From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752966Ab0CIHJP (ORCPT ); Tue, 9 Mar 2010 02:09:15 -0500 Received: from mtagate1.de.ibm.com ([195.212.17.161]:40124 "EHLO mtagate1.de.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751732Ab0CIHJM (ORCPT ); Tue, 9 Mar 2010 02:09:12 -0500 Date: Tue, 9 Mar 2010 08:09:21 +0100 From: Heiko Carstens To: Tejun Heo Cc: Oleg Nesterov , linux-kernel@vger.kernel.org, rusty@rustcorp.com.au, sivanich@sgi.com, torvalds@linux-foundation.org, mingo@elte.hu, peterz@infradead.org, dipankar@in.ibm.com, josh@freedesktop.org, paulmck@linux.vnet.ibm.com, akpm@linux-foundation.org, Martin Schwidefsky Subject: Re: [PATCH 2/4] stop_machine: reimplement using cpuhog Message-ID: <20100309070921.GA2360@osiris.boeblingen.de.ibm.com> References: <1268063603-7425-1-git-send-email-tj@kernel.org> <1268063603-7425-3-git-send-email-tj@kernel.org> <20100308171020.GC2557@osiris.boeblingen.de.ibm.com> <20100308182708.GA8504@redhat.com> <20100308193737.GA2466@osiris.boeblingen.de.ibm.com> <4B958ABC.3010401@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4B958ABC.3010401@kernel.org> 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 Tue, Mar 09, 2010 at 08:39:40AM +0900, Tejun Heo wrote: > Hello, > > On 03/09/2010 04:37 AM, Heiko Carstens wrote: > >> How cpuhog can make a difference? Afaics, we shouldn't pass a > >> blocking callback to hog_cpus/hog_one_cpu. > > > > Well, it might me true that this shouldn't be done. But I don't see > > a reason why in general it wouldn't work to pass a function that > > would block. So it's just a matter of time until somebody uses it > > for such a purpose. For the current stop_machine implementation it > > would be broken to pass a blocking function (preemption disabled, > > interrupts disabled). > > Well, all current users don't block and it definitely can be enforced > by turning off preemption around the callback. stop_machine() uses > busy waiting for every state transition so something else blocking on > a cpu could waste a lot of cpu cycles on other cpus even if the wait > is guaranteed to be finite. Would that sooth your concern? Yes, enforcing non blocking functions would be good. Thanks!