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 3D49BC55178 for ; Wed, 21 Oct 2020 13:49:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D892721481 for ; Wed, 21 Oct 2020 13:49:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2443065AbgJUNtC (ORCPT ); Wed, 21 Oct 2020 09:49:02 -0400 Received: from mail2-relais-roc.national.inria.fr ([192.134.164.83]:52344 "EHLO mail2-relais-roc.national.inria.fr" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2439375AbgJUNtC (ORCPT ); Wed, 21 Oct 2020 09:49:02 -0400 X-IronPort-AV: E=Sophos;i="5.77,401,1596492000"; d="scan'208";a="473693564" Received: from 173.121.68.85.rev.sfr.net (HELO hadrien) ([85.68.121.173]) by mail2-relais-roc.national.inria.fr with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 21 Oct 2020 15:48:59 +0200 Date: Wed, 21 Oct 2020 15:48:59 +0200 (CEST) From: Julia Lawall X-X-Sender: jll@hadrien To: Mel Gorman cc: Julia Lawall , 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 In-Reply-To: <20201021131827.GF32041@suse.de> Message-ID: References: <1603211879-1064-1-git-send-email-Julia.Lawall@inria.fr> <20201021112038.GC32041@suse.de> <20201021122532.GA30733@vingu-book> <20201021124700.GE32041@suse.de> <20201021131827.GF32041@suse.de> User-Agent: Alpine 2.22 (DEB 394 2020-01-19) MIME-Version: 1.0 Content-Type: multipart/mixed; boundary="8323329-484522562-1603288140=:57356" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org This message is in MIME format. The first part should be readable text, while the remaining parts are likely unreadable without MIME-aware tools. --8323329-484522562-1603288140=:57356 Content-Type: text/plain; charset=iso-8859-15 Content-Transfer-Encoding: 8BIT On Wed, 21 Oct 2020, Mel Gorman wrote: > 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. Could it be possible to check p->recent_used_cpu? If that is prev (or on the same socket?), then prev could be a good choice. If that is on the same socket as the waker, then maybe the waker would be better. julia --8323329-484522562-1603288140=:57356--