Re: [RFC][PATCH 16/17] sched: Move the second half of ttwu() tothe remote cpu

From: Peter Zijlstra
Date: Tue Jan 04 2011 - 09:47:35 EST


On Tue, 2011-01-04 at 15:28 +0100, Oleg Nesterov wrote:
> On 12/24, Peter Zijlstra wrote:
> >
> > +static void
> > +ttwu_do_activate(struct rq *rq, struct task_struct *p, int wake_flags)
> > +{
> > +#ifdef CONFIG_SMP
> > + if (task_cpu(p) != cpu_of(rq))
> > + set_task_cpu(p, cpu_of(rq));
> > +#endif
>
> This looks a bit suspicious.
>
> If this is called by sched_ttwu_pending() we are holding rq->lock,
> not task_rq_lock(). It seems, we can race with, say, migration
> thread running on task_cpu().

I don't think so, nobody should be migrating a TASK_WAKING task.

> OK, p->state = TASK_WAKING protects us against, say, set_cpus_allowed_ptr()
> which does task_rq_lock(p) and thus checks task_is_waking().
>
> But, at the same time,
>
> > +#ifdef CONFIG_SMP
> > +static void ttwu_queue_remote(struct task_struct *p, int cpu)
> > +{
> > + struct task_struct *next = NULL;
> > + struct rq *rq = cpu_rq(cpu);
> > +
> > + for (;;) {
> > + struct task_struct *old = next;
> > +
> > + p->wake_entry = next;
> > + next = cmpxchg(&rq->wake_list, old, p);
> > + if (next == old)
> > + break;
> > + }
> > +
> > + if (!next)
> > + smp_send_reschedule(cpu);
>
> what if that cpu does set_cpus_allowed_ptr(p) ?
>
> It spins with irq disabled. Once the caller, try_to_wake_up(),
> drops ->pi_lock it will wait for !task_is_waking() forever, no?

Ah, it appears I've already fixed that, let me clean up my current
series and repost.
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