From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752511Ab2GTTi0 (ORCPT ); Fri, 20 Jul 2012 15:38:26 -0400 Received: from ogre.sisk.pl ([193.178.161.156]:50171 "EHLO ogre.sisk.pl" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751573Ab2GTTiZ (ORCPT ); Fri, 20 Jul 2012 15:38:25 -0400 From: "Rafael J. Wysocki" To: Tejun Heo Subject: Re: [PATCHSET] workqueue: reimplement CPU hotplug to keep idle workers Date: Fri, 20 Jul 2012 21:44:04 +0200 User-Agent: KMail/1.13.6 (Linux/3.5.0-rc5+; KDE/4.6.0; x86_64; ; ) Cc: Peter Zijlstra , linux-kernel@vger.kernel.org, torvalds@linux-foundation.org, tglx@linutronix.de, linux-pm@vger.kernel.org References: <1342545149-3515-1-git-send-email-tj@kernel.org> <1342808550.2583.48.camel@twins> <20120720183400.GL32763@google.com> In-Reply-To: <20120720183400.GL32763@google.com> MIME-Version: 1.0 Content-Type: Text/Plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Message-Id: <201207202144.05154.rjw@sisk.pl> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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