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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Thu, 1 Feb 2018 18:25:56 +0100
Message-ID: <CAKfTPtAVBbM5XpbZ6iv5gyYz5cpJeJ3=RAZyA80jcLc_8A7ThA@mail.gmail.com>
Subject: Re: [RFC PATCH 2/5] sched: Add NOHZ_STATS_KICK
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Ingo Molnar <mingo@kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Brendan Jackman <brendan.jackman@arm.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Morten Rasmussen <morten.rasmussen@arm.com>
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On 1 February 2018 at 17:52, Peter Zijlstra <peterz@infradead.org> wrote:
> On Mon, Jan 15, 2018 at 09:26:09AM +0100, Vincent Guittot wrote:
>
> Would've probably been easier to read if you'd not included the revert
> of that timer patch...
>
>> @@ -9258,21 +9255,11 @@ void nohz_balance_enter_idle(int cpu)
>>       set_cpu_sd_state_idle(cpu);
>>
>>       /*
>> -      * implies a barrier such that if the stats_state update is observed
>> -      * the above updates are also visible. Pairs with stuff in
>> -      * update_sd_lb_stats() and nohz_idle_balance().
>> +      * Each time a cpu enter idle, we assume that it has blocked load and
>> +      * enable the periodic update of the load of idle cpus
>>        */
>> -     val = atomic_read(&nohz.stats_state);
>> -     do {
>> -             new = val + 2;
>> -             new |= 1;
>> -     } while (!atomic_try_cmpxchg(&nohz.stats_state, &val, new));
>> +     atomic_set(&nohz.stats_state, 1);
>>
>
>> @@ -9422,7 +9408,6 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
>>               return false;
>>       }
>>
>> -     stats_seq = atomic_read(&nohz.stats_state);
>>       /*
>>        * barrier, pairs with nohz_balance_enter_idle(), ensures ...
>>        */
>> @@ -9432,6 +9417,16 @@ 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 stats state. If a cpu enters idle in the mean time, it will
>> +      * set the stats state and trig another update of idle load.
>> +      * Because a cpu that becomes idle, is added to idle_cpus_mask before
>> +      * setting the stats state, we are sure to not clear the state and not
>> +      * check the load of an idle cpu.
>> +      */
>> +     atomic_set(&nohz.stats_state, 0);
>> +
>>       for_each_cpu(balance_cpu, nohz.idle_cpus_mask) {
>>               if (balance_cpu == this_cpu || !idle_cpu(balance_cpu))
>>                       continue;
>> @@ -9441,8 +9436,10 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
>>                * work being done for other cpus. Next load
>>                * balancing owner will pick it up.
>>                */
>> -             if (need_resched())
>> +             if (need_resched()) {
>> +                     has_blocked_load = true;
>>                       break;
>> +             }
>>
>>               rq = cpu_rq(balance_cpu);
>>
>> @@ -9477,12 +9474,12 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
>>       if (flags & NOHZ_BALANCE_KICK)
>>               rebalance_domains(this_rq, CPU_IDLE);
>>
>> -     if (has_blocked_load ||
>> -         !atomic_try_cmpxchg(&nohz.stats_state, &stats_seq, 0)) {
>> -             WRITE_ONCE(nohz.next_stats,
>> -                             now + msecs_to_jiffies(LOAD_AVG_PERIOD));
>> -             mod_timer(&nohz.timer, nohz.next_stats);
>> -     }
>> +     WRITE_ONCE(nohz.next_stats,
>> +             now + msecs_to_jiffies(LOAD_AVG_PERIOD));
>> +
>> +     /* There is still blocked load, enable periodic update */
>> +     if (has_blocked_load)
>> +             atomic_set(&nohz.stats_state, 1);
>>
>>       /*
>>        * next_balance will be updated only when there is a need.
>
> After this there is no point for stats_state to be atomic anymore. Also
> a better name.

Ok

>
> Maybe if I drop the last two patches (and you re-introduce the bits
> from: Subject: sched: Optimize nohz stats, that you do need) this all
> becomes more readable?

Yes. we can do like that