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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 685ABC4167B for ; Wed, 29 Nov 2023 12:16:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233272AbjK2MQa (ORCPT ); Wed, 29 Nov 2023 07:16:30 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:51254 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233253AbjK2MQ1 (ORCPT ); Wed, 29 Nov 2023 07:16:27 -0500 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 3A82310C3 for ; Wed, 29 Nov 2023 04:16:34 -0800 (PST) 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 2239C2F4; Wed, 29 Nov 2023 04:17:21 -0800 (PST) Received: from [192.168.178.6] (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 3BFB63F5A1; Wed, 29 Nov 2023 04:16:32 -0800 (PST) Message-ID: <764e015b-b987-4722-b088-56da22848644@arm.com> Date: Wed, 29 Nov 2023 13:16:30 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] sched/fair: Use all little CPUs for CPU-bound workload Content-Language: en-US To: Pierre Gondois , linux-kernel@vger.kernel.org Cc: Qais Yousef , Vincent Guittot , Ingo Molnar , Peter Zijlstra , Juri Lelli , Steven Rostedt , Ben Segall , Mel Gorman , Daniel Bristot de Oliveira , Valentin Schneider References: <20231124153323.3202444-1-pierre.gondois@arm.com> <980a1a52-80a1-4d3b-8931-5f9a6546bbf5@arm.com> From: Dietmar Eggemann In-Reply-To: <980a1a52-80a1-4d3b-8931-5f9a6546bbf5@arm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 29/11/2023 11:48, Pierre Gondois wrote: > > > On 11/29/23 09:55, Dietmar Eggemann wrote: >> On 24/11/2023 16:33, Pierre Gondois wrote: [...] >>> Testing workload ([1]): >>> Spawn 6 CPU-bound tasks. During the first 100ms (step 1), each tasks >>> is affine to a CPU, except for: >>> - one little CPU which is left idle. >>> - one big CPU which has 2 tasks affine. >>> After the 100ms (step 2), remove the cpumask affinity. >>> >>> Before patch: >>> During step 2, the load balancer running from the idle CPU tags sched >>> domains as: >>> - little CPUs: 'group_has_spare'. Indeed, 3 CPU-bound tasks run on a >>>    4 CPUs sched-domain, and the idle CPU provides enough spare >>>    capacity. >> >> What is meant by 'idle CPU provides enough spare capacity? I thought the >> task (util_avg ~ 512_ does not fit on the sched group [1,3-5] when we >> consider util_avg/capacity (383) > > Right, I meant that when evaluating the 'group_type', there is enough spare > capacity when summing the utilization of CPUs in the the MC sched domain: > > --- > group_has_capacity() > { >     [...] >     if ((sgs->group_capacity * 100) > >             (sgs->group_util * imbalance_pct)) >         return true; >     [...] > } I see. But doesn't group_has_capacity() already return true because of `if (sgs->sum_nr_running < sgs->group_weight)` in this case (3 < 4)? Maybe you can make this clearer when sending the v3 with the Reviewed-By tags? > --- > >> >> The calculated imbalance of ~350 is too small for the task-size and >> that's why we need the 'shr_bound(util, env->sd->nr_balance_failed)' to >> let the task load-balance if nr_balance_failed = 2? > > Yes exact, the tasks are too big and cannot fit this imbalance value > (representing the available spare capacity in the little CPUs in this > case). > > 'shr_bound(...)' allows to progressively reduce the size of the tasks and > allow migrations after having tried to balance 'nr_balance_failed' times. [...]