linux-kernel - Re: [RFC PATCH 5/6] sched/fair: trigger the update of blocked load on newly idle cpu

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Message-ID: <jhjpn19tap8.mognet@arm.com>
Date:   Tue, 09 Feb 2021 17:25:39 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Qais Yousef <qais.yousef@....com>
Subject: Re: [RFC PATCH 5/6] sched/fair: trigger the update of blocked load on newly idle cpu

On 09/02/21 14:57, Vincent Guittot wrote:
> On Tue, 9 Feb 2021 at 14:09, Valentin Schneider
> <valentin.schneider@....com> wrote:
>> On 05/02/21 12:48, Vincent Guittot wrote:
>> > @@ -261,6 +261,12 @@ static void cpuidle_idle_call(void)
>> >  static void do_idle(void)
>> >  {
>> >       int cpu = smp_processor_id();
>> > +
>> > +     /*
>> > +      * Check if we need to update some blocked load
>> > +      */
>> > +     nohz_run_idle_balance(cpu);
>> > +
>>
>> What do we gain from doing this here vs having a stats update in
>> newidle_balance()?
>
> As mentioned by Joel, newidle_balance is called in the schedule
> context with preempt and irq off  which prevent any local activity
> like irq/timer. Whereas in this new place, we have the same condition
> as during ILB with only preemptoff and _nohz_idle_balance() regularly
> checks if it has to abort because something has been scheduled on the
> cpu.
>

Gotcha, that's already hinted at in the cover letter. Could you point this
out in the changelog? Other than that, I don't see anything wrong with this
approach.