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

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Date:   Thu, 17 Sep 2020 18:30:01 +0200
From:   Sebastian Siewior <bigeasy@...utronix.de>
To:     peterz@...radead.org
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 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?

> 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.

Looking at my queue, FPU related is crypto. And here we break the loops
mostly due to the construct:
	kernel_fpu_begin();
	while (bytes)
		crypto_thingy();
		skcipher_walk_done()

and skcipher_walk_done() could allocate/free/map memory. This is
independent.

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()")

Sebastian