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From:   Valentin Schneider <valentin.schneider@arm.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     tglx@linutronix.de, mingo@kernel.org, bigeasy@linutronix.de,
        swood@redhat.com, juri.lelli@redhat.com,
        vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
        rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
        bristot@redhat.com, vincent.donnefort@arm.com, qais.yousef@arm.com,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/3] sched: Use cpu_dying() to fix balance_push vs hotplug-rollback
In-Reply-To: <YHgAYef83VQhKdC2@hirez.programming.kicks-ass.net>
References: <20210310145258.899619710@infradead.org> <20210310150109.259726371@infradead.org> <871rclu3jz.mognet@e113632-lin.i-did-not-set--mail-host-address--so-tickle-me> <YHQ3Iy7QfL+0UoM0@hirez.programming.kicks-ass.net> <87r1jfmn8d.mognet@arm.com> <YHU/a9HvGLYpOLKZ@hirez.programming.kicks-ass.net> <YHgAYef83VQhKdC2@hirez.programming.kicks-ass.net>
Date:   Thu, 15 Apr 2021 15:32:11 +0100
Message-ID: <87a6pzmxec.mognet@arm.com>
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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@arm.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)
>  {