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=-5.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 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 F1716C433DB for ; Tue, 16 Mar 2021 19:07:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B7D0E65145 for ; Tue, 16 Mar 2021 19:07:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240287AbhCPTHS (ORCPT ); Tue, 16 Mar 2021 15:07:18 -0400 Received: from foss.arm.com ([217.140.110.172]:57300 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S240312AbhCPTGM (ORCPT ); Tue, 16 Mar 2021 15:06:12 -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 6ADDFD6E; Tue, 16 Mar 2021 12:06:12 -0700 (PDT) Received: from [192.168.178.6] (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 994123F70D; Tue, 16 Mar 2021 12:06:10 -0700 (PDT) Subject: Re: [PATCH v3 1/7] sched/fair: Ignore percpu threads for imbalance pulls To: Valentin Schneider , linux-kernel@vger.kernel.org Cc: Lingutla Chandrasekhar , Peter Zijlstra , Ingo Molnar , Vincent Guittot , Morten Rasmussen , Qais Yousef , Quentin Perret , Pavan Kondeti , Rik van Riel References: <20210311120527.167870-1-valentin.schneider@arm.com> <20210311120527.167870-2-valentin.schneider@arm.com> <87pmzzronf.mognet@arm.com> From: Dietmar Eggemann Message-ID: <0377d32d-52ac-76e1-4f73-8fa426136e8c@arm.com> Date: Tue, 16 Mar 2021 20:06:09 +0100 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: <87pmzzronf.mognet@arm.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 16/03/2021 18:31, Valentin Schneider wrote: > On 16/03/21 16:49, Dietmar Eggemann wrote: >> On 11/03/2021 13:05, Valentin Schneider wrote: >>> From: Lingutla Chandrasekhar >>> >>> In load balancing, when balancing group is unable to pull task >>> due to ->cpus_ptr constraints from busy group, then it sets >>> LBF_SOME_PINNED to lb env flags, as a consequence, sgc->imbalance >>> is set for its parent domain level. which makes the group >>> classified as imbalance to get help from another balancing cpu. >>> >>> Consider a 4-CPU big.LITTLE system with CPUs 0-1 as LITTLEs and >> >> Does it have to be a big.LITTLE system? I assume this issue also happens >> on an SMP system. >> > > Aye, though the consequences are "worse" on asym CPU capacity systems. I can only think of higher group_type 'group_imbalanced' eclipses 'group_misfit_task' here? > >>> CPUs 2-3 as Bigs with below scenario: >>> - CPU0 doing newly_idle balancing >>> - CPU1 running percpu kworker and RT task (small tasks) >> >> What's the role of the small RT task here in the story? >> > > I don't think it matters much here. Chandra already mentioned that this is part of the story, namely to start trying to move task on lb MC CPU1->CPU0 (if (busiest->nr_running > 1)). [...] >> This sentence mentioned per-cpu threads (and so does the patch name) but >> the implementation (only) deals with per-cpu kernel threads. IMHO, it >> would be good to align this. >> > > Tell you what, I'll go for: > 1) how can pcpu kthreads cause LBF_SOME_PINNED > 2) why we may not want this, but still ignore !kthread pcpu tasks > 3) why this is even more important for big.LITTLE LGTM.