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=-13.0 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,NICE_REPLY_A, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 88BCCC433DF for ; Mon, 3 Aug 2020 08:16:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6CBB820678 for ; Mon, 3 Aug 2020 08:16:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726075AbgHCIQu (ORCPT ); Mon, 3 Aug 2020 04:16:50 -0400 Received: from foss.arm.com ([217.140.110.172]:53810 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725855AbgHCIQu (ORCPT ); Mon, 3 Aug 2020 04:16:50 -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 DAEF5D6E; Mon, 3 Aug 2020 01:16:49 -0700 (PDT) Received: from [192.168.178.2] (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id D98573F718; Mon, 3 Aug 2020 01:16:38 -0700 (PDT) From: Dietmar Eggemann Subject: Re: [PATCH] sched/fair: reduce preemption with IDLE tasks runable To: Jiang Biao , mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com, vincent.guittot@linaro.org Cc: rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, linux-kernel@vger.kernel.org, Jiang Biao References: <20200801023248.90104-1-benbjiang@gmail.com> Message-ID: <5ed0fd46-3a3d-3c1a-5d75-03a74864e640@arm.com> Date: Mon, 3 Aug 2020 10:16:37 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20200801023248.90104-1-benbjiang@gmail.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 01/08/2020 04:32, Jiang Biao wrote: > From: Jiang Biao > > No need to preempt when there are only one runable CFS task with > other IDLE tasks on runqueue. The only one CFS task would always > be picked in that case. > > Signed-off-by: Jiang Biao > --- > kernel/sched/fair.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 04fa8dbcfa4d..8fb80636b010 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -4527,7 +4527,7 @@ entity_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr, int queued) > return; > #endif > > - if (cfs_rq->nr_running > 1) > + if (cfs_rq->nr_running > cfs_rq.idle_h_nr_running + 1) cfs_rq is a pointer. > check_preempt_tick(cfs_rq, curr); > } You can't compare cfs_rq->nr_running with cfs_rq->idle_h_nr_running! There is a difference between cfs_rq->h_nr_running and cfs_rq->nr_running. The '_h_' stands for hierarchical. The former gives you hierarchical task accounting whereas the latter is the number of sched entities (representing tasks or taskgroups) enqueued in cfs_rq. In entity_tick(), cfs_rq->nr_running has to be used for the condition to call check_preempt_tick(). We want to check if curr has to be preempted by __pick_first_entity(cfs_rq) on this cfs_rq. entity_tick() is called for each sched entity (and so for each cfs_rq_of(se)) of the task group hierarchy (e.g. task p running in taskgroup /A/B : se(p) -> se(A/B) -> se(A)).