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=-5.2 required=3.0 tests=BAYES_00, 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 91191C388F9 for ; Wed, 21 Oct 2020 13:18:33 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3F3F122249 for ; Wed, 21 Oct 2020 13:18:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2440861AbgJUNSc (ORCPT ); Wed, 21 Oct 2020 09:18:32 -0400 Received: from mx2.suse.de ([195.135.220.15]:41942 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2436971AbgJUNSb (ORCPT ); Wed, 21 Oct 2020 09:18:31 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id C7B91AD12; Wed, 21 Oct 2020 13:18:29 +0000 (UTC) Date: Wed, 21 Oct 2020 14:18:27 +0100 From: Mel Gorman To: Julia Lawall Cc: Vincent Guittot , Ingo Molnar , kernel-janitors@vger.kernel.org, Peter Zijlstra , Juri Lelli , Dietmar Eggemann , Steven Rostedt , Ben Segall , Daniel Bristot de Oliveira , linux-kernel@vger.kernel.org, Valentin Schneider , Gilles.Muller@inria.fr Subject: Re: [PATCH] sched/fair: check for idle core Message-ID: <20201021131827.GF32041@suse.de> References: <1603211879-1064-1-git-send-email-Julia.Lawall@inria.fr> <20201021112038.GC32041@suse.de> <20201021122532.GA30733@vingu-book> <20201021124700.GE32041@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Oct 21, 2020 at 02:56:06PM +0200, Julia Lawall wrote: > > > On Wed, 21 Oct 2020, Mel Gorman wrote: > > > On Wed, Oct 21, 2020 at 02:25:32PM +0200, Vincent Guittot wrote: > > > > I see Vincent already agreed with the patch so I could be wrong. Vincent, > > > > did I miss something stupid? > > > > > > This patch fixes the problem that we don't favor anymore the prev_cpu when it is idle since > > > commit 11f10e5420f6ce because load is not null when cpu is idle whereas runnable_load was > > > And this is important because this will then decide in which LLC we will looks for a cpu > > > > > > > Ok, that is understandable but I'm still concerned that the fix simply > > trades one problem for another by leaving related tasks remote to each > > other and increasing cache misses and remote data accesses. > > > > wake_affine_weight is a giant pain because really we don't care about the > > load on the waker CPU or its available, we care about whether it has idle > > siblings that can be found quickly. As tempting as ripping it out is, > > it never happened because sometimes it makes the right decision. > > My goal was to restore the previous behavior, when runnable load was used. > The patch removing the use of runnable load (11f10e5420f6) presented it > basically as that load balancing was using it, so wakeup should use it > too, and any way it didn't matter because idle CPUS were checked for > anyway. > Which is fair. > Is your point of view that the proposed change is overkill? Or is it that > the original behavior was not desirable? > I worry it's overkill because prev is always used if it is idle even if it is on a node remote to the waker. It cuts off the option of a wakee moving to a CPU local to the waker which is not equivalent to the original behaviour. -- Mel Gorman SUSE Labs