linux-kernel - Re: [RFC][PATCH 08/17] sched: Drop the rq argument to sched_class::select_task

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Date:	Mon, 03 Jan 2011 12:16:49 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Yong Zhang <yong.zhang0@...il.com>
Cc:	Chris Mason <chris.mason@...cle.com>,
	Frank Rowand <frank.rowand@...sony.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mike Galbraith <efault@....de>,
	Oleg Nesterov <oleg@...hat.com>, Paul Turner <pjt@...gle.com>,
	Jens Axboe <axboe@...nel.dk>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 08/17] sched: Drop the rq argument to
 sched_class::select_task_rq()

On Wed, 2010-12-29 at 22:31 +0800, Yong Zhang wrote:
> On Fri, Dec 24, 2010 at 01:23:46PM +0100, Peter Zijlstra wrote:
> > In preparation of calling select_task_rq() without rq->lock held, drop
> > the dependency on the rq argument.
> > 
> > Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> > ---
> > @@ -3416,27 +3409,22 @@ void sched_exec(void)
> >  {
> >  	struct task_struct *p = current;
> >  	unsigned long flags;
> > -	struct rq *rq;
> >  	int dest_cpu;
> >  
> > -	rq = task_rq_lock(p, &flags);
> > -	dest_cpu = p->sched_class->select_task_rq(rq, p, SD_BALANCE_EXEC, 0);
> > +	raw_spin_lock_irqsave(&p->pi_lock, flags);
> 
> Seems this should go to patch 07/17 ;)

Ah, the reason its here is that this patch removes the rq argument and
thus we no longer need rq->lock. So this part relies on the property
introduced by patch 7.

> > +	dest_cpu = p->sched_class->select_task_rq(p, SD_BALANCE_EXEC, 0);
> >  	if (dest_cpu == smp_processor_id())
> >  		goto unlock;
> >  
> > -	/*
> > -	 * select_task_rq() can race against ->cpus_allowed
> > -	 */
> > -	if (cpumask_test_cpu(dest_cpu, &p->cpus_allowed) &&
> > -	    likely(cpu_active(dest_cpu)) && migrate_task(p, rq)) {
> > +	if (likely(cpu_active(dest_cpu)) && need_migrate_task(p)) {
> 
> If we drop rq_lock, need_migrate_task() maybe return true but
> p is already running on other cpu. Thus we do a wrong migration
> call.

Yeah, too bad.. ;-) exec load balancing is more an optimistic thing
anyway, if it got rebalanced under out feet we don't care.
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