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 BE5CAC4363A for ; Wed, 21 Oct 2020 11:21:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 50B7521741 for ; Wed, 21 Oct 2020 11:21:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2441951AbgJULVT (ORCPT ); Wed, 21 Oct 2020 07:21:19 -0400 Received: from mx2.suse.de ([195.135.220.15]:41610 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2410667AbgJULUo (ORCPT ); Wed, 21 Oct 2020 07:20:44 -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 CC476ACB8; Wed, 21 Oct 2020 11:20:41 +0000 (UTC) Date: Wed, 21 Oct 2020 12:20:38 +0100 From: Mel Gorman To: Julia Lawall Cc: Ingo Molnar , kernel-janitors@vger.kernel.org, Peter Zijlstra , Juri Lelli , Vincent Guittot , 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: <20201021112038.GC32041@suse.de> References: <1603211879-1064-1-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: <1603211879-1064-1-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 Tue, Oct 20, 2020 at 06:37:59PM +0200, Julia Lawall wrote: > On a thread wakeup, the change [1] from runnable load average to load > average for comparing candidate cores means that recent short-running > daemons on the core where a thread ran previously can be considered to > have a higher load than the core performing the wakeup, even when the > core where the thread ran previously is currently idle. This can > cause a thread to migrate, taking the place of some other thread that > is about to wake up, and so on. To avoid unnecessary migrations, > extend wake_affine_idle to check whether the core where the thread > previously ran is currently idle, and if so return that core as the > target. > > [1] commit 11f10e5420f6ce ("sched/fair: Use load instead of runnable > load in wakeup path") > > This particularly has an impact when using passive (intel_cpufreq) > power management, 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/active v5.9+patch/active > 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/passive v5.9+patch/passive > 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. > > Signed-off-by: Julia Lawall I suspect that the benefit of this patch is due to avoiding the overhead of wake_affine_weight() check because the following check exists in select_idle_sibling /* * If the previous CPU is cache affine and idle, don't be stupid: */ if (prev != target && cpus_share_cache(prev, target) && (available_idle_cpu(prev) || sched_idle_cpu(prev))) return prev; Still, the concept makes some sense to avoid wake_affine_weight but look at the earlier part of wake_affine_idle() if (available_idle_cpu(this_cpu) && cpus_share_cache(this_cpu, prev_cpu)) return available_idle_cpu(prev_cpu) ? prev_cpu : this_cpu; This thing is almost completely useless because this_cpu is only going to be idle if it's a wakeup from interrupt context when the CPU was otherwise idle *but* it takes care to only use the CPU if this and prev share LLC. The patch as it stands may leave a task on a remote node when it should have been pulled local to the waker because prev happened to be idle. This is not guaranteed because a node could have multiple LLCs and prev is still appropriate but that's a different problem entirely and requires much deeper surgery. Still, not pulling a task from a remote node is a change in expected behaviour. While it's possible that NUMA domains will not even reach this path, it depends on the NUMA distance as can be seen in sd_init() for the setting of SD_WAKE_AFFINE so I think the cpus_share_cache check is necessary. I think it would be more appropriate to rework that block that checks this_cpu to instead check if the CPUs share cache first and then return one of them (preference to prev based on the comment above it about avoiding a migration) if either one is idle. I see Vincent already agreed with the patch so I could be wrong. Vincent, did I miss something stupid? -- Mel Gorman SUSE Labs