linux-kernel - Re: [RFC][PATCH 5/5] sched: Reduce ttwu rq->lock contention

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Date:	Fri, 17 Dec 2010 18:43:01 +0100
From:	Peter Zijlstra <peterz@...radead.org>
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>, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 5/5] sched: Reduce ttwu rq->lock contention

On Fri, 2010-12-17 at 17:54 +0100, Oleg Nesterov wrote:
> On 12/16, Peter Zijlstra wrote:
> >
> > It does the state and on_rq checks first, if we find on_rq,
> 
> The problem is, somehow we should check both on_rq and state
> at the same time,
> 
> > +try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
> >  {
> > -	int cpu, orig_cpu, this_cpu, success = 0;
> > +	int cpu, load, ret = 0;
> >  	unsigned long flags;
> > -	unsigned long en_flags = ENQUEUE_WAKEUP;
> > -	struct rq *rq;
> >
> > -	this_cpu = get_cpu();
> > +	smp_mb();
> 
> Yes, we need the full mb(). without subsequent spin_lock(), wmb()
> can't act as a smp_store_load_barrier() (which we don't have).
> 
> > +	if (p->se.on_rq && ttwu_force(p, state, wake_flags))
> > +		return 1;
> 
> 	----- WINDOW -----
> 
> > +	for (;;) {
> > +		unsigned int task_state = p->state;
> > +
> > +		if (!(task_state & state))
> > +			goto out;
> > +
> > +		load = task_contributes_to_load(p);
> > +
> > +		if (cmpxchg(&p->state, task_state, TASK_WAKING) == task_state)
> > +			break;
> 
> Suppose that we have a task T sleeping in TASK_INTERRUPTIBLE state,
> and this cpu does try_to_wake_up(TASK_INTERRUPTIBLE). on_rq == false.
> try_to_wake_up() starts the "for (;;)" loop.
> 
> However, in the WINDOW above, it is possible that somebody else wakes
> it up, and then this task changes its state to TASK_INTERRUPTIBLE again.
> 
> Then we set ->state = TASK_WAKING, but this (still running) T restores
> TASK_RUNNING after us.

See, there's a reason I CC'ed you ;-)

Hrmph, so is it only about serializing concurrent wakeups? If so, we
could possibly hold p->pi_lock over the wakeup.
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