linux-kernel - Re: [PATCH] sched: Simplify cpu-hot-unplug task migration

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Date:	Thu, 18 Nov 2010 15:24:09 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Raistlin <raistlin@...ux.it>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Chris Friesen <cfriesen@...tel.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Darren Hart <darren@...art.com>,
	Johan Eker <johan.eker@...csson.com>,
	"p.faure" <p.faure@...tech.ch>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Claudio Scordino <claudio@...dence.eu.com>,
	michael trimarchi <trimarchi@...is.sssup.it>,
	Fabio Checconi <fabio@...dalf.sssup.it>,
	Tommaso Cucinotta <cucinotta@...up.it>,
	Juri Lelli <juri.lelli@...il.com>,
	Nicola Manica <nicola.manica@...i.unitn.it>,
	Luca Abeni <luca.abeni@...tn.it>,
	Dhaval Giani <dhaval@...is.sssup.it>,
	Harald Gustafsson <hgu1972@...il.com>,
	paulmck <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH] sched: Simplify cpu-hot-unplug task migration

On Thu, 2010-11-18 at 15:05 +0100, Oleg Nesterov wrote:
> > -     /* Wait for it to sleep (leaving idle task). */
> > -     while (!idle_cpu(cpu))
> > -             yield();
> > +     /*
> > +      * The migration_call() CPU_DYING callback will have removed all
> > +      * runnable tasks from the cpu, there's only the idle task left now
> > +      * that the migration thread is done doing the stop_machine thing.
> > +      */
> > +     BUG_ON(!idle_cpu(cpu));
> 
> I am not sure.
> 
> Yes, we know for sure rhat the only runnable task is rq->idle.
> But only after migration thread calls schedule() and switches to the
> idle thread.
> 
> However, I see nothing which can guarantee this. Migration thread
> running on the dead cpu wakes up the caller of stop_cpus() before
> it calls schedule(), _cpu_down() can check rq->curr before it was
> changed.
> 
> No?
> 
> 
> 
> Hmm. In fact, I think it is possible that cpu_stopper_thread() can
> have more cpu_stop_work's queued when __stop_machine() returns.
> This has nothing to do with this patch, but I think it makes sense
> to clear stopper->enabled at CPU_DYING stage as well (of course,
> this needs a separate patch). 

Hmm, I think you're right, although I haven't hit that case during
testing.

There is no firm guarantee the dying cpu actually got to running the
idle thread (there's a guarantee it will at some point), so we ought to
maintain that wait-loop, possibly using cpu_relax(), I don't see the
point in calling yield() here.
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