From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-10.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 4958CC433ED for ; Tue, 4 May 2021 09:27:25 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 10483613AA for ; Tue, 4 May 2021 09:27:25 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230115AbhEDJ2S (ORCPT ); Tue, 4 May 2021 05:28:18 -0400 Received: from foss.arm.com ([217.140.110.172]:56158 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229903AbhEDJ2R (ORCPT ); Tue, 4 May 2021 05:28:17 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 9B177ED1; Tue, 4 May 2021 02:27:22 -0700 (PDT) Received: from e120877-lin.cambridge.arm.com (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id D042B3F73B; Tue, 4 May 2021 02:27:20 -0700 (PDT) Date: Tue, 4 May 2021 10:27:15 +0100 From: Vincent Donnefort To: Pierre.Gondois@arm.com Cc: linux-kernel@vger.kernel.org, xuewen.yan@unisoc.com, qperret@qperret.net, dietmar.eggemann@arm.com, Lukasz.Luba@arm.com, mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com, vincent.guittot@linaro.org, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, bristot@redhat.com Subject: Re: [PATCH v2 2/2] sched/fair: Fix negative energy delta in find_energy_efficient_cpu() Message-ID: <20210504092714.GA100203@e120877-lin.cambridge.arm.com> References: <20210429101948.31224-1-Pierre.Gondois@arm.com> <20210429101948.31224-3-Pierre.Gondois@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210429101948.31224-3-Pierre.Gondois@arm.com> User-Agent: Mutt/1.5.24 (2015-08-30) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 29, 2021 at 11:19:48AM +0100, Pierre.Gondois@arm.com wrote: > From: Pierre Gondois > > find_energy_efficient_cpu() (feec()) searches the best energy CPU > to place a task on. To do so, compute_energy() estimates the energy > impact of placing the task on a CPU, based on CPU and task utilization > signals. > > Utilization signals can be concurrently updated while evaluating a > performance domain (pd). In some cases, this leads to having a > 'negative delta', i.e. placing the task in the pd is seen as an > energy gain. Thus, any further energy comparison is biased. > > In case of a 'negative delta', return prev_cpu since: > 1. a 'negative delta' happens in less than 0.5% of feec() calls, > on a Juno with 6 CPUs (4 little, 2 big) > 2. it is unlikely to have two consecutive 'negative delta' for > a task, so if the first call fails, feec() will correctly > place the task in the next feec() call > 3. EAS current behavior tends to select prev_cpu if the task > doesn't raise the OPP of its current pd. prev_cpu is EAS's > generic decision > 4. prev_cpu should be preferred to returning an error code. > In the latter case, select_idle_sibling() would do the placement, > selecting a big (and not energy efficient) CPU. As 3., the task > would potentially reside on the big CPU for a long time > > Reported-by: Xuewen Yan > Suggested-by: Xuewen Yan > Signed-off-by: Pierre Gondois > --- I've been testing this patch on the Google's Pixel4, with a modified kernel that we are using to evalute mailine performance and energy consumption for a "real-life" mobile usage. As always, I ran the Work2.0 workload from PCMark on Android. With that setup I haven't observed any statistically significant performance change neither any CPU Idle residency modification. Nevertheless, this code protected against ~600 bad computations (and by extent bad placements) during a single PCMark iteration and by looking at the traces, this is saving from spurious wake-ups that would otherwise happen on the biggest CPUs of the system. + Reviewed-by: Vincent Donnefort