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

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Date:	Tue, 04 Jan 2011 15:47:21 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Oleg Nesterov <oleg@...hat.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>, Paul Turner <pjt@...gle.com>,
	Jens Axboe <axboe@...nel.dk>,
	Yong Zhang <yong.zhang0@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 16/17] sched: Move the second half of ttwu() to
 the remote cpu

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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