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 5B6BB7DA9E; Thu, 15 Aug 2024 13:48:40 +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=1723729720; cv=none; b=qyxRDMRiRAJ558l+4c/ROyHQ1ZLQE77eKHrvw6ikM+3SSz55fYI5jwzxxXspxaL+ryf79rQB+0/hg8io53qPq7n6RAm0y7KsZhM43ryk5ptf7fg4W/m/OGQUfFtl2z6fKov5lAj4BvND3yC0qYeTnYVpRPNqdlTnrHFsY3+VKV0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1723729720; c=relaxed/simple; bh=FOd74EXQqUuy17xnoit/dt3rWWiIWGLmBfQKXAid1KY=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=TuKxtNUfc0oNfLgAeN5WmxbiD60fyS4cv0QMMLeWz3c86E7LUmEqVEcUemDmkqJToQfXAKA5UPXaCZISuYhkbEI4ynImo0eRU+lX7A7mnFxINrLv1Kqkib8RKRcydenb20X5luAbkMgx1yH01c5Z+2WRoo4DQsKoSasGbxSsV4U= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=PoshCPiW; 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="PoshCPiW" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D4E53C32786; Thu, 15 Aug 2024 13:48:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1723729720; bh=FOd74EXQqUuy17xnoit/dt3rWWiIWGLmBfQKXAid1KY=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=PoshCPiW/eOPMYLPB457jJ3E8rBAF4VKkLkFwTNSZTCag3Fwh3Qgn0l8Nl57/ZJLN jygfGyw0wKs0awtuyaNDLfO49wjGM1d8akATumZfC89hHmGGDrFqkF+l0/1UB3Rfl4 AZ0xf+JTk4Ph88pM8aNFVjADjBxtYthLewVr4evc= 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 Subject: [PATCH 5.15 186/484] sched/fair: Use all little CPUs for CPU-bound workloads Date: Thu, 15 Aug 2024 15:20:44 +0200 Message-ID: <20240815131948.600122473@linuxfoundation.org> X-Mailer: git-send-email 2.46.0 In-Reply-To: <20240815131941.255804951@linuxfoundation.org> References: <20240815131941.255804951@linuxfoundation.org> User-Agent: quilt/0.67 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 5.15-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 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 @@ -8241,7 +8241,7 @@ 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;