From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754105Ab2GWIjI (ORCPT ); Mon, 23 Jul 2012 04:39:08 -0400 Received: from hqemgate03.nvidia.com ([216.228.121.140]:13423 "EHLO hqemgate03.nvidia.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751691Ab2GWIjG (ORCPT ); Mon, 23 Jul 2012 04:39:06 -0400 X-PGP-Universal: processed; by hqnvupgp07.nvidia.com on Mon, 23 Jul 2012 01:33:59 -0700 Date: Mon, 23 Jul 2012 11:38:18 +0300 From: Peter De Schrijver To: "Rafael J. Wysocki" CC: Tejun Heo , Peter Zijlstra , "linux-kernel@vger.kernel.org" , "torvalds@linux-foundation.org" , "tglx@linutronix.de" , "linux-pm@vger.kernel.org" Subject: Re: [PATCHSET] workqueue: reimplement CPU hotplug to keep idle workers Message-ID: <20120723083818.GA12034@tbergstrom-lnx.Nvidia.com> References: <1342545149-3515-1-git-send-email-tj@kernel.org> <1342808550.2583.48.camel@twins> <20120720183400.GL32763@google.com> <201207202144.05154.rjw@sisk.pl> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: <201207202144.05154.rjw@sisk.pl> X-NVConfidentiality: public User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jul 20, 2012 at 09:44:04PM +0200, Rafael J. Wysocki wrote: > 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. I don't see how we can get rid of hotplug (or some other state where some processors are completely quiesient) in systems which have CPU clusters with a different number of CPUs in each cluster. (eg 4 and 1 in case of Tegra30). Cheers, Peter.