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Subject: Re: [RFC][PATCH 08/17] sched: Drop the rq argument to
 sched_class::select_task_rq()
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Yong Zhang <yong.zhang0@gmail.com>, Chris Mason <chris.mason@oracle.com>,
        Frank Rowand <frank.rowand@am.sony.com>, Ingo Molnar <mingo@elte.hu>,
        Thomas Gleixner <tglx@linutronix.de>, Mike Galbraith <efault@gmx.de>,
        Paul Turner <pjt@google.com>, Jens Axboe <axboe@kernel.dk>,
        Steven Rostedt <rostedt@goodmis.org>, linux-kernel@vger.kernel.org
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Date: Mon, 03 Jan 2011 16:21:01 +0100
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On Mon, 2011-01-03 at 15:59 +0100, Oleg Nesterov wrote:
> On 01/03, Peter Zijlstra wrote:
> >
> > On Wed, 2010-12-29 at 22:31 +0800, Yong Zhang wrote:
> > > > -	/*
> > > > -	 * 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.
> 
> I don't understand this need_migrate_task() at all (with or without
> the patch). This task is current/running, it should always return T.

This is true for the sched_exec() case, yes.

> I guess, migrate_task() was needed before to initialize migration_req.

But afaict you can call set_cpus_allowed_ptr() on !self.