linux-kernel - Re: [patch 09/10] sched/core: Add migrate

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Message-ID: <20200918082843.GM1362448@hirez.programming.kicks-ass.net>
Date:   Fri, 18 Sep 2020 10:28:43 +0200
From:   peterz@...radead.org
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Sebastian Siewior <bigeasy@...utronix.de>,
        Qais Yousef <qais.yousef@....com>,
        Scott Wood <swood@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Ingo Molnar <mingo@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Vincent Donnefort <vincent.donnefort@....com>
Subject: Re: [patch 09/10] sched/core: Add migrate_disable/enable()

On Fri, Sep 18, 2020 at 09:00:03AM +0200, Thomas Gleixner wrote:

> >> +void migrate_disable(void)
> >> +{
> >> +	unsigned long flags;
> >> +
> >> +	if (!current->migration_ctrl.disable_cnt) {
> >> +		raw_spin_lock_irqsave(&current->pi_lock, flags);
> >> +		current->migration_ctrl.disable_cnt++;
> >> +		raw_spin_unlock_irqrestore(&current->pi_lock, flags);
> >> +	} else {
> >> +		current->migration_ctrl.disable_cnt++;
> >> +	}
> >> +}
> >
> > That pi_lock seems unfortunate, and it isn't obvious what the point of
> > it is.
> 
> Indeed. That obviously lacks a big fat comment.
> 
> current->migration_ctrl.disable_cnt++ is obviously a RMW operation. So
> you end up with the following:
> 
> CPU0                                            CPU1
> migrate_disable()
>    R = current->migration_ctrl.disable_cnt;
>                                                 set_cpus_allowed_ptr()
>                                                   task_rq_lock();
>                                                   if
>                                                   (!p->migration_ctrl.disable_cnt) {
>    current->migration_ctrl.disable_cnt = R + 1;
>    							stop_one_cpu();
> ---> stopper_thread()
>         BUG_ON(task->migration_ctrl.disable_cnt);
> 
> I tried to back out from that instead of BUG(), but that ended up being
> even more of a hacky trainwreck than just biting the bullet and taking
> pi_lock.

You don't need the load-store for that I think, pure timing will do.
Blergh, lemme prepare more wake-up juice and think about that.