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=-8.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SPF_PASS,USER_AGENT_GIT 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 BFD70C43387 for ; Thu, 17 Jan 2019 15:37:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 98F8D20855 for ; Thu, 17 Jan 2019 15:37:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727716AbfAQPhg (ORCPT ); Thu, 17 Jan 2019 10:37:36 -0500 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:41980 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725928AbfAQPhg (ORCPT ); Thu, 17 Jan 2019 10:37:36 -0500 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id F39F7EBD; Thu, 17 Jan 2019 07:37:35 -0800 (PST) Received: from e113632-lin.cambridge.arm.com (e113632-lin.cambridge.arm.com [10.1.194.37]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPA id 9CB1E3F694; Thu, 17 Jan 2019 07:37:34 -0800 (PST) From: Valentin Schneider To: linux-kernel@vger.kernel.org Cc: mingo@kernel.org, peterz@infradead.org, vincent.guittot@linaro.org, morten.rasmussen@arm.com, Dietmar.Eggemann@arm.com Subject: [PATCH 0/5] sched/fair: NOHZ cleanups and misfit improvement Date: Thu, 17 Jan 2019 15:34:06 +0000 Message-Id: <20190117153411.2390-1-valentin.schneider@arm.com> X-Mailer: git-send-email 2.20.1 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org In commit 5fbdfae5221a ("sched/fair: Kick nohz balance if rq->misfit_task_load") was added a trigger for nohz kicks, which is required to offload misfit tasks from LITTLE to big CPUs. However, those kicks could be issued a lot more frequently than what is strictly needed. This patch-set brings some unrelated nohz code cleanups, and tunes down unneeded nohz kicks. - Patches [1-3] do some cleaning up of the current nohz code - Patches [4-5] tweak the nohz kick conditions for asymmetric systems * Testing ** kick_ilb() hits This causes a large reduction in calls to kick_ilb() (and thus subsequent rescheduling interrupts & useless nohz balance calls) in most scenarios. The "best case" one is running NR_BIG_CPUS big tasks, which I tested with 4 50% periodic tasks running for 5 seconds on my HiKey960 (4x4 big.LITTLE): | CPU | hits (baseline) | hits (patchset) | |-----+-----------------+-----------------| | 0 | 31 | 41 | | 1 | 21 | 3 | | 2 | 35 | 2 | | 3 | 9 | 4 | |-----+-----------------+-----------------| | 4 | 170 | 4 | | 5 | 573 | 4 | | 6 | 544 | 4 | | 7 | 579 | 4 | Something a bit less idealistic with NR_CPUS-1 big tasks still shows some improvements (7 100% tasks running for 5 seconds on my HiKey960): | CPU | hits (baseline) | hits (patchset) | |-----+-----------------+-----------------| | 0 | 14 | 122 | | 1 | 47 | 162 | | 2 | 11 | 156 | | 3 | 9 | 3 | |-----+-----------------+-----------------| | 4 | 53 | 6 | | 5 | 276 | 13 | | 6 | 312 | 7 | | 7 | 250 | 11 | I was surprised to see such an increase in calls to kick_ilb() from LITTLE CPUs ([0-3]), but after a bit of investigation it turns out that the big CPUs would always run nohz_balancer_kick() a jiffy before the LITTLEs, so the LITTLEs would always bail out because nohz.next_balance had just been updated before they called nohz_balancer_kick(). IOW, time_before(now, nohz.next_balance) would always be true on CPUs [0-3] during my workload. Quieting the kicks issued by the big CPUs allowed the LITTLEs to execute nohz_balancer_kick() past that condition, explaining the higher number of kicks issued from LITTLE CPUs. ** misfit behaviour For good measure I also ran the usual misfit tests [1] which showed no particular change. * Notes Since patch 05 introduces yet another label in kick_nohz_balancer(), I benchmarked splitting its "slowpath" (all of the rcu derefs) into its own function and didn't find any real overhead. If folks don't hate the idea I can add it to a v2 or post it separately. [1]: https://github.com/ARM-software/lisa/blob/next/lisa/tests/kernel/scheduler/misfit.py Valentin Schneider (5): sched/fair: Use for_each_cpu_and for asym-packing nohz kicks sched/fair: Explain LLC nohz kick condition sched/fair: Prune nohz_balancer_kick() comment block sched/fair: Tune down misfit nohz kicks sched/fair: Skip LLC nohz logic for asymmetric systems kernel/sched/fair.c | 62 ++++++++++++++++++++++++++++++++------------- 1 file changed, 44 insertions(+), 18 deletions(-) -- 2.20.1