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SubjectRe: [PATCH v2 1/3] sched: Stop nohz stats when decayed
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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