus); >> >> set_cpu_sd_state_idle(cpu); > > /* > * Ensures that if nohz_idle_balance() fails to observe our > * @idle_cpus_mask store, it must observe the @has_blocked > * store. > */ > smp_mb__after_atomic(); > >> + >> +out: >> + /* >> + * Each time a cpu enter idle, we assume that it has blocked load and >> + * enable the periodic update of the load of idle cpus >> + */ >> + WRITE_ONCE(nohz.has_blocked, 1); >> } > > > >> @@ -9374,6 +9407,16 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle) >> >> SCHED_WARN_ON((flags & NOHZ_KICK_MASK) == NOHZ_BALANCE_KICK); >> >> + /* >> + * We assume there will be no idle load after this update and clear >> + * the has_blocked flag. If a cpu enters idle in the mean time, it will >> + * set the has_blocked flag and trig another update of idle load. >> + * Because a cpu that becomes idle, is added to idle_cpus_mask before >> + * setting the flag, we are sure to not clear the state and not >> + * check the load of an idle cpu. >> + */ >> + WRITE_ONCE(nohz.has_blocked, 0); > > /* > * Ensures that if we miss the CPU, we must see the has_blocked > * store from nohz_balance_enter_idle(). > */ > smp_mb(); > >> for_each_cpu(balance_cpu, nohz.idle_cpus_mask) { >> if (balance_cpu == this_cpu || !idle_cpu(balance_cpu)) >> continue; > > > I _think_, but my brain isn't quite willing to turn on today. > > Without this ordering I think it would be possible to loose has_blocked > and not observe the CPU either.
I think that you are right. I also wondered if some barriers were necessary but wrongly concluded that set operation on nohz.idle_cpus_mask and WRITE_ONCE with volatile would be enough to ensure the right ordering
> >
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