From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752173AbeBHQDx (ORCPT ); Thu, 8 Feb 2018 11:03:53 -0500 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:36934 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751094AbeBHQDh (ORCPT ); Thu, 8 Feb 2018 11:03:37 -0500 Date: Thu, 8 Feb 2018 16:03:41 +0000 From: Will Deacon To: Peter Zijlstra Cc: Vincent Guittot , mingo@kernel.org, linux-kernel@vger.kernel.org, valentin.schneider@arm.com, morten.rasmussen@foss.arm.com, brendan.jackman@arm.com, dietmar.eggemann@arm.com Subject: Re: [PATCH v2 1/3] sched: Stop nohz stats when decayed Message-ID: <20180208160341.GD17775@arm.com> References: <1517944987-343-1-git-send-email-vincent.guittot@linaro.org> <1517944987-343-2-git-send-email-vincent.guittot@linaro.org> <20180208140005.GH25201@hirez.programming.kicks-ass.net> <20180208153030.GB17775@arm.com> <20180208154643.GB25181@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180208154643.GB25181@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Feb 08, 2018 at 04:46:43PM +0100, Peter Zijlstra wrote: > On Thu, Feb 08, 2018 at 03:30:31PM +0000, Will Deacon wrote: > > On Thu, Feb 08, 2018 at 03:00:05PM +0100, Peter Zijlstra wrote: > > > > Without this ordering I think it would be possible to loose has_blocked > > > and not observe the CPU either. > > > > I had a quick look at this, and I think you're right. This looks very much > > like an 'R'-shaped test, which means it's smp_mb() all round otherwise Power > > will go wrong. That also means the smp_mb__after_atomic() in > > nohz_balance_enter_idle *cannot* be an smp_wmb(), so you might want a > > comment stating that explicitly. > > Thanks Will. BTW, where does that 'R' shape nomenclature come from? > This is the first I've heard of it. I don't know where it originates from, but the imfamous "test6.pdf" has it: https://www.cl.cam.ac.uk/~pes20/ppc-supplemental/test6.pdf half way down the first page on the left. It says "needs sync+sync" which is about as bad as it gets for Power (compare with "2+2w", which gets away with lwsync+lwsync). See also: http://materials.dagstuhl.de/files/16/16471/16471.DerekWilliams.Slides.pdf for a light-hearted, yet technically accurate story about the latter. Will