From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753350AbZAZRQw (ORCPT ); Mon, 26 Jan 2009 12:16:52 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751691AbZAZRQo (ORCPT ); Mon, 26 Jan 2009 12:16:44 -0500 Received: from mx2.mail.elte.hu ([157.181.151.9]:39673 "EHLO mx2.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751595AbZAZRQn (ORCPT ); Mon, 26 Jan 2009 12:16:43 -0500 Date: Mon, 26 Jan 2009 18:16:18 +0100 From: Ingo Molnar To: Andrew Morton Cc: Rusty Russell , Mike Travis , Ingo Molnar , Dave Jones , cpufreq@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/3] work_on_cpu: Use our own workqueue. Message-ID: <20090126171618.GA32091@elte.hu> References: <20090116191108.135927000@polaris-admin.engr.sgi.com> <20090116191108.533053000@polaris-admin.engr.sgi.com> <20090124001537.7cfde78e.akpm@linux-foundation.org> <200901261711.43943.rusty@rustcorp.com.au> <20090125230130.bcdab2e5.akpm@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20090125230130.bcdab2e5.akpm@linux-foundation.org> User-Agent: Mutt/1.5.18 (2008-05-17) X-ELTE-VirusStatus: clean X-ELTE-SpamScore: -1.5 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-1.5 required=5.9 tests=BAYES_00 autolearn=no SpamAssassin version=3.2.3 -1.5 BAYES_00 BODY: Bayesian spam probability is 0 to 1% [score: 0.0000] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Andrew Morton wrote: > > > Yet another kernel thread for each CPU. All because of some dung > > > way down in arch/x86/kernel/cpu/cpufreq/acpi-cpufreq.c. > > > > > > Is there no other way? > > > > Perhaps, but this works. Trying to be clever got me into this mess in > > the first place. > > > > We could stop using workqueues and change work_on_cpu to create a > > thread every time, which would give it a new failure mode so I don't > > know that everyone could use it any more. Or we could keep a single > > thread around to do all the cpus, and duplicate much of the workqueue > > code. > > > > None of these options are appealing... > > Can we try harder please? 10 screenfuls of kernel threads in the ps > output is just irritating. > > How about banning the use of work_on_cpu() from schedule_work() handlers > and then fixing that driver somehow? Yes, but that's fundamentally fragile: anyone who happens to stick the wrong thing into keventd (and it's dead easy because schedule_work() is easy to use) will lock up work_on_cpu() users. work_on_cpu() is an important (and lowlevel enough) facility to be isolated from casual interaction like that. > What _is_ the bug anyway? The only description we were given was > > Impact: remove potential clashes with generic kevent workqueue > > Annoyingly, some places we want to use work_on_cpu are already in > workqueues. As per Ingo's suggestion, we create a different > workqueue for work_on_cpu. > > which didn't bother telling anyone squat. > > When was this bug added? Was it added into that driver or was it due to > infrastructural changes? This fixes lockups during bootup caused by the cpumask changes/cleanups which changed set_cpus_allowed()+on-kernel-stack-cpumask_t to work_on_cpu(). Which was fine except it didnt take into account the interaction with the kevents workqueue and the very wide cross section for worklet dependencies that this brings with itself. work_on_cpu() was rarely used before so this didnt show up. Ingo