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

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Date:   Thu, 17 Sep 2020 17:54:10 +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 05:13:41PM +0200, Sebastian Siewior wrote:
> On 2020-09-17 16:49:37 [+0200], peterz@...radead.org wrote:
> > 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?
> 
> Of course it would be better not to need the duct tape. 
> Also symmetrical locking is what you want but clearly futex is one of
> a kind.
> 
> I'm currently not aware of any issues in the FPU code in regard to this.
> A few weeks ago, I was looking for this kind of usage and only futex
> popped up.

I'm not sure what the problem with FPU was, I was throwing alternatives
at tglx to see what would stick, in part to (re)discover the design
constraints of this thing.

One reason for not allowing migrate_disable() to sleep was: FPU code.

Could it be it does something like:

	preempt_disable();
	spin_lock();

	spin_unlock();
	preempt_enable();

Where we'll never get preempted while migrate_disable()'d and thus never
trigger any of the sleep paths?