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=-15.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_2 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 73476C433ED for ; Mon, 19 Apr 2021 02:18:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 48369610E7 for ; Mon, 19 Apr 2021 02:18:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237260AbhDSCSb (ORCPT ); Sun, 18 Apr 2021 22:18:31 -0400 Received: from shelob.surriel.com ([96.67.55.147]:55336 "EHLO shelob.surriel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233084AbhDSCSa (ORCPT ); Sun, 18 Apr 2021 22:18:30 -0400 Received: from [2603:3005:d05:2b00:6e0b:84ff:fee2:98bb] (helo=imladris.surriel.com) by shelob.surriel.com with esmtpsa (TLS1.2) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.94) (envelope-from ) id 1lYJU3-0001eJ-PD; Sun, 18 Apr 2021 22:17:55 -0400 Date: Sun, 18 Apr 2021 22:17:51 -0400 From: Rik van Riel To: linux-kernel@vger.kernel.org Cc: kernel-team@fb.com, Peter Zijlstra , Ingo Molnar , Vincent Guittot , Dietmar Eggemann , Mel Gorman Subject: [PATCH] sched,fair: skip newidle_balance if a wakeup is pending Message-ID: <20210418221751.7edfc03b@imladris.surriel.com> X-Mailer: Claws Mail 3.17.6 (GTK+ 2.24.32; x86_64-redhat-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: riel@shelob.surriel.com Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org The try_to_wake_up function has an optimization where it can queue a task for wakeup on its previous CPU, if the task is still in the middle of going to sleep inside schedule(). Once schedule() re-enables IRQs, the task will be woken up with an IPI, and placed back on the runqueue. If we have such a wakeup pending, there is no need to search other CPUs for runnable tasks. Just skip (or bail out early from) newidle balancing, and run the just woken up task. For a memcache like workload test, this reduces total CPU use by about 2%, proportionally split between user and system time, and p99 and p95 application response time by 2-3% on average. The schedstats run_delay number shows a similar improvement. Signed-off-by: Rik van Riel --- kernel/sched/fair.c | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 69680158963f..19a92c48939f 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -7163,6 +7163,14 @@ done: __maybe_unused; if (!rf) return NULL; + /* + * We have a woken up task pending here. No need to search for ones + * elsewhere. This task will be enqueued the moment we unblock irqs + * upon exiting the scheduler. + */ + if (rq->ttwu_pending) + return NULL; + new_tasks = newidle_balance(rq, rf); /* @@ -10661,7 +10669,8 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf) * Stop searching for tasks to pull if there are * now runnable tasks on this rq. */ - if (pulled_task || this_rq->nr_running > 0) + if (pulled_task || this_rq->nr_running > 0 || + this_rq->ttwu_pending) break; } rcu_read_unlock(); -- 2.25.4