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=-13.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,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 87152C388F9 for ; Fri, 23 Oct 2020 08:40:23 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 280FB2168B for ; Fri, 23 Oct 2020 08:40:23 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S460731AbgJWIkV (ORCPT ); Fri, 23 Oct 2020 04:40:21 -0400 Received: from mx2.suse.de ([195.135.220.15]:45756 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S460684AbgJWIkV (ORCPT ); Fri, 23 Oct 2020 04:40:21 -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 95BF3ABF4; Fri, 23 Oct 2020 08:40:19 +0000 (UTC) Date: Fri, 23 Oct 2020 09:40:16 +0100 From: Mel Gorman To: Julia Lawall Cc: Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Daniel Bristot de Oliveira , linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] sched/fair: check for idle core Message-ID: <20201023084016.GP32041@suse.de> References: <1603372550-14680-1-git-send-email-Julia.Lawall@inria.fr> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <1603372550-14680-1-git-send-email-Julia.Lawall@inria.fr> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Oct 22, 2020 at 03:15:50PM +0200, Julia Lawall wrote: > In the case of a thread wakeup, wake_affine determines whether a core > will be chosen for the thread on the socket where the thread ran > previously or on the socket of the waker. This is done primarily by > comparing the load of the core where th thread ran previously (prev) > and the load of the waker (this). > > commit 11f10e5420f6 ("sched/fair: Use load instead of runnable load > in wakeup path") changed the load computation from the runnable load > to the load average, where the latter includes the load of threads > that have already blocked on the core. > > When a short-running daemon processes happens to run on prev, this > change raised the situation that prev could appear to have a greater > load than this, even when prev is actually idle. When prev and this > are on the same socket, the idle prev is detected later, in > select_idle_sibling. But if that does not hold, prev is completely > ignored, causing the waking thread to move to the socket of the waker. > In the case of N mostly active threads on N cores, this triggers other > migrations and hurts performance. > > In contrast, before commit 11f10e5420f6, the load on an idle core > was 0, and in the case of a non-idle waker core, the effect of > wake_affine was to select prev as the target for searching for a core > for the waking thread. > > To avoid unnecessary migrations, extend wake_affine_idle to check > whether the core where the thread previously ran is currently idle, > and if so simply return that core as the target. > target > [1] commit 11f10e5420f6ce ("sched/fair: Use load instead of runnable > load in wakeup path") > > This particularly has an impact when using the ondemand power manager, > where kworkers run every 0.004 seconds on all cores, increasing the > likelihood that an idle core will be considered to have a load. > > The following numbers were obtained with the benchmarking tool > hyperfine (https://github.com/sharkdp/hyperfine) on the NAS parallel > benchmarks (https://www.nas.nasa.gov/publications/npb.html). The > tests were run on an 80-core Intel(R) Xeon(R) CPU E7-8870 v4 @ > 2.10GHz. Active (intel_pstate) and passive (intel_cpufreq) power > management were used. Times are in seconds. All experiments use all > 160 hardware threads. > > v5.9/intel-pstate v5.9+patch/intel-pstate > bt.C.c 24.725724+-0.962340 23.349608+-1.607214 > lu.C.x 29.105952+-4.804203 25.249052+-5.561617 > sp.C.x 31.220696+-1.831335 30.227760+-2.429792 > ua.C.x 26.606118+-1.767384 25.778367+-1.263850 > > v5.9/ondemand v5.9+patch/ondemand > bt.C.c 25.330360+-1.028316 23.544036+-1.020189 > lu.C.x 35.872659+-4.872090 23.719295+-3.883848 > sp.C.x 32.141310+-2.289541 29.125363+-0.872300 > ua.C.x 29.024597+-1.667049 25.728888+-1.539772 > > On the smaller data sets (A and B) and on the other NAS benchmarks > there is no impact on performance. > > This also has a major impact on the splash2x.volrend benchmark of the > parsec benchmark suite that goes from 1m25 without this patch to 0m45, > in active (intel_pstate) mode. > > Fixes: 11f10e5420f6 ("sched/fair: Use load instead of runnable load in wakeup path") > Signed-off-by: Julia Lawall > Reviewed-by Vincent Guittot > In principal, I think the patch is ok after the recent discussion. I'm holding off an ack until a battery of tests on loads with different levels of utilisation and wakeup patterns makes its way through a test grid. It's based on Linus's tree mid-merge window that includes what is in the scheduler pipeline -- Mel Gorman SUSE Labs