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From: Sebastian Siewior <bigeasy@linutronix.de>
To: peterz@infradead.org
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 10:48:35 +0200	[thread overview]
Message-ID: <20200918084835.onjipzofxac5epe2@linutronix.de> (raw)
In-Reply-To: <20200918082232.GL1362448@hirez.programming.kicks-ass.net>

On 2020-09-18 10:22:32 [+0200], peterz@infradead.org wrote:
> > > 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. Me stupid. fpregs_lock() does

	preempt_disable();
	local_bh_disable();

which is more or less the "official" pattern. As of today
local_bh_disable() does migrate_disable() / spin_lock(). Not sure what
we end up with for local_bh_disable() in the end.
We used not have a BLK here on RT but ended up in all kind of locking
problems because vanilla treats local_bh_disable() as a BLK and uses it
for locking.
Today we have a per-CPU spinlock_t in local_bh_disable() to emulate the
BKL. But this pattern above isn't working due to the atomic part…

Sebastian

  reply	other threads:[~2020-09-18  8:48 UTC|newest]

Thread overview: 21+ 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
2020-09-17  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
2020-09-17  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
2020-09-18  8:48                 ` Sebastian Siewior [this message]
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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