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=-16.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_GIT 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 3E837C433DB for ; Thu, 11 Mar 2021 12:06:33 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id DA1C164DCC for ; Thu, 11 Mar 2021 12:06:32 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233033AbhCKMGE (ORCPT ); Thu, 11 Mar 2021 07:06:04 -0500 Received: from foss.arm.com ([217.140.110.172]:34024 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232638AbhCKMFh (ORCPT ); Thu, 11 Mar 2021 07:05:37 -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 28F17ED1; Thu, 11 Mar 2021 04:05:37 -0800 (PST) Received: from e113632-lin.cambridge.arm.com (e113632-lin.cambridge.arm.com [10.1.194.46]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPA id 94A6A3F793; Thu, 11 Mar 2021 04:05:35 -0800 (PST) From: Valentin Schneider To: linux-kernel@vger.kernel.org Cc: Lingutla Chandrasekhar , Peter Zijlstra , Ingo Molnar , Vincent Guittot , Dietmar Eggemann , Morten Rasmussen , Qais Yousef , Quentin Perret , Pavan Kondeti , Rik van Riel Subject: [PATCH v3 1/7] sched/fair: Ignore percpu threads for imbalance pulls Date: Thu, 11 Mar 2021 12:05:21 +0000 Message-Id: <20210311120527.167870-2-valentin.schneider@arm.com> X-Mailer: git-send-email 2.25.1 In-Reply-To: <20210311120527.167870-1-valentin.schneider@arm.com> References: <20210311120527.167870-1-valentin.schneider@arm.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Lingutla Chandrasekhar In load balancing, when balancing group is unable to pull task due to ->cpus_ptr constraints from busy group, then it sets LBF_SOME_PINNED to lb env flags, as a consequence, sgc->imbalance is set for its parent domain level. which makes the group classified as imbalance to get help from another balancing cpu. Consider a 4-CPU big.LITTLE system with CPUs 0-1 as LITTLEs and CPUs 2-3 as Bigs with below scenario: - CPU0 doing newly_idle balancing - CPU1 running percpu kworker and RT task (small tasks) - CPU2 running 2 big tasks - CPU3 running 1 medium task While CPU0 is doing newly_idle load balance at MC level, it fails to pull percpu kworker from CPU1 and sets LBF_SOME_PINNED to lb env flag and set sgc->imbalance at DIE level domain. As LBF_ALL_PINNED not cleared, it tries to redo the balancing by clearing CPU1 in env cpus, but it don't find other busiest_group, so CPU0 stops balacing at MC level without clearing 'sgc->imbalance' and restart the load balacing at DIE level. And CPU0 (balancing cpu) finds LITTLE's group as busiest_group with group type as imbalance, and Bigs that classified the level below imbalance type would be ignored to pick as busiest, and the balancing would be aborted without pulling any tasks (by the time, CPU1 might not have running tasks). It is suboptimal decision to classify the group as imbalance due to percpu threads. So don't use LBF_SOME_PINNED for per cpu threads. Signed-off-by: Lingutla Chandrasekhar [Use kthread_is_per_cpu() rather than p->nr_cpus_allowed] Signed-off-by: Valentin Schneider --- kernel/sched/fair.c | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 2e2ab1e00ef9..83aea97fbf22 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -7565,6 +7565,10 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env) if (throttled_lb_pair(task_group(p), env->src_cpu, env->dst_cpu)) return 0; + /* Disregard pcpu kthreads; they are where they need to be. */ + if ((p->flags & PF_KTHREAD) && kthread_is_per_cpu(p)) + return 0; + if (!cpumask_test_cpu(env->dst_cpu, p->cpus_ptr)) { int cpu; -- 2.25.1