linux-kernel - Re: [PATCH 2/2] sched/hotplug: Ensure only per-cpu kthreads run during hotplug

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Message-ID: <20200911122922.GJ1362448@hirez.programming.kicks-ass.net>
Date:   Fri, 11 Sep 2020 14:29:22 +0200
From:   peterz@...radead.org
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     mingo@...nel.org, vincent.guittot@...aro.org, tglx@...utronix.de,
        linux-kernel@...r.kernel.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bristot@...hat.com, swood@...hat.com,
        Vincent Donnefort <Vincent.Donnefort@....com>
Subject: Re: [PATCH 2/2] sched/hotplug: Ensure only per-cpu kthreads run
 during hotplug

On Fri, Sep 11, 2020 at 01:17:45PM +0100, Valentin Schneider wrote:

> > @@ -6968,6 +7064,8 @@ int sched_cpu_deactivate(unsigned int cp
> >        */
> >       synchronize_rcu();
> >
> > +	balance_push_set(cpu, true);
> > +
> 
> IIUC this is going to make every subsequent finish_lock_switch()
> migrate the switched-to task if it isn't a pcpu kthread. So this is going
> to lead to a dance of
> 
> switch_to(<task0>) -> switch_to(<stopper>) -> switch_to(<task1>) -> switch_to(<stopper>) ...
> 
> Wouldn't it be better to batch all those in a migrate_tasks() sibling that
> skips pcpu kthreads?

That's 'difficult', this is hotplug, performance is not a consideration.

Basically we don't have an iterator for the runqueues, so finding these
tasks is hard.