linux-kernel - Re: [PATCH v4 3/3] sched: optimize migration by forcing rmb() and updating to be called once

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Message-ID: <20151110010905.GD4164@byungchulpark-X58A-UD3R>
Date:	Tue, 10 Nov 2015 10:09:05 +0900
From:	Byungchul Park <byungchul.park@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org,
	yuyang.du@...el.com, pjt@...gle.com, efault@....de,
	tglx@...utronix.de
Subject: Re: [PATCH v4 3/3] sched: optimize migration by forcing rmb() and
 updating to be called once

On Mon, Nov 09, 2015 at 02:29:14PM +0100, Peter Zijlstra wrote:
> On Sat, Oct 24, 2015 at 01:16:21AM +0900, byungchul.park@....com wrote:
> > +++ b/kernel/sched/core.c
> > @@ -1264,6 +1264,8 @@ EXPORT_SYMBOL_GPL(set_cpus_allowed_ptr);
> >  
> >  void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
> >  {
> > +	unsigned int prev_cpu = task_cpu(p);
> > +
> >  #ifdef CONFIG_SCHED_DEBUG
> >  	/*
> >  	 * We should never call set_task_cpu() on a blocked task,
> > @@ -1289,15 +1291,14 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
> >  #endif
> >  
> >  	trace_sched_migrate_task(p, new_cpu);
> > +	__set_task_cpu(p, new_cpu);
> >  
> > -	if (task_cpu(p) != new_cpu) {
> > +	if (prev_cpu != new_cpu) {
> >  		if (p->sched_class->migrate_task_rq)
> > -			p->sched_class->migrate_task_rq(p, new_cpu);
> > +			p->sched_class->migrate_task_rq(p, prev_cpu);
> >  		p->se.nr_migrations++;
> >  		perf_event_task_migrate(p);
> >  	}
> > -
> > -	__set_task_cpu(p, new_cpu);
> >  }
> 
> I don't think this is safe, see the comment in __set_task_cpu(). We want
> that to be last.

I am sorry but I don't understand what you said. I checked the comment in 
__set_task_cpu().

	/*
	 * After ->cpu is set up to a new value, task_rq_lock(p, ...) can be
	 * successfuly executed on another CPU. We must ensure that updates of
	 * per-task data have been completed by this moment.
	 */

Of course, ->cpu should be set up to a new value for task_rq_lock() to be
executed successfully on another CPU. Is this the case? Is there something
i missed? I think it would be ok if task->pi_lock can work correctly within
"if" statement in set_task_cpu(). Is there problem to do that?

> 
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