linux-kernel - Re: [PATCH -mm 2/3] Freezer: Use wait queue instead of busy looping (updated)

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Date:	Thu, 2 Aug 2007 22:40:02 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Nigel Cunningham <nigel@...el.suspend2.net>,
	Pavel Machek <pavel@....cz>,
	pm list <linux-pm@...ts.linux-foundation.org>
Subject: Re: [PATCH -mm 2/3] Freezer: Use wait queue instead of busy looping (updated)

On 08/02, Rafael J. Wysocki wrote:
>
> @@ -171,6 +186,10 @@ static int try_to_freeze_tasks(int freez
>  
>  	end_time = jiffies + TIMEOUT;
>  	do {
> +		DEFINE_WAIT(wait);
> +
> +		add_wait_queue(&refrigerator_waitq, &wait);

Hmm. In that case I'd sugest to use prepare_to_wait(). This means that
multiple wakeups from refrigerator() won't do unnecessary work, and

> +
>  		todo = 0;
>  		read_lock(&tasklist_lock);
>  		do_each_thread(g, p) {
> @@ -189,7 +208,12 @@ static int try_to_freeze_tasks(int freez
>  				todo++;
>  		} while_each_thread(g, p);
>  		read_unlock(&tasklist_lock);
> -		yield();			/* Yield is okay here */
> +
> +		set_current_state(TASK_UNINTERRUPTIBLE);
> +		if (todo && !list_empty_careful(&wait.task_list))
> +			schedule_timeout(WAIT_TIME);

we don't need to check list_empty_careful() before schedule, prepare_to_wait()
sets TASK_UNINTERRUPTIBLE under wait_queue_head_t->lock.

Still, I personally agree with Pavel. Perhaps it is better to just replace
yield() with schedule_timeout(a_bit).

Oleg.

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