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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no 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 9539CC3A5A9 for ; Mon, 4 May 2020 15:48:15 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 790392063A for ; Mon, 4 May 2020 15:48:15 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728757AbgEDPsO (ORCPT ); Mon, 4 May 2020 11:48:14 -0400 Received: from foss.arm.com ([217.140.110.172]:47696 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725941AbgEDPsO (ORCPT ); Mon, 4 May 2020 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 8E5CB1FB; Mon, 4 May 2020 08:48:13 -0700 (PDT) Received: from e113632-lin (e113632-lin.cambridge.arm.com [10.1.194.46]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 1856A3F68F; Mon, 4 May 2020 08:48:11 -0700 (PDT) References: <20200503083407.GA27766@iZj6chx1xj0e0buvshuecpZ> User-agent: mu4e 0.9.17; emacs 26.3 From: Valentin Schneider To: Vincent Guittot Cc: Peng Liu , linux-kernel , Ingo Molnar , Peter Zijlstra , Juri Lelli , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman Subject: Re: [PATCH] sched/fair: Fix nohz.next_balance update In-reply-to: Date: Mon, 04 May 2020 16:48:06 +0100 Message-ID: MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 04/05/20 16:17, Vincent Guittot wrote: > On Sun, 3 May 2020 at 10:34, Peng Liu wrote: >> >> commit c5afb6a87f23 ("sched/fair: Fix nohz.next_balance update") >> During idle load balance, this_cpu(ilb) do load balance for the other >> idle CPUs, also gather the earliest (nohz.)next_balance. >> >> Since commit: >> 'b7031a02ec75 ("sched/fair: Add NOHZ_STATS_KICK")' >> >> We update nohz.next_balance like this: >> >> _nohz_idle_balance() { >> for_each_cpu(nohz.idle_cpus_mask) { >> rebalance_domains() { >> update nohz.next_balance <-- compare and update >> } >> } >> rebalance_domains(this_cpu) { >> update nohz.next_balance <-- compare and update >> } >> update nohz.next_balance <-- unconditionally update >> } >> >> For instance, nohz.idle_cpus_mask spans {cpu2,3,5,8}, and this_cpu is >> cpu5. After the above loop we could gather the earliest *next_balance* >> among {cpu2,3,8}, then rebalance_domains(this_cpu) update >> nohz.next_balance with this_rq->next_balance, but finally overwrite >> nohz.next_balance with the earliest *next_balance* among {cpu2,3,8}, >> we may end up with not getting the earliest next_balance. >> >> Since we can gather all the updated rq->next_balance, including this_cpu, >> in _nohz_idle_balance(), it's safe to remove the extra lines in >> rebalance_domains() which are originally intended for this_cpu. And >> finally the updating only happen in _nohz_idle_balance(). > > I'm not sure that's always true. Nothing prevents nohz_idle_balance() > to return false . Then run_rebalance_domains() calls > rebalance_domains(this_rq ,SCHED_IDLE) outside _nohz_idle_balance(). > In this case we must keep the code in rebalance_domains(). > > For example when the tick is not stopped when entering idle. Or when > need_resched() returns true. > I had missed that, good points. > So instead of removing the code from rebalance_domains, you should > move the one in _nohz_idle_balance() to make sure that the "if > (likely(update_next_balance)) ..." is called before calling > rebalance_domains for the local cpu > Why not just get rid of the update in _nohz_idle_balance() entirely then? The nohz.next_balance update in rebalance_domains() will always happen if it is required (and we have idle == CPU_IDLE), so the extra update in _nohz_idle_balance() doesn't seem to be any useful.