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 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8F991C433F5 for ; Tue, 5 Oct 2021 14:13:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 78ABE611C0 for ; Tue, 5 Oct 2021 14:13:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236632AbhJEOPd (ORCPT ); Tue, 5 Oct 2021 10:15:33 -0400 Received: from Galois.linutronix.de ([193.142.43.55]:51280 "EHLO galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235889AbhJEOOJ (ORCPT ); Tue, 5 Oct 2021 10:14:09 -0400 Date: Tue, 05 Oct 2021 14:12:16 -0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1633443137; h=from:from:sender:sender:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=YUVeoxtJ8R5MVDOLzU/tizWswHflWbFyYnPycLT0/Vo=; b=0CcHuRjSEB6zaQAJfTv2SadRxOVJH91Fs8jq4H5WCbm0AsI0TiyPyam3Wb7MpIKURdds6p vKdKO4SvwdpVyPPIg7BWKuYbd4P0qJ1XCa/66DG1FBXQnPM7GWYpnV+Fp/ItxrA6PhPhZw reCqPH2uuzE/84n1aAtiJF4u7GZTXoszyWs69SvBgeaU8KPMHMwQCCNQPY2ca5U40vIvVp DXlP5F0czcBHoOzzP0ddGvmdvMwfhlySxfF4XHC5bs9mKjH5AUGEtIZNDRH4sn5hTenLo0 Samm2y3clvVGWyAvqMGLElJ8SkCHdSqLtw3lkdtUy/0KvjlPn5LJ5UKaXLu3PA== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1633443137; h=from:from:sender:sender:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=YUVeoxtJ8R5MVDOLzU/tizWswHflWbFyYnPycLT0/Vo=; b=/c9Zl/n15Z0qNchgIjIs/yJldJrpvLmnnlcaZOrH/c1aXqogPWN03oX1/xn6+THvya/+Ql +dUK/vGL2PCtgqCA== From: "tip-bot2 for Valentin Schneider" Sender: tip-bot2@linutronix.de Reply-to: linux-kernel@vger.kernel.org To: linux-tip-commits@vger.kernel.org Subject: [tip: sched/core] sched/fair: Trigger nohz.next_balance updates when a CPU goes NOHZ-idle Cc: Valentin Schneider , "Peter Zijlstra (Intel)" , Vincent Guittot , x86@kernel.org, linux-kernel@vger.kernel.org In-Reply-To: <20210823111700.2842997-3-valentin.schneider@arm.com> References: <20210823111700.2842997-3-valentin.schneider@arm.com> MIME-Version: 1.0 Message-ID: <163344313681.25758.6556207958432267712.tip-bot2@tip-bot2> Robot-ID: Robot-Unsubscribe: Contact to get blacklisted from these emails Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org The following commit has been merged into the sched/core branch of tip: Commit-ID: 7fd7a9e0caba10829b4f8db1aa7711b558681fd4 Gitweb: https://git.kernel.org/tip/7fd7a9e0caba10829b4f8db1aa7711b558681fd4 Author: Valentin Schneider AuthorDate: Mon, 23 Aug 2021 12:17:00 +01:00 Committer: Peter Zijlstra CommitterDate: Tue, 05 Oct 2021 15:51:31 +02:00 sched/fair: Trigger nohz.next_balance updates when a CPU goes NOHZ-idle 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 Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Vincent Guittot Link: https://lkml.kernel.org/r/20210823111700.2842997-3-valentin.schneider@arm.com --- 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 f4de7f5..6cc958e 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -5787,6 +5787,7 @@ 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; @@ -10450,6 +10451,9 @@ 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); } @@ -10546,12 +10550,13 @@ 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 @@ -10600,13 +10605,17 @@ 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 @@ -10630,6 +10639,8 @@ 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; }