linux-kernel - Re: [PATCH v3 1/3] sched: Stop nohz stats when decayed

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Date:   Mon, 12 Feb 2018 12:02:49 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Morten Rasmussen <morten.rasmussen@...s.arm.com>,
        Brendan Jackman <brendan.jackman@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>
Subject: Re: [PATCH v3 1/3] sched: Stop nohz stats when decayed

On 12 February 2018 at 11:45, Peter Zijlstra <peterz@...radead.org> wrote:
> On Mon, Feb 12, 2018 at 09:07:52AM +0100, Vincent Guittot wrote:
>> @@ -9383,11 +9450,16 @@ 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())
>> -                     break;
>> +             if (need_resched()) {
>> +                     has_blocked_load = true;
>> +                     goto abort;
>> +             }
>>
>>               rq = cpu_rq(balance_cpu);
>>
>> +             update_blocked_averages(rq->cpu);
>
> Does that want to be update_nohz_stats() ?

I was about to say yesy but the test if (!time_after(jiffies,
rq->last_blocked_load_update_tick)) worry me a bit because it can skip
the update and the chance to clear nohz.has_blocked.
As an example the tick is 10ms long on arm 32bits so we can easily
reach the 32ms decay within the current tick even if we have already
updated it

>
>> +             has_blocked_load |= rq->has_blocked_load;
>> +
>>               /*
>>                * If time for next balance is due,
>>                * do the balance.