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 81A81C33CB2 for ; Wed, 29 Jan 2020 12:10:58 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5347320716 for ; Wed, 29 Jan 2020 12:10:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726622AbgA2MK5 (ORCPT ); Wed, 29 Jan 2020 07:10:57 -0500 Received: from foss.arm.com ([217.140.110.172]:40314 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726271AbgA2MK5 (ORCPT ); Wed, 29 Jan 2020 07:10:57 -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 C50041FB; Wed, 29 Jan 2020 04:10:56 -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 B747E3F67D; Wed, 29 Jan 2020 04:10:55 -0800 (PST) Subject: Re: [PATCH v3 1/3] sched/fair: Add asymmetric CPU capacity wakeup scan To: Dietmar Eggemann , linux-kernel@vger.kernel.org Cc: 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> From: Valentin Schneider Message-ID: <40bfa77b-b695-5f53-848a-b72b67b33d69@arm.com> Date: Wed, 29 Jan 2020 12:10:55 +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: 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 11:04, Dietmar Eggemann wrote: >> + /* >> + * It would be silly to keep looping when we've found a CPU >> + * of highest available capacity. Just check that it's not been >> + * too pressured lately. >> + */ >> + if (rq->cpu_capacity_orig == READ_ONCE(rq->rd->max_cpu_capacity) && > > There is a similar check in check_misfit_status(). Common helper function? Mright, and check_misfit_status() is missing the READ_ONCE(). That said... > >> + !check_cpu_capacity(rq, sd)) >> + return cpu; > > I wonder how this special treatment of a big CPU behaves in (LITTLE, > medium, big) system like Pixel4 (Snapdragon 855): > > flame:/ $ cat /sys/devices/system/cpu/cpu*/cpu_capacity > > 261 > 261 > 261 > 261 > 871 > 871 > 871 > 1024 > > Or on legacy systems where the sd->imbalance_pct is 25% instead of 17%? > ... This is a very valid point. When I wrote this bit I had the good old big.LITTLE split in mind where there are big differences between the capacity values. As you point out, that's not so true with DynamIQ systems sporting > 2 capacity values. The issue here is that we could bail early picking a (slightly) pressured big (1024 capacity_orig) when there was a non-pressured idle medium (871 capacity orig). It's borderline in this example - the threshold for a big to be seen as pressured by check_cpu_capacity(), assuming a flat topology with just an MC domain, is ~ 875. If we have e.g. mediums at 900 and bigs at 1024, this logic is broken. So this is pretty much a case of my trying to be too clever for my own good, I'll remove that "fastpath" in v4. Thanks for pointing it out!