linux-kernel - Re: [PATCH v2 1/2] sched: force update of blocked load of idle cpus

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Date:   Wed, 20 Dec 2017 15:27:05 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Brendan Jackman <brendan.jackman@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Morten Rasmussen <morten.rasmussen@....com>
Subject: Re: [PATCH v2 1/2] sched: force update of blocked load of idle cpus

On 20 December 2017 at 15:09, Peter Zijlstra <peterz@...radead.org> wrote:
> On Fri, Dec 01, 2017 at 06:01:56PM +0000, Brendan Jackman wrote:
>
>> @@ -9210,7 +9256,15 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
>>                       cpu_load_update_idle(rq);
>>                       rq_unlock_irq(rq, &rf);
>>
>> -                     rebalance_domains(rq, CPU_IDLE);
>> +                     update_blocked_averages(balance_cpu);
>> +                     /*
>> +                      * This idle load balance softirq may have been
>> +                      * triggered only to update the blocked load and shares
>> +                      * of idle CPUs (which we have just done for
>> +                      * balance_cpu). In that case skip the actual balance.
>> +                      */
>> +                     if (!in_nohz_stats_kick(this_cpu))
>> +                             rebalance_domains(rq, idle);
>>               }
>>
>>               if (time_after(next_balance, rq->next_balance)) {
>
>> @@ -9336,7 +9396,12 @@ static __latent_entropy void run_rebalance_domains(struct softirq_action *h)
>>        * and abort nohz_idle_balance altogether if we pull some load.
>>        */
>>       nohz_idle_balance(this_rq, idle);
>> -     rebalance_domains(this_rq, idle);
>> +     update_blocked_averages(this_rq->cpu);
>> +     if (!in_nohz_stats_kick(this_rq->cpu))
>> +             rebalance_domains(this_rq, idle);
>> +#ifdef CONFIG_NO_HZ_COMMON
>> +     clear_bit(NOHZ_STATS_KICK, nohz_flags(this_rq->cpu));
>> +#endif
>>  }
>>
>>  /*
>
> You're doing the same thing to both (all) callsites of
> rebalance_domains(), does that not suggest doing it inside and leaving
> update_blocked_averages() where it is?

The goal of moving update_blocked_averages() outside rebalance_domains
is to not add a new parameter or use a special  cpu_idle_type value in
rebalance_domains parameters in order to abort the rebalance sequence
just after updating blocked load

>
>