From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751031AbeBIJvf (ORCPT ); Fri, 9 Feb 2018 04:51:35 -0500 Received: from mail-wr0-f194.google.com ([209.85.128.194]:41618 "EHLO mail-wr0-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750789AbeBIJvd (ORCPT ); Fri, 9 Feb 2018 04:51:33 -0500 X-Google-Smtp-Source: AH8x227OQLaju+ZQ0i/3xK+/cRB5TQx3SaxeXB0c8fc90nHlGufMiqhbdaXfNFJ4Hi9imd2rEhrgqA== Date: Fri, 9 Feb 2018 10:51:25 +0100 From: Andrea Parri To: Will Deacon Cc: Peter Zijlstra , 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: <20180209095125.GA17357@andrea> 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> <20180208160341.GD17775@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180208160341.GD17775@arm.com> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Feb 08, 2018 at 04:03:41PM +0000, Will Deacon wrote: > 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 Indeed. As a curiosity: I've never _observed_ R+lwsync+sync (the lwsync separating the two writes), and other people who tried found the same http://moscova.inria.fr/~maranget/cats7/linux/hard.html#unseen http://www.cl.cam.ac.uk/~pes20/ppc-supplemental/ppc051.html#toc8 . It would be interesting to hear about different results ... ;-) Andrea > 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