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

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Message-Id: <200708022313.32944.rjw@sisk.pl>
Date:	Thu, 2 Aug 2007 23:13:32 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Oleg Nesterov <oleg@...sign.ru>
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 Thursday, 2 August 2007 20:40, Oleg Nesterov wrote:
> 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,

I'm not sure what you mean.

Do you mean that if we are TASK_UNINTERRUPTIBLE, then the first wake up
should remove us from the queue?

> 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.

Yes.

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

Hmm, I think that we shouldn't wait if that's not necessary.

Greetings,
Rafael


-- 
"Premature optimization is the root of all evil." - Donald Knuth
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