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 15:23:01 +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 15:03, Peter Zijlstra <peterz@...radead.org> wrote:
> On Fri, Dec 01, 2017 at 06:01:56PM +0000, Brendan Jackman wrote:
>> @@ -8955,8 +8964,20 @@ static void nohz_balancer_kick(void)
>>       if (ilb_cpu >= nr_cpu_ids)
>>               return;
>>
>> -     if (test_and_set_bit(NOHZ_BALANCE_KICK, nohz_flags(ilb_cpu)))
>> +     if (test_and_set_bit(NOHZ_BALANCE_KICK, nohz_flags(ilb_cpu))) {
>> +             if (!only_update) {
>> +                     /*
>> +                      * There's a pending/ongoing nohz kick/balance. If it's
>> +                      * just for stats, convert it to a proper load balance.
>> +                      */
>> +                     clear_bit(NOHZ_STATS_KICK, nohz_flags(ilb_cpu));
>> +             }
>>               return;
>> +     }
>> +
>> +     if (only_update)
>> +             set_bit(NOHZ_STATS_KICK, nohz_flags(ilb_cpu));
>> +
>>       /*
>>        * Use smp_send_reschedule() instead of resched_cpu().
>>        * This way we generate a sched IPI on the target cpu which
>
> This looks racy.. if its not we don't need atomic ops, if it is but is
> still fine it needs a comment.

NOHZ_STATS_KICK modification is protected by test_and_set_bit(NOHZ_BALANCE_KICK.
You're right that we don't need atomics ops and __set_bit() is enough