linux-kernel - Re: [PATCHSET] workqueue: reimplement CPU hotplug to keep idle workers

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Message-Id: <201207202144.05154.rjw@sisk.pl>
Date:	Fri, 20 Jul 2012 21:44:04 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Tejun Heo <tj@...nel.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	tglx@...utronix.de, linux-pm@...r.kernel.org
Subject: Re: [PATCHSET] workqueue: reimplement CPU hotplug to keep idle workers

On Friday, July 20, 2012, Tejun Heo wrote:
> Hello,
> 
> On Fri, Jul 20, 2012 at 08:22:30PM +0200, Peter Zijlstra wrote:
> > I really think people who use hotplug at high frequencies are on drugs
> > and doing it wrong.
> 
> I don't know.  It does make some sense.  It's not like we have any
> other mechanism to keep some processors completely quiesient, which
> could make a noticeable difference from powersaving POV compared to
> mostly idle.  Rafael, can you please chime in and explain how / where
> / how freqeuntly / etc CPU hotplug is used for powersaving?

Well, there are use cases I'm not really familiar with.

Pretty much the only use case I'm sufficiently familiar with is
suspend/hibernate where we unplug all of the nonboot CPUs at one point.

The other use cases, which I don't really think are entirely valid,
are on some ARM platforms where CPUs are unplugged instead of being put into
C-states or equivalent (because we don't have a good mechanism for handling
multiprocessor C-states; there's a set of patches for that waiting for
the merge window in the Len's tree).  I'm hoping to get rid of those
use cases in future entirely.

Thanks,
Rafael
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