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=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,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 27B5CC43603 for ; Thu, 19 Dec 2019 11:46:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 03A4124672 for ; Thu, 19 Dec 2019 11:46:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726789AbfLSLqN (ORCPT ); Thu, 19 Dec 2019 06:46:13 -0500 Received: from foss.arm.com ([217.140.110.172]:37406 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726668AbfLSLqM (ORCPT ); Thu, 19 Dec 2019 06:46:12 -0500 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id E60F131B; Thu, 19 Dec 2019 03:46:11 -0800 (PST) Received: from [10.1.194.46] (e113632-lin.cambridge.arm.com [10.1.194.46]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 940973F719; Thu, 19 Dec 2019 03:46:08 -0800 (PST) Subject: Re: [PATCH] sched, fair: Allow a small degree of load imbalance between SD_NUMA domains To: Peter Zijlstra Cc: Mel Gorman , Vincent Guittot , Ingo Molnar , pauld@redhat.com, srikar@linux.vnet.ibm.com, quentin.perret@arm.com, dietmar.eggemann@arm.com, Morten.Rasmussen@arm.com, hdanton@sina.com, parth@linux.ibm.com, riel@surriel.com, LKML References: <20191218154402.GF3178@techsingularity.net> <8f049805-3e97-09bb-2d32-0718be1dec9b@arm.com> <20191219100232.GY2844@hirez.programming.kicks-ass.net> From: Valentin Schneider Message-ID: <83a9363b-a044-845a-d37c-bf2ca7c8a09e@arm.com> Date: Thu, 19 Dec 2019 11:46:03 +0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.9.0 MIME-Version: 1.0 In-Reply-To: <20191219100232.GY2844@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 19/12/2019 10:02, Peter Zijlstra wrote: > On Wed, Dec 18, 2019 at 06:50:52PM +0000, Valentin Schneider wrote: >> I'm quite sure you have reasons to have written it that way, but I was >> hoping we could squash it down to something like: >> --- >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c >> index 08a233e97a01..f05d09a8452e 100644 >> --- a/kernel/sched/fair.c >> +++ b/kernel/sched/fair.c >> @@ -8680,16 +8680,27 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s >> env->migration_type = migrate_task; >> lsub_positive(&nr_diff, local->sum_nr_running); >> env->imbalance = nr_diff >> 1; >> - return; >> + } else { >> + >> + /* >> + * If there is no overload, we just want to even the number of >> + * idle cpus. >> + */ >> + env->migration_type = migrate_task; >> + env->imbalance = max_t(long, 0, (local->idle_cpus - >> + busiest->idle_cpus) >> 1); >> } >> >> /* >> - * If there is no overload, we just want to even the number of >> - * idle cpus. >> + * Allow for a small imbalance between NUMA groups; don't do any >> + * of it if there is at least half as many tasks / busy CPUs as >> + * there are available CPUs in the busiest group >> */ >> - env->migration_type = migrate_task; >> - env->imbalance = max_t(long, 0, (local->idle_cpus - >> - busiest->idle_cpus) >> 1); >> + if (env->sd->flags & SD_NUMA && >> + (busiest->sum_nr_running < busiest->group_weight >> 1) && >> + (env->imbalance < busiest->group_weight * (env->sd->imbalance_pct - 100) / 100)) > > Note that this form allows avoiding the division. Every time I see that > /100 I'm thinking we should rename and make imbalance_pct a base-2 > thing. > Right, I kept the original form but we can turn that into env->imbalance * 100 < busiest->group_weight * (env->sd->imbalance_pct - 100) As for the base-2 imbalance; I think you've mentioned that in the past. Looking at check_cpu_capacity() as a lambda imbalance_pct user, we could turn that from: rq->cpu_capacity * sd->imbalance_pct < rq->cpu_capacity_orig * 100 to: rq->cpu_capacity_orig - rq->cpu_capacity < rq->cpu_capacity_orig >> sd->imbalance_shift And here we could just go with env->imbalance < busiest->group_weight >> sd->imbalance_shift As for picking values, right now we have 125 (default) / 117 (LLC domain) / 110 (SMT domain) We could have >> 2 (25%), >> 3 (12.5%), >> 4 (6.25%). It's not strictly equivalent but IMO the whole imbalance_pct thing isn't very precise anyway; just needs to be good enough on a sufficient number of topologies. >> + env->imbalance = 0; >> + >> return; >> } >>