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=-8.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS 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 D0AB3C07E9B for ; Mon, 19 Jul 2021 18:12:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A5E7B610FB for ; Mon, 19 Jul 2021 18:12:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1377890AbhGSRcP (ORCPT ); Mon, 19 Jul 2021 13:32:15 -0400 Received: from foss.arm.com ([217.140.110.172]:36276 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1346338AbhGSPsO (ORCPT ); Mon, 19 Jul 2021 11:48:14 -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 3B00F1FB; Mon, 19 Jul 2021 09:28:50 -0700 (PDT) Received: from e113632-lin (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 6C87B3F73D; Mon, 19 Jul 2021 09:28:49 -0700 (PDT) From: Valentin Schneider To: Dietmar Eggemann , linux-kernel@vger.kernel.org Cc: Peter Zijlstra , Ingo Molnar , Vincent Guittot Subject: Re: [PATCH v2 2/2] sched/fair: Trigger nohz.next_balance updates when a CPU goes NOHZ-idle In-Reply-To: References: <20210719103117.3624936-1-valentin.schneider@arm.com> <20210719103117.3624936-3-valentin.schneider@arm.com> Date: Mon, 19 Jul 2021 17:28:44 +0100 Message-ID: <878s22mfnn.mognet@arm.com> MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 19/07/21 17:24, Dietmar Eggemann wrote: > On 19/07/2021 12:31, Valentin Schneider wrote: > > [...] > >> @@ -10351,6 +10352,9 @@ static void nohz_balancer_kick(struct rq *rq) >> unlock: >> rcu_read_unlock(); >> out: >> + if (READ_ONCE(nohz.needs_update)) >> + flags |= NOHZ_NEXT_KICK; >> + > > Since NOHZ_NEXT_KICK is part of NOHZ_KICK_MASK, some conditions above > will already set it in flags. Is this intended? So if no kick would be issued (e.g. flags == 0 because nohz.next_balance is later in the future), then this does the right thing and issues a NOHZ_NEXT_KICK one. However you're right to point out that even if nohz.needs_update is false, we can set NOHZ_NEXT_KICK into the ilb rq's NOHZ flags due to it being included in NOHZ_KICK_MASK, which I think is a mistake. Looking at it now, it shouldn't be part of NOHZ_KICK_MASK. > >> if (flags) >> kick_ilb(flags); >> } >> @@ -10447,12 +10451,13 @@ 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 >> @@ -10501,13 +10506,17 @@ static void _nohz_idle_balance(struct rq *this_rq, unsigned int flags, > > function header would need update to incorporate the new 'update > nohz.next_balance' functionality. It only talks about 'update of blocked > load' and 'complete load balance' so far. > >> /* >> * 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 >> @@ -10531,6 +10540,8 @@ static void _nohz_idle_balance(struct rq *this_rq, unsigned int flags, >> if (need_resched()) { >> if (flags & NOHZ_STATS_KICK) >> has_blocked_load = true; > > This looks weird now? 'has_blocked_load = true' vs > 'WRITE_ONCE(nohz.needs_update, 1)'. > Well, has_blocked_load lets us factorize the nohz.has_blocked write (one is needed either when aborting or at the tail of the cpumask iteration), whereas there is just a single write for nohz.needs_update (when aborting). >> + if (flags & NOHZ_NEXT_KICK) >> + WRITE_ONCE(nohz.needs_update, 1); >> goto abort; >> } >> >>