linux-kernel - Re: [PATCH 3/3] sched: Use cpu_dying() to fix balance

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Message-ID: <87a6pzmxec.mognet@arm.com>
Date:   Thu, 15 Apr 2021 15:32:11 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     tglx@...utronix.de, mingo@...nel.org, bigeasy@...utronix.de,
        swood@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, vincent.donnefort@....com, qais.yousef@....com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] sched: Use cpu_dying() to fix balance_push vs hotplug-rollback

On 15/04/21 10:59, Peter Zijlstra wrote:
> Can't make sense of what I did.. I've removed that hunk. Patch now looks
> like this.
>

Small nit below, but regardless feel free to apply to the whole lot:
Reviewed-by: Valentin Schneider <valentin.schneider@....com>

@VincentD, ISTR you had tested the initial version of this with your fancy
shmancy hotplug rollback stresser. Feel like doing this

> So instead, make sure balance_push is enabled between
> sched_cpu_deactivate() and sched_cpu_activate() (eg. when
> !cpu_active()), and gate it's utility with cpu_dying().

I'd word that "is enabled below sched_cpu_activate()", since
sched_cpu_deactivate() is now out of the picture.

[...]
> @@ -7639,6 +7639,9 @@ static DEFINE_PER_CPU(struct cpu_stop_wo
>
>  /*
>   * Ensure we only run per-cpu kthreads once the CPU goes !active.
> + *
> + * This is active/set between sched_cpu_deactivate() / sched_cpu_activate().

Ditto

> + * But only effective when the hotplug motion is down.
>   */
>  static void balance_push(struct rq *rq)
>  {