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

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Date:   Thu, 17 Sep 2020 16:49:37 +0200
From:   peterz@...radead.org
To:     Sebastian Siewior <bigeasy@...utronix.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        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 Thu, Sep 17, 2020 at 04:38:50PM +0200, Sebastian Siewior wrote:
> On 2020-09-17 16:24:38 [+0200], peterz@...radead.org wrote:
> > And if I'm not mistaken, the above migrate_enable() *does* require being
> > able to schedule, and our favourite piece of futex:
> > 
> > 	raw_spin_lock_irq(&q.pi_state->pi_mutex.wait_lock);
> > 	spin_unlock(q.lock_ptr);
> > 
> > is broken. Consider that spin_unlock() doing migrate_enable() with a
> > pending sched_setaffinity().
> 
> There are two instances of the above and only in the futex code and we
> have sort of duct tape for that by manually balancing the migrate
> counter so that it does not come to this.
> But yes, not having to do the manual balance is a plus.

I'm aware of the duct-tape :-) But I was under the impression that we
didn't want the duct-tape, and that there was lots of issues with the
FPU code, or was that another issue?