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 24A17C197A0 for ; Thu, 16 Nov 2023 16:34:10 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1344251AbjKPQeL (ORCPT ); Thu, 16 Nov 2023 11:34:11 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42194 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230473AbjKPQeJ (ORCPT ); Thu, 16 Nov 2023 11:34:09 -0500 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id E409119B for ; Thu, 16 Nov 2023 08:34:05 -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 92C461595; Thu, 16 Nov 2023 08:34:51 -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 7FF603F73F; Thu, 16 Nov 2023 08:34:03 -0800 (PST) Message-ID: <3249da56-b5b8-4f10-a148-8a221cdf269a@arm.com> Date: Thu, 16 Nov 2023 17:34:01 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] sched/fair: Use all little CPUs for CPU-bound workload Content-Language: en-US To: Pierre Gondois , linux-kernel@vger.kernel.org Cc: Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Steven Rostedt , Ben Segall , Mel Gorman , Daniel Bristot de Oliveira , Valentin Schneider References: <20231110125902.2152380-1-pierre.gondois@arm.com> From: Dietmar Eggemann In-Reply-To: <20231110125902.2152380-1-pierre.gondois@arm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/11/2023 13:59, Pierre Gondois wrote: > Running n CPU-bound tasks on an n CPUs platform with asymmetric CPU > capacity 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. I used your workload on my Juno-r0 with LISA (rt-app overwrites the mainline CPU capacity values [446 1024 1024 446 446 466] to [675 1024 1024 675 675 675] to adapt to rt-app busy loop's instruction mix. Here I can't see the issue you bring up here. The two tasks sharing a CPU have util_avg = ~512 and the load-balance from one task to the idle little CPU is happening, I assume it's the diff in CPU capacity of the little CPUs: 383 < 512 < 675 ? > > 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. > - big CPUs: 'group_overloaded'. Indeed, 3 tasks run on a 2 CPUs > sched-domain, so the following path is used: > group_is_overloaded() > \-if (sgs->sum_nr_running <= sgs->group_weight) return true; > > The following path which would change the migration type to > 'migrate_task' is not taken: > calculate_imbalance() > \-if (env->idle != CPU_NOT_IDLE && env->imbalance == 0) > as the local group has some spare capacity, so the imbalance > is not 0. > > The migration type requested is 'migrate_util' and the busiest > runqueue is the big CPU's runqueue having 2 tasks (each having a > utilization of 512). The idle little CPU cannot pull one of these > task as its capacity is too small for the task. The following path > is used: Ah, here you're describing the issue I mentioned above. > detach_tasks() > \-case migrate_util: > \-if (util > env->imbalance) goto next; > > After patch: > When the local group has spare capacity and the busiest group is at > least tagged as 'group_fully_busy', if the local group has more CPUs > than CFS tasks and the busiest group more CFS tasks than CPUs, > request a 'migrate_task' type migration. > > Improvement: > Running the testing workload [1] with the step 2 representing > a ~10s load for a big CPU: > Before patch: ~19.3s > After patch: ~18s (-6.7%) > > The issue only happens at the DIE level on platforms able to have > 'migrate_util' migration types, i.e. no DynamIQ systems where > SD_SHARE_PKG_RESOURCES is set. Right, mainline Arm DynamIQ should only have 1 SD level (MC). Android might still be affected since they run MC and DIE. > > Signed-off-by: Pierre Gondois > --- > kernel/sched/fair.c | 17 +++++++++++++++++ > 1 file changed, 17 insertions(+) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index df348aa55d3c..5a215c96d420 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -10495,6 +10495,23 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s > env->imbalance = max(local->group_capacity, local->group_util) - > local->group_util; > > + /* > + * On an asymmetric system with CPU-bound tasks, a > + * migrate_util balance might not be able to migrate a > + * task from a big to a little CPU, letting a little > + * CPU unused. > + * If local has an empty CPU and busiest is overloaded, > + * balance one task with a migrate_task migration type > + * instead. > + */ > + if (env->sd->flags & SD_ASYM_CPUCAPACITY && > + local->sum_nr_running < local->group_weight && > + busiest->sum_nr_running > busiest->group_weight) { > + env->migration_type = migrate_task; > + env->imbalance = 1; > + return; > + } > + > /* > * In some cases, the group's utilization is max or even > * higher than capacity because of migrations but the