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From: peterz@infradead.org
To: Sebastian Siewior <bigeasy@linutronix.de>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	LKML <linux-kernel@vger.kernel.org>,
	Qais Yousef <qais.yousef@arm.com>, Scott Wood <swood@redhat.com>,
	Valentin Schneider <valentin.schneider@arm.com>,
	Ingo Molnar <mingo@kernel.org>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Vincent Donnefort <vincent.donnefort@arm.com>
Subject: Re: [patch 09/10] sched/core: Add migrate_disable/enable()
Date: Fri, 18 Sep 2020 10:22:32 +0200	[thread overview]
Message-ID: <20200918082232.GL1362448@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20200917163001.5ksl5vjwi35ozzsv@linutronix.de>

On Thu, Sep 17, 2020 at 06:30:01PM +0200, Sebastian Siewior wrote:
> On 2020-09-17 17:54:10 [+0200], peterz@infradead.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.

  reply	other threads:[~2020-09-18  8:23 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-17  9:42 [patch 00/10] sched: Migrate disable support for RT Thomas Gleixner
2020-09-17  9:42 ` [patch 01/10] sched: Fix balance_callback() Thomas Gleixner
2020-09-19 22:48   ` kernel test robot
2020-09-17  9:42 ` [patch 02/10] sched/hotplug: Ensure only per-cpu kthreads run during hotplug Thomas Gleixner
2020-09-17  9:42 ` [patch 03/10] sched/core: Wait for tasks being pushed away on hotplug Thomas Gleixner
2020-09-17  9:42 ` [patch 04/10] sched/hotplug: Consolidate task migration on CPU unplug Thomas Gleixner
2020-09-21  8:33   ` [sched/hotplug] fc8cae04ec: kernel_BUG_at_kernel/sched/core.c kernel test robot
2020-09-17  9:42 ` [patch 05/10] sched/core: Split __set_cpus_allowed_ptr() Thomas Gleixner
2020-09-17  9:42 ` [patch 06/10] sched: Add task components for migration control Thomas Gleixner
2020-09-17  9:42 ` [patch 07/10] sched/core: Add mechanism to wait for affinity setting to complete Thomas Gleixner
2020-09-17  9:42 ` [patch 08/10] sched: Add update_migratory() callback to scheduler classes Thomas Gleixner
2020-09-17  9:42 ` [patch 09/10] sched/core: Add migrate_disable/enable() Thomas Gleixner
2020-09-17 14:24   ` peterz
2020-09-17 14:38     ` Sebastian Siewior
2020-09-17 14:49       ` peterz
2020-09-17 15:13         ` Sebastian Siewior
2020-09-17 15:54           ` peterz
2020-09-17 16:30             ` Sebastian Siewior
2020-09-18  8:22               ` peterz [this message]
2020-09-18  8:48                 ` Sebastian Siewior
2020-09-18  7:00     ` Thomas Gleixner
2020-09-18  8:28       ` peterz
2020-09-17  9:42 ` [patch 10/10] sched/core: Make migrate disable and CPU hotplug cooperative Thomas Gleixner

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