From nobody Tue Dec 16 05:42:24 2025 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 C845EC4167B for ; Wed, 6 Dec 2023 09:01:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1377152AbjLFJA5 (ORCPT ); Wed, 6 Dec 2023 04:00:57 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41258 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230357AbjLFJAv (ORCPT ); Wed, 6 Dec 2023 04:00:51 -0500 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 5756C1048D for ; Wed, 6 Dec 2023 01:00:53 -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 7B29F139F; Wed, 6 Dec 2023 01:01:38 -0800 (PST) Received: from e126645.nice.arm.com (e126645.nice.arm.com [10.34.100.101]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPA id 88BD73F762; Wed, 6 Dec 2023 01:00:50 -0800 (PST) From: Pierre Gondois To: linux-kernel@vger.kernel.org Cc: Qais Yousef , Pierre Gondois , Vincent Guittot , Dietmar Eggemann , Ingo Molnar , Peter Zijlstra , Juri Lelli , Steven Rostedt , Ben Segall , Mel Gorman , Daniel Bristot de Oliveira , Valentin Schneider Subject: [PATCH v3] sched/fair: Use all little CPUs for CPU-bound workload Date: Wed, 6 Dec 2023 10:00:43 +0100 Message-Id: <20231206090043.634697-1-pierre.gondois@arm.com> X-Mailer: git-send-email 2.25.1 MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" Running n CPU-bound tasks on an n CPUs platform: - with asymmetric CPU capacity - not being a DynamIq system (i.e. having a PKG level sched domain without the SD_SHARE_PKG_RESOURCES flag set) 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'. Cf. group_has_capacity() and group_is_overloaded(), 3 CPU-bound tasks run on a 4 CPUs sched-domain, and the idle CPU provides enough spare capacity regarding the imbalance_pct - 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 <=3D 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 !=3D CPU_NOT_IDLE && env->imbalance =3D=3D 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: detach_tasks() \-case migrate_util: \-if (util > env->imbalance) goto next; After patch: As the number of failed balancing attempts grows (with 'nr_balance_failed'), progressively make it easier to migrate a big task to the idling little CPU. A similar mechanism is used for the 'migrate_load' migration type. 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%) 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/ v2: https://lore.kernel.org/all/20231124153323.3202444-1-pierre.gondois@arm.com/ Suggested-by: Vincent Guittot Signed-off-by: Pierre Gondois Reviewed-by: Vincent Guittot Reviewed-by: Dietmar Eggemann --- Notes: v2: - Used Vincent's approach. v3: - Updated commit message. - Added Reviewed-by tags kernel/sched/fair.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index d7a3c63a2171..9481b8cff31b 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -9060,7 +9060,7 @@ static int detach_tasks(struct lb_env *env) case migrate_util: util =3D task_util_est(p); =20 - if (util > env->imbalance) + if (shr_bound(util, env->sd->nr_balance_failed) > env->imbalance) goto next; =20 env->imbalance -=3D util; --=20 2.25.1