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

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Message-ID: <20200918082232.GL1362448@hirez.programming.kicks-ass.net>
Date:   Fri, 18 Sep 2020 10:22:32 +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 06:30:01PM +0200, Sebastian Siewior wrote:
> On 2020-09-17 17:54:10 [+0200], peterz@...radead.org wrote:
> > 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.
> 
> was this recent or distant in the time line?

The past few weeks :-) Thomas and me have been bickering about this
stuff on IRC on and off.

> > 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?
> 
> I try to get rid of something like that. This doesn't work either way
> because the spin_lock() may block which it can't with disabled
> preemption.

Yeah, that obviously should have been migrate_disable/enable instead of
spin_lock/unlock :/

> Ah. We used to have migrate_disable() in pagefault_disable(). The x86
> FPU code does
> 	preempt_disable();
> 	…
> 	pagefault_disable();
> 
> but that migrate_disable() was moved from pagefault_disable() to
> kmap_atomic(). We shouldn't have
> 	preempt_disable(); || local_irq_disable();
> 	kmap_atomic();
> 
> on RT. I've been running around removing those. See
>    a10dcebacdb0c ("fs/ntfs/aops.c: don't disable interrupts during kmap_atomic()")
>    ce1e518190ea7 ("ide: don't disable interrupts during kmap_atomic()")
>    f3a1075e5fc34 ("block: don't disable interrupts during kmap_atomic()")

Hmm, okay.