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SubjectRe: [PATCH v3 1/3] sched: Stop nohz stats when decayed
On 12 February 2018 at 10:41, Peter Zijlstra <peterz@infradead.org> wrote:
> On Mon, Feb 12, 2018 at 09:07:52AM +0100, Vincent Guittot wrote:
>> @@ -9222,6 +9259,20 @@ 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);
>> }
>> #else
>> static inline void nohz_balancer_kick(struct rq *rq) { }
>
> I moved the barrier up one statement, such that its right after the
> atomic_inc(). Otherwise people will get all itchy about __after_atomic()
> semantics and we really only care about the cpumask_set_cpu() vs
> WRITE_ONCE() ordering, so it doesn't really matter _where_ that barrier
> lands in between.

ok. make sense

>
>> @@ -9374,6 +9425,22 @@ 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;
>
> Fixed that white space damage for you ;-)

Thanks

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