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Message-ID: <1294068061.2016.84.camel@laptop>
Date: Mon, 03 Jan 2011 16:21:01 +0100
From: Peter Zijlstra <a.p.zijlstra@...llo.nl>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Yong Zhang <yong.zhang0@...il.com>,
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>, 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 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.
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