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=-19.4 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, 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 F219FC43616 for ; Mon, 10 May 2021 11:10:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id DAD5261458 for ; Mon, 10 May 2021 11:10:48 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234709AbhEJLJk (ORCPT ); Mon, 10 May 2021 07:09:40 -0400 Received: from mail.kernel.org ([198.145.29.99]:34814 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235380AbhEJLAu (ORCPT ); Mon, 10 May 2021 07:00:50 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 79EA561A2B; Mon, 10 May 2021 10:53:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1620644001; bh=JV2NK6R0gKiSejI5fjveQ7DPkt6Rt4IaZLiPZ87IL60=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=nKEZdYs0P0PKQPD9Dg7BJ6vGCuJM1NdEo+cDsgxH6snpz8ZlNjTVBBGLCmMDElevu VnFHAA1+uCQKRMC9kWeh812wSDJGgmNkPnDH15vRHTNNfvYek64QogjRayg5eRDnhd zkXsXiVVa0JgovA4C/I3BbBDaaBQCwpuBCTB2EvA= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Lingutla Chandrasekhar , Valentin Schneider , "Peter Zijlstra (Intel)" , Dietmar Eggemann , Vincent Guittot , Sasha Levin Subject: [PATCH 5.11 212/342] sched/fair: Ignore percpu threads for imbalance pulls Date: Mon, 10 May 2021 12:20:02 +0200 Message-Id: <20210510102017.097484637@linuxfoundation.org> X-Mailer: git-send-email 2.31.1 In-Reply-To: <20210510102010.096403571@linuxfoundation.org> References: <20210510102010.096403571@linuxfoundation.org> User-Agent: quilt/0.66 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org From: Lingutla Chandrasekhar [ Upstream commit 9bcb959d05eeb564dfc9cac13a59843a4fb2edf2 ] During load balance, LBF_SOME_PINNED will be set if any candidate task cannot be detached due to CPU affinity constraints. This can result in setting env->sd->parent->sgc->group_imbalance, which can lead to a group being classified as group_imbalanced (rather than any of the other, lower group_type) when balancing at a higher level. In workloads involving a single task per CPU, LBF_SOME_PINNED can often be set due to per-CPU kthreads being the only other runnable tasks on any given rq. This results in changing the group classification during load-balance at higher levels when in reality there is nothing that can be done for this affinity constraint: per-CPU kthreads, as the name implies, don't get to move around (modulo hotplug shenanigans). It's not as clear for userspace tasks - a task could be in an N-CPU cpuset with N-1 offline CPUs, making it an "accidental" per-CPU task rather than an intended one. KTHREAD_IS_PER_CPU gives us an indisputable signal which we can leverage here to not set LBF_SOME_PINNED. Note that the aforementioned classification to group_imbalance (when nothing can be done) is especially problematic on big.LITTLE systems, which have a topology the likes of: DIE [ ] MC [ ][ ] 0 1 2 3 L L B B arch_scale_cpu_capacity(L) < arch_scale_cpu_capacity(B) Here, setting LBF_SOME_PINNED due to a per-CPU kthread when balancing at MC level on CPUs [0-1] will subsequently prevent CPUs [2-3] from classifying the [0-1] group as group_misfit_task when balancing at DIE level. Thus, if CPUs [0-1] are running CPU-bound (misfit) tasks, ill-timed per-CPU kthreads can significantly delay the upgmigration of said misfit tasks. Systems relying on ASYM_PACKING are likely to face similar issues. Signed-off-by: Lingutla Chandrasekhar [Use kthread_is_per_cpu() rather than p->nr_cpus_allowed] [Reword changelog] Signed-off-by: Valentin Schneider Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Dietmar Eggemann Reviewed-by: Vincent Guittot Link: https://lkml.kernel.org/r/20210407220628.3798191-2-valentin.schneider@arm.com Signed-off-by: Sasha Levin --- kernel/sched/fair.c | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index dfb65140eb2d..8c82019d9c6f 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -7577,6 +7577,10 @@ 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; -- 2.30.2