From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753997AbaHERyn (ORCPT ); Tue, 5 Aug 2014 13:54:43 -0400 Received: from g2t2353.austin.hp.com ([15.217.128.52]:37544 "EHLO g2t2353.austin.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753482AbaHERyl (ORCPT ); Tue, 5 Aug 2014 13:54:41 -0400 X-Greylist: delayed 141493 seconds by postgrey-1.27 at vger.kernel.org; Tue, 05 Aug 2014 13:54:41 EDT Message-ID: <53E11A5D.2060303@hp.com> Date: Tue, 05 Aug 2014 13:54:37 -0400 From: Waiman Long User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:10.0.12) Gecko/20130109 Thunderbird/10.0.12 MIME-Version: 1.0 To: Peter Zijlstra CC: Ingo Molnar , linux-kernel@vger.kernel.org, linux-api@vger.kernel.org, linux-doc@vger.kernel.org, Davidlohr Bueso , Jason Low , Scott J Norton Subject: Re: [PATCH 1/7] locking/rwsem: don't resched at the end of optimistic spinning References: <1407119782-41119-1-git-send-email-Waiman.Long@hp.com> <1407119782-41119-2-git-send-email-Waiman.Long@hp.com> <20140804075528.GI9918@twins.programming.kicks-ass.net> <53DFD2B3.3090101@hp.com> <20140804204824.GT3935@laptop> In-Reply-To: <20140804204824.GT3935@laptop> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 08/04/2014 04:48 PM, Peter Zijlstra wrote: > On Mon, Aug 04, 2014 at 02:36:35PM -0400, Waiman Long wrote: >> On 08/04/2014 03:55 AM, Peter Zijlstra wrote: >>> On Sun, Aug 03, 2014 at 10:36:16PM -0400, Waiman Long wrote: >>>> For a fully preemptive kernel, a call to preempt_enable() could >>>> potentially trigger a task rescheduling event. In the case of rwsem >>>> optimistic spinning, the task has either gotten the lock or is going >>>> to sleep soon. So there is no point to do rescheduling here. >>> Uh what? Why shouldn't we preempt if we've gotten the lock? What if a >>> FIFO task just woke up? >> I didn't mean that we shouldn't preempt if there is a higher priority task. >> I am sure that there will be other preemption points along the way that a >> higher priority task can take over the CPU. I just want to say that doing it >> here may not be the best place especially if the task is going to sleep >> soon. >> >> If you think this patch does not make sense, I can remove it as other >> patches in the set has no dependency on this one. > Yeah, its actively harmful, you delay preemption by an unspecified > amount of time in case of the spin-acquire. We've had such bugs in -rt > and they're not fun. > > Basically the only time you should use no_resched is if the very next > statement is schedule(). Thank for the clarification. I will remove patch 1 from the patch set. -Longman