linux-kernel - Re: [RFC][PATCH]: wait: don't resched in prepare_to

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Date:	Fri, 06 Mar 2009 16:47:04 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	Nick Piggin <npiggin@...e.de>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH]: wait: don't resched in prepare_to_wait()

On Fri, 2009-03-06 at 16:41 +0100, Oleg Nesterov wrote:
> On 03/06, Peter Zijlstra wrote:
> >
> > After prepare to wait, we either call schedule() or find !cond in which
> > case we'll call finish_wait() which contains another preemption check.
> >
> > @@ -73,7 +73,9 @@ prepare_to_wait(wait_queue_head_t *q, wait_queue_t *wait, int state)
> >  	if (list_empty(&wait->task_list))
> >  		__add_wait_queue(q, wait);
> >  	set_current_state(state);
> > +	preempt_disable();
> >  	spin_unlock_irqrestore(&q->lock, flags);
> > +	preempt_enable_no_resched();
> 
> But this only helps if the task gets TIF_NEED_RESCHED while it was in
> spin_lock_irqsave() section, right? This window should be very small
> unless we have the contention.
> 
> Or do you have something else in mind?

No, that was exactly it, we then get preempted only to go to sleep right
afterwards, which would cause a schedule anyway.

So its avoiding a schedule.

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