linux-kernel - Re: [PATCH 01/19] freezer: don't get over-anxious while waiting

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Date:	Thu, 1 Oct 2009 23:04:00 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Pavel Machek <pavel@....cz>, Tejun Heo <tj@...nel.org>
Cc:	jeff@...zik.org, mingo@...e.hu, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, jens.axboe@...cle.com,
	rusty@...tcorp.com.au, cl@...ux-foundation.org,
	dhowells@...hat.com, arjan@...ux.intel.com,
	pm list <linux-pm@...ts.linux-foundation.org>,
	Oren Laadan <orenl@...rato.com>
Subject: Re: [PATCH 01/19] freezer: don't get over-anxious while waiting

On Thursday 01 October 2009, Pavel Machek wrote:
> 
> > Freezing isn't exactly the most latency sensitive operation and
> > there's no reason to burn cpu cycles and power waiting for it to
> > complete.  msleep(10) instead of yield().  This should improve
> > reliability of emergency hibernation.
> 
> i don't see how it improves reliability, but its probably ok.
> 
> Well... for hibernation anyway. I can imagine cgroup users where
> freeze is so fast that this matters. rjw cc-ed.		pavel

Thanks.  I'd like to hear from the cgroup freezer people about that.

> > Signed-off-by: Tejun Heo <tj@...nel.org>
> > ---
> >  kernel/power/process.c |   13 +++++++++----
> >  1 files changed, 9 insertions(+), 4 deletions(-)
> > 
> > diff --git a/kernel/power/process.c b/kernel/power/process.c
> > index cc2e553..9d26a0a 100644
> > --- a/kernel/power/process.c
> > +++ b/kernel/power/process.c
> > @@ -41,7 +41,7 @@ static int try_to_freeze_tasks(bool sig_only)
> >  	do_gettimeofday(&start);
> >  
> >  	end_time = jiffies + TIMEOUT;
> > -	do {
> > +	while (true) {
> >  		todo = 0;
> >  		read_lock(&tasklist_lock);
> >  		do_each_thread(g, p) {
> > @@ -62,10 +62,15 @@ static int try_to_freeze_tasks(bool sig_only)
> >  				todo++;
> >  		} while_each_thread(g, p);
> >  		read_unlock(&tasklist_lock);
> > -		yield();			/* Yield is okay here */
> > -		if (time_after(jiffies, end_time))
> > +		if (!todo || time_after(jiffies, end_time))
> >  			break;
> > -	} while (todo);
> > +
> > +		/*
> > +		 * We need to retry.  There's no reason to be
> > +		 * over-anxious about it and waste power.
> > +		 */

The comment above looks like it's only meaningful in the context of the patch.
After it's been applied the meaning of the comment won't be so obvious, I'm
afraid.

> > +		msleep(10);
> > +	}
> >  
> >  	do_gettimeofday(&end);
> >  	elapsed_csecs64 = timeval_to_ns(&end) - timeval_to_ns(&start);

Thanks,
Rafael
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