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Date:   Fri, 13 Dec 2019 09:14:28 +0100
From:   Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To:     Scott Wood <swood@redhat.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        linux-rt-users <linux-rt-users@vger.kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH RT] sched: migrate_enable: Busy loop until the migration
 request is completed
Message-ID: <20191213081428.mw6bqjg6m7djwhby@linutronix.de>
References: <20191212112717.2tzoqbe3xeknoyvs@linutronix.de>
 <30ab713901ef0e1f23c1ca387373788a4a73639f.camel@redhat.com>
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On 2019-12-13 00:44:22 [-0600], Scott Wood wrote:
> > @@ -8239,7 +8239,10 @@ void migrate_enable(void)
> >  		stop_one_cpu_nowait(task_cpu(p), migration_cpu_stop,
> >  				    &arg, &work);
> >  		__schedule(true);
> > -		WARN_ON_ONCE(!arg.done && !work.disabled);
> > +		if (!work.disabled) {
> > +			while (!arg.done)
> > +				cpu_relax();
> > +		}
> 
> We should enable preemption while spinning -- besides the general badness
> of spinning with it disabled, there could be deadlock scenarios if
> multiple CPUs are spinning in such a loop.  Long term maybe have a way to
> dequeue the no-longer-needed work instead of waiting.

Hmm. My plan was to use per-CPU memory and spin before the request is
enqueued if the previous isn't done yet (which should not happen™).
Then we could remove __schedule() here and rely on preempt_enable()
doing that. With that change we wouldn't care about migrate-disable
level vs preempt-disable level and could drop the hacks we have in futex
code for instance (where we have an extra migrate_disable() in advance
so they are later balanced). 

> -Scott

Sebastian