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 03AFCC07CB1 for ; Wed, 29 Nov 2023 10:48:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230012AbjK2Ksh (ORCPT ); Wed, 29 Nov 2023 05:48:37 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33554 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229498AbjK2Ksg (ORCPT ); Wed, 29 Nov 2023 05:48:36 -0500 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 559DA10CE for ; Wed, 29 Nov 2023 02:48:41 -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 3E3632F4; Wed, 29 Nov 2023 02:49:28 -0800 (PST) Received: from [10.34.100.101] (e126645.nice.arm.com [10.34.100.101]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 719BA3F5A1; Wed, 29 Nov 2023 02:48:39 -0800 (PST) Message-ID: <980a1a52-80a1-4d3b-8931-5f9a6546bbf5@arm.com> Date: Wed, 29 Nov 2023 11:48:34 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] sched/fair: Use all little CPUs for CPU-bound workload To: Dietmar Eggemann , 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> Content-Language: en-US From: Pierre Gondois In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/29/23 09:55, Dietmar Eggemann wrote: > On 24/11/2023 16:33, Pierre Gondois wrote: >> Running n CPU-bound tasks on an n CPUs platform: >> - with asymmetric CPU capacity >> - not having SD_SHARE_PKG_RESOURCES flag set at the DIE > > nit: DIE is now called PKG on tip sched/core. > > f577cd57bfaa - sched/topology: Rename 'DIE' domain to 'PKG' (2023-10-12 > Peter Zijlstra) > >> sched domain level (i.e. not DynamIQ systems) >> might result in a task placement where two tasks run on a big CPU >> and none on a little CPU. This placement could be more optimal by >> using all CPUs. >> >> Testing platform: >> Juno-r2: >> - 2 big CPUs (1-2), maximum capacity of 1024 >> - 4 little CPUs (0,3-5), maximum capacity of 383 >> >> 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; [...] } --- > > 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. > > [...] > >> Similar issue reported at: >> https://lore.kernel.org/lkml/20230716014125.139577-1-qyousef@layalina.io/ >> >> v1: >> https://lore.kernel.org/all/20231110125902.2152380-1-pierre.gondois@arm.com/ >> >> Suggested-by: Vincent Guittot >> Signed-off-by: Pierre Gondois > > Even though this cures only classical big.LITTLE it might have a > positive effect on today's Arm DynamIQ Android systems with Phantom SDs > when running benchmarks like Geekbench. > > [...] > > Reviewed-by: Dietmar Eggemann >