From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755323AbZA0AnZ (ORCPT ); Mon, 26 Jan 2009 19:43:25 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752834AbZA0AnR (ORCPT ); Mon, 26 Jan 2009 19:43:17 -0500 Received: from smtp1.linux-foundation.org ([140.211.169.13]:52639 "EHLO smtp1.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752648AbZA0AnQ (ORCPT ); Mon, 26 Jan 2009 19:43:16 -0500 Date: Mon, 26 Jan 2009 16:42:37 -0800 From: Andrew Morton To: Ingo Molnar Cc: oleg@redhat.com, a.p.zijlstra@chello.nl, rusty@rustcorp.com.au, travis@sgi.com, mingo@redhat.com, davej@redhat.com, cpufreq@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/3] work_on_cpu: Use our own workqueue. Message-Id: <20090126164237.1dc45156.akpm@linux-foundation.org> In-Reply-To: <20090126235331.GA8726@elte.hu> References: <20090126212727.GA13670@elte.hu> <20090126133551.fab5e27a.akpm@linux-foundation.org> <20090126214516.GA22142@elte.hu> <20090126140116.35f9c173.akpm@linux-foundation.org> <20090126220537.GA6755@elte.hu> <20090126141605.707877bb.akpm@linux-foundation.org> <20090126222002.GB10215@elte.hu> <20090126145003.b29b81d7.akpm@linux-foundation.org> <20090126225957.GA3999@elte.hu> <20090126154217.b312e278.akpm@linux-foundation.org> <20090126235331.GA8726@elte.hu> X-Mailer: Sylpheed version 2.2.4 (GTK+ 2.8.20; i486-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 27 Jan 2009 00:53:31 +0100 Ingo Molnar wrote: > > * Andrew Morton wrote: > > > > The problem is the intrinsic utility of work_on_cpu(): we _really_ > > > want such a generic facility to be usable from any (blockable) > > > context, just like on_each_cpu(func, info) does for atomic functions, > > > without restrictions on locking context. > > > > Do we? work_on_cpu() is some last-gasp oh-i-screwed-my-code-up thing. > > We _really_ want people to use on_each_cpu()! > > why? on_each_cpu() is limited and runs in IRQ context. It's worked OK for the great majority of callers. > Is there a > requirement that worklets need to be atomic? Blocking leads to deadlocks. > > We should bust a gut to keep the number of callers to the > > resource-intensive (deadlocky!) work_on_cpu() to a minimum. > > i wouldnt call +10K 'resource intensive'. per CPU. Plus there's the `ps aux | wth?' effect. We've busted a gut over far, far less. Plus the bugfixed, undeadlockable version will be more expensive still. > > (And to think that adding add_timer_on() creeped me out). > > > > hm. None of that was very helpful. How to move forward? > > > > I think I disagree that work_on_cpu() should be made into some robust, > > smiled-upon core kernel facility. It _is_ slow, it _is_ deadlockable. > > uhm, why is it slow? It could be faster in fact in some cases: the main > overhead in on_each_cpu() is having to wait for the IPIs - with a thread > based approach if the other CPUs are idle we can get an IPI-less wakeup. spose so, if the CPU can do mwait? If the CPU was idle, etc. If a CPU was busy then the call could take a long time. > > It should be positioned as something which is only used as a last > > resort. And if you _have_ to use it, sort out your locking! > > > > Plus the number of code sites which want to fiddle with other CPUs in > > this manner will always be small. cpufreq, MCE, irq-affinity, things > > like that. > > > > What is the deadlock in acpi-cpufreq? Which lock, and who is the > > "other" holder of that lock? > > a quick look suggests that it's dbs_mutex. > Can't see it. In fact all work_on_cpu() handlers in arch/x86/kernel/cpu/cpufreq/acpi-cpufreq.c appear to be atomic. Couldn't the whole thing be converted to use smp_call_function_many()?