linux-kernel - Re: On migrate_disable() and latencies

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Date:	Thu, 28 Jul 2011 09:01:23 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Yong Zhang <yong.zhang0@...il.com>
cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-rt-users <linux-rt-users@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>, Carsten Emde <ce@...g.ch>,
	Clark Williams <williams@...hat.com>,
	Kumar Gala <galak@...e.crashing.org>,
	Ralf Baechle <ralf@...ux-mips.org>,
	rostedt <rostedt@...dmis.org>,
	Nicholas Mc Guire <der.herr@...r.at>
Subject: Re: On migrate_disable() and latencies

On Thu, 28 Jul 2011, Yong Zhang wrote:

> On Wed, Jul 27, 2011 at 11:30:08AM -0700, Paul E. McKenney wrote:
> > o	Tasks awakening outside of migrate-disable regions will pick
> > 	the CPU running the lowest-priority task, whether or not this
> > 	task is in migrate-disable state.  (At least I don't see
> > 	anything in 3.0-rt3 that looks like a scheduling decision
> > 	based on ->migrate_disable, perhaps due to blindness.)
> 
> I'm also confused here, seems we just disable migration for RT task.
> migrate_disable()
> {
> 	...
> 		if (p->sched_class->set_cpus_allowed)
> 			p->sched_class->set_cpus_allowed(p, mask);
> 		p->rt.nr_cpus_allowed =	cpumask_weight(mask);
> 	...
> }
> 
> Shouldn't we also forbid migration on !RT task?

We do. Just RT is the only sched class which has a set_cpus_allowed()
callback implemented and want's an update to its rt.nr_cpus_allowed
field.

       if (!p->migrate_disable) {
                if (p->sched_class && p->sched_class->set_cpus_allowed)
                        p->sched_class->set_cpus_allowed(p, new_mask);
                p->rt.nr_cpus_allowed = cpumask_weight(new_mask);
        }

The general part is here:

       cpumask_copy(&p->cpus_allowed, new_mask);

And tsk_cpus_allowed() does:
{
        if (p->migrate_disable)
                return cpumask_of(task_cpu(p));

        return &p->cpus_allowed;
}

which is the relevant information to keep any task independent of it's
sched class pinned.

Thanks,

	tglx
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