Messages in this thread | | | Date | Thu, 8 Feb 2018 15:30:31 +0000 | From | Will Deacon <> | Subject | Re: [PATCH v2 1/3] sched: Stop nohz stats when decayed |
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On Thu, Feb 08, 2018 at 03:00:05PM +0100, Peter Zijlstra wrote: > On Tue, Feb 06, 2018 at 08:23:05PM +0100, Vincent Guittot wrote: > > @@ -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 had a quick look at this, and I think you're right. This looks very much like an 'R'-shaped test, which means it's smp_mb() all round otherwise Power will go wrong. That also means the smp_mb__after_atomic() in nohz_balance_enter_idle *cannot* be an smp_wmb(), so you might want a comment stating that explicitly.
On arm64, release/acquire would work, but that's basically not the case for anybody else including x86, so let's not go there.
Will
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