linux-kernel - Re: [PATCH v5]: Improve performance of LZO hibernation

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Date:	Wed, 28 Sep 2011 17:29:07 +1000
From:	Bojan Smojver <bojan@...ursive.com>
To:	Pekka Enberg <penberg@...helsinki.fi>
Cc:	linux-kernel@...r.kernel.org, "Rafael J. Wysocki" <rjw@...k.pl>
Subject: Re: [PATCH v5]: Improve performance of LZO hibernation

On Wed, 2011-09-28 at 10:22 +0300, Pekka Enberg wrote:

> > +       while(1) {
> > +               wait_event(d->go, atomic_read(&d->ready) ||
> > +                                 kthread_should_stop());
> > +               if (kthread_should_stop())
> > +                       break;
> 
> So... what happens to the hibernation process when 'kthread_should_stop()'
> returns true?

The compression/decompression threads stop by breaking out of the loop.
At least they should, right? Did I misread some docs here?

PS. I'm not really a kernel programmer, so I'm kinda stumbling my way
through all this.

> > +       nthr = num_online_cpus() - 1;
> > +       nthr = nthr > LZO_THREADS ? LZO_THREADS : (nthr < 1 ? 1 : nthr);
> 
> That's probably one of the most unreadable uses of the ternary
> operator I've ever seen!

Sorry about that. I can simplify.

> What's going on here anyway? Why "num_online_cpus() - 1"? What's wrong with
> 
>   nr_threads = num_online_cpus();
>   if (nr_threads > LZO_THREADS)
>     nr_threads = LZO_THREADS;

We want to keep at least one CPU free for that I/O and for pulling the
other threads into sync when they are done (that is if we have more than
one), right?

> [ And yes, please use less cryptic variable names. ]

OK, been pulled over for that before. Will fix.

> Overall, I really like your patch!

Thanks, hopefully it doesn't blow up too many file systems :-)

-- 
Bojan

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