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

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Message-ID: <20090306154144.GA7841@redhat.com>
Date:	Fri, 6 Mar 2009 16:41:44 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 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?

Oleg.

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