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 8D849C388F9 for ; Fri, 23 Oct 2020 10:05:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3F207221F9 for ; Fri, 23 Oct 2020 10:05:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S461894AbgJWKFz (ORCPT ); Fri, 23 Oct 2020 06:05:55 -0400 Received: from mx2.suse.de ([195.135.220.15]:60466 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S373632AbgJWKFz (ORCPT ); Fri, 23 Oct 2020 06:05:55 -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 81990AC82; Fri, 23 Oct 2020 10:05:53 +0000 (UTC) Date: Fri, 23 Oct 2020 11:05:51 +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: <20201023100551.GR32041@suse.de> References: <1603372550-14680-1-git-send-email-Julia.Lawall@inria.fr> <20201023084016.GP32041@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline 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 Fri, Oct 23, 2020 at 11:21:50AM +0200, Julia Lawall wrote: > > > On Fri, 23 Oct 2020, Mel Gorman wrote: > > > 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 > > OK, if it doesn't work out, it would be interesting to know what goes > badly. > Yep, if something goes wrong, I'll make the full logs available, details on reproducing it etc. -- Mel Gorman SUSE Labs