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

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Date:	Fri, 3 Aug 2007 02:05:41 +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:
>
> On Thursday, 2 August 2007 23:23, Oleg Nesterov wrote:
> > On 08/02, Rafael J. Wysocki wrote:
> > >
> > > 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?
> > 
> > No, not because we are TASK_UNINTERRUPTIBLE, but yes, first wake up will
> > remove us because DEFINE_WAIT() uses autoremove_wake_function().
> 
> Yes, it does, but the prepare_to_wait() version would only cause current to
> become TASK_UNINTERRUPTIBLE before it sends freezing requests, the other
> differences don't seem to matter.  I'm trying to understand why it would change
> the behavior in the way you have described.

Ugh, this is not the first time when I didn't read your patch carefully, sorry!

I missed that your patch already uses DEFINE_WAIT(), and I was confused by the
add_wait_queue() which is usually used along with DECLARE_WAITQUEUE().

Oleg.

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