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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 07444C33CB2 for ; Wed, 29 Jan 2020 12:10:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D7C1720716 for ; Wed, 29 Jan 2020 12:10:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726551AbgA2MKI (ORCPT ); Wed, 29 Jan 2020 07:10:08 -0500 Received: from foss.arm.com ([217.140.110.172]:40284 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726177AbgA2MKH (ORCPT ); Wed, 29 Jan 2020 07:10:07 -0500 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 6B5CB1FB; Wed, 29 Jan 2020 04:10:07 -0800 (PST) Received: from [10.0.2.15] (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 2C1093F67D; Wed, 29 Jan 2020 04:10:06 -0800 (PST) Subject: Re: [PATCH v3 1/3] sched/fair: Add asymmetric CPU capacity wakeup scan To: Dietmar Eggemann , Pavan Kondeti Cc: linux-kernel@vger.kernel.org, mingo@redhat.com, peterz@infradead.org, vincent.guittot@linaro.org, morten.rasmussen@arm.com, qperret@google.com, adharmap@codeaurora.org References: <20200126200934.18712-1-valentin.schneider@arm.com> <20200126200934.18712-2-valentin.schneider@arm.com> <20200128062245.GA27398@codeaurora.org> <1ed322d6-0325-ecac-cc68-326a14b8c1dd@arm.com> <1aa14491-517e-92d2-08b0-568338d75812@arm.com> From: Valentin Schneider Message-ID: Date: Wed, 29 Jan 2020 12:10:05 +0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1 MIME-Version: 1.0 In-Reply-To: <1aa14491-517e-92d2-08b0-568338d75812@arm.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 29/01/2020 10:38, Dietmar Eggemann wrote: > On 28/01/2020 12:30, Valentin Schneider wrote: >> Hi Pavan, >> >> On 28/01/2020 06:22, Pavan Kondeti wrote: >>> Hi Valentin, >>> >>> On Sun, Jan 26, 2020 at 08:09:32PM +0000, Valentin Schneider wrote: > > [...] > >>>> + >>>> + if (!static_branch_unlikely(&sched_asym_cpucapacity)) >>>> + return -1; > > We do need this one to bail out quickly on non CPU asym systems. (1) > >>>> + sd = rcu_dereference(per_cpu(sd_asym_cpucapacity, target)); >>>> + if (!sd) >>>> + return -1; > > And I assume we can't return target here because of exclusive cpusets > which can form symmetric CPU capacities islands on a CPU asymmetric > system? (2) > Precisely, the "canonical" check for asymmetry is static key + SD pointer. In terms of functionality we could "just" check sd_asym_cpucapacity (it can't be set without having the static key set, though the reverse isn't true), but we *want* to use the static key here to make SMP people happy. >> That's not the case anymore, so indeed we may be able to bail out of >> select_idle_sibling() right after select_idle_capacity() (or after the >> prev / recent_used_cpu checks). Our only requirement here is that sd_llc >> remains a subset of sd_asym_cpucapacity. > > How do you distinguish '-1' in (1), (2) and 'best_cpu = -1' (3)? > > In (1) and (2) you want to check if target is idle (or sched_idle) but > in (3) you probably only want to check 'recent_used_cpu'? > Right, when we come back from select_idle_capacity(), and we did go through the CPU loop, but we still returned -1, it means all CPUs in sd_asym_cpucapacity were not idle. This includes 'target' of course, so we shouldn't need to check it again. In those cases we might still not have evaluated 'prev' or 'recent_used_cpu'. It's somewhat of a last ditch attempt to find an idle CPU, and they'll only help when they aren't in sd_asym_cpucapacity. I'm actually curious as to how much the 'recent_used_cpu' thing helps, I've never paid it much attention. So yeah my options are (for asym CPU capacity topologies): a) just bail out after select_idle_capacity() b) try to squeeze out a bit more out of select_idle_sibling() by also doing the 'prev' & 'recent_used_cpu' checks. a) is quite easy to implement; I can just inline the static key and sd checks in select_idle_sibling() and return unconditionally once I'm past those checks. b) is more intrusive and I don't have a clear picture yet as to how much it will really bring to the table. I'll think about it and try to play around with both of these.