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

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Date:   Mon, 19 Apr 2021 11:56:30 +0100
From:   Vincent Donnefort <vincent.donnefort@....com>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>, 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, 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 Thu, Apr 15, 2021 at 03:32:11PM +0100, Valentin Schneider wrote:
> 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

I indeed wrote a test to verify all the rollback cases, up and down.

It seems I encounter an intermitent issue while running several iterations of
that test ... but I need more time to debug and figure-out where it is blocking.

> 
> > 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)
> >  {