From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752406AbeBHPqv (ORCPT ); Thu, 8 Feb 2018 10:46:51 -0500 Received: from merlin.infradead.org ([205.233.59.134]:47712 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752375AbeBHPqs (ORCPT ); Thu, 8 Feb 2018 10:46:48 -0500 Date: Thu, 8 Feb 2018 16:46:43 +0100 From: Peter Zijlstra To: Will Deacon 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: <20180208154643.GB25181@hirez.programming.kicks-ass.net> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180208153030.GB17775@arm.com> User-Agent: Mutt/1.9.2 (2017-12-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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.