linux-kernel - Re: [PATCH RT 1/2] sched: migrate_enable: Use per-cpu cpu_stop

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Message-ID: <20200124103822.76ab320f@gandalf.local.home>
Date:   Fri, 24 Jan 2020 10:38:22 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Scott Wood <swood@...hat.com>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH RT 1/2] sched: migrate_enable: Use per-cpu cpu_stop_work

On Fri, 24 Jan 2020 06:11:46 -0500
Scott Wood <swood@...hat.com> wrote:

> Commit e6c287b1512d ("sched: migrate_enable: Use stop_one_cpu_nowait()")
> adds a busy wait to deal with an edge case where the migrated thread
> can resume running on another CPU before the stopper has consumed
> cpu_stop_work.  However, this is done with preemption disabled and can
> potentially lead to deadlock.
> 
> While it is not guaranteed that the cpu_stop_work will be consumed before
> the migrating thread resumes and exits the stack frame, it is guaranteed
> that nothing other than the stopper can run on the old cpu between the
> migrating thread scheduling out and the cpu_stop_work being consumed.
> Thus, we can store cpu_stop_work in per-cpu data without it being
> reused too early.
> 

Makes sense to me.

Reviewed-by: Steven Rostedt (VMware) <rostedt@...dmis.org>

-- Steve