From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id CFB6734D92B; Fri, 21 Nov 2025 13:37:24 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1763732244; cv=none; b=dVwDRuAotnvdkx/dXdmM1aIIdzjxmgS47XRo7Pt7xTGMERc8s3NXLY+JBysWrKH1bBctDrt3IQPGjmSFdbSmdHD4MS6eMH7epgVyOnFWJDu9vRrqNdv7YSTbA0CRniz+cCU1Ya8NM6kU8yjhkGNb68aQK7qvj/SWsuXfTuA6fiM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1763732244; c=relaxed/simple; bh=BzBRfG9dRhn7kJrHSZUg8JCRFH3F51W5ga0ScgNPKZw=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=K6qlf5rWgCMGNBHOoAeSbmH75atczkoXelpgXivlUagR8cDspcCxZh/o0abVBA/Pgsw4YfJ4m9pWrpWtDGpRRVUKOtorxQtn14W7YEFIlubRvZOBQ0imi5eKjuJUY2AC1beVbmkRyObsXO+zchgEdSsLJ8ej11sGgDnxHaLYFms= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=AgjIS5lp; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="AgjIS5lp" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 4C272C4CEFB; Fri, 21 Nov 2025 13:37:24 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1763732244; bh=BzBRfG9dRhn7kJrHSZUg8JCRFH3F51W5ga0ScgNPKZw=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=AgjIS5lplZcFm41WnsPL+Ca88JxxH2SmGLqLIME5m5QI21RH0vlR1XiKTJzh5+XhH FfXIASAt4Fsq7pqp2DChsmlQTKKXhsOrUwaHrm/bsnzdJ0cvLXTf3NzvVeWcSdHQve 9YwUroPudGqFVfMJgOVRbgB+WYob+UF1CJDiEFws= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Vincent Guittot , Pierre Gondois , Ingo Molnar , Dietmar Eggemann , Qais Yousef , John Stultz Subject: [PATCH 6.6 052/529] sched/fair: Use all little CPUs for CPU-bound workloads Date: Fri, 21 Nov 2025 14:05:51 +0100 Message-ID: <20251121130232.863450294@linuxfoundation.org> X-Mailer: git-send-email 2.52.0 In-Reply-To: <20251121130230.985163914@linuxfoundation.org> References: <20251121130230.985163914@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.6-stable review patch. If anyone has any objections, please let me know. ------------------ From: Pierre Gondois commit 3af7524b14198f5159a86692d57a9f28ec9375ce upstream. 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. Behavior before the 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 <= 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: detach_tasks() \-case migrate_util: \-if (util > env->imbalance) goto next; After the 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/ Suggested-by: Vincent Guittot Signed-off-by: Pierre Gondois Signed-off-by: Ingo Molnar Reviewed-by: Vincent Guittot Reviewed-by: Dietmar Eggemann Acked-by: Qais Yousef Link: https://lore.kernel.org/r/20231206090043.634697-1-pierre.gondois@arm.com Cc: John Stultz Signed-off-by: Greg Kroah-Hartman --- kernel/sched/fair.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -9165,7 +9165,7 @@ static int detach_tasks(struct lb_env *e case migrate_util: util = task_util_est(p); - if (util > env->imbalance) + if (shr_bound(util, env->sd->nr_balance_failed) > env->imbalance) goto next; env->imbalance -= util;