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,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 D32D1C432BE for ; Mon, 23 Aug 2021 11:17:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B9BD3610D0 for ; Mon, 23 Aug 2021 11:17:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236404AbhHWLRx (ORCPT ); Mon, 23 Aug 2021 07:17:53 -0400 Received: from foss.arm.com ([217.140.110.172]:51998 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236347AbhHWLRu (ORCPT ); Mon, 23 Aug 2021 07:17:50 -0400 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 D4F02143D; Mon, 23 Aug 2021 04:17:07 -0700 (PDT) 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 F036A3F66F; Mon, 23 Aug 2021 04:17:06 -0700 (PDT) From: Valentin Schneider To: linux-kernel@vger.kernel.org Cc: Peter Zijlstra , Ingo Molnar , Vincent Guittot , Dietmar Eggemann Subject: [PATCH v3 2/2] sched/fair: Trigger nohz.next_balance updates when a CPU goes NOHZ-idle Date: Mon, 23 Aug 2021 12:17:00 +0100 Message-Id: <20210823111700.2842997-3-valentin.schneider@arm.com> X-Mailer: git-send-email 2.25.1 In-Reply-To: <20210823111700.2842997-1-valentin.schneider@arm.com> References: <20210823111700.2842997-1-valentin.schneider@arm.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Consider a system with some NOHZ-idle CPUs, such that nohz.idle_cpus_mask = S nohz.next_balance = T When a new CPU k goes NOHZ idle (nohz_balance_enter_idle()), we end up with: nohz.idle_cpus_mask = S \U {k} nohz.next_balance = T Note that the nohz.next_balance hasn't changed - it won't be updated until a NOHZ balance is triggered. This is problematic if the newly NOHZ idle CPU has an earlier rq.next_balance than the other NOHZ idle CPUs, IOW if: cpu_rq(k).next_balance < nohz.next_balance In such scenarios, the existing nohz.next_balance will prevent any NOHZ balance from happening, which itself will prevent nohz.next_balance from being updated to this new cpu_rq(k).next_balance. Unnecessary load balance delays of over 12ms caused by this were observed on an arm64 RB5 board. Use the new nohz.needs_update flag to mark the presence of newly-idle CPUs that need their rq->next_balance to be collated into nohz.next_balance. Trigger a NOHZ_NEXT_KICK when the flag is set. Signed-off-by: Valentin Schneider --- kernel/sched/fair.c | 15 +++++++++++++-- 1 file changed, 13 insertions(+), 2 deletions(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 4a91f3027c92..081a9e54058a 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -5754,6 +5754,7 @@ static struct { cpumask_var_t idle_cpus_mask; atomic_t nr_cpus; int has_blocked; /* Idle CPUS has blocked load */ + int needs_update; /* Newly idle CPUs need their next_balance collated */ unsigned long next_balance; /* in jiffy units */ unsigned long next_blocked; /* Next update of blocked load in jiffies */ } nohz ____cacheline_aligned; @@ -10417,6 +10418,9 @@ static void nohz_balancer_kick(struct rq *rq) unlock: rcu_read_unlock(); out: + if (READ_ONCE(nohz.needs_update)) + flags |= NOHZ_NEXT_KICK; + if (flags) kick_ilb(flags); } @@ -10513,12 +10517,13 @@ void nohz_balance_enter_idle(int cpu) /* * Ensures that if nohz_idle_balance() fails to observe our * @idle_cpus_mask store, it must observe the @has_blocked - * store. + * and @needs_update stores. */ smp_mb__after_atomic(); set_cpu_sd_state_idle(cpu); + WRITE_ONCE(nohz.needs_update, 1); out: /* * Each time a cpu enter idle, we assume that it has blocked load and @@ -10567,13 +10572,17 @@ static void _nohz_idle_balance(struct rq *this_rq, unsigned int flags, /* * We assume there will be no idle load after this update and clear * the has_blocked flag. If a cpu enters idle in the mean time, it will - * set the has_blocked flag and trig another update of idle load. + * set the has_blocked flag and trigger another update of idle load. * Because a cpu that becomes idle, is added to idle_cpus_mask before * setting the flag, we are sure to not clear the state and not * check the load of an idle cpu. + * + * Same applies to idle_cpus_mask vs needs_update. */ if (flags & NOHZ_STATS_KICK) WRITE_ONCE(nohz.has_blocked, 0); + if (flags & NOHZ_NEXT_KICK) + WRITE_ONCE(nohz.needs_update, 0); /* * Ensures that if we miss the CPU, we must see the has_blocked @@ -10597,6 +10606,8 @@ static void _nohz_idle_balance(struct rq *this_rq, unsigned int flags, if (need_resched()) { if (flags & NOHZ_STATS_KICK) has_blocked_load = true; + if (flags & NOHZ_NEXT_KICK) + WRITE_ONCE(nohz.needs_update, 1); goto abort; } -- 2.25.1