From nobody Sat Dec 27 03:14:22 2025 Received: from galois.linutronix.de (Galois.linutronix.de [193.142.43.55]) (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 A01DCFC03; Sat, 23 Dec 2023 16:09:56 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linutronix.de Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linutronix.de Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="jipBEsw2"; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="a7otgDy5" Date: Sat, 23 Dec 2023 16:09:48 -0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1703347789; h=from:from:sender:sender:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=bT6h0tGdiOTZ3K5/IhOutd8TItu/QUf8/c+iv/bvYKQ=; b=jipBEsw2xZ9YofDD8+6CrKiJwWABMlDR8SJPq78Q2hBZJKLc/TyRUdqMb782SKUG6acF6l 2Xb1IvW3OD8wLs0W5620pi/fZbmTiDELOiwbgdW2mzsFPNPP95jmDKPgxbpPb66nLr8Iux orxOyVkveMSxM3daCGCNF1yEdnPuED/IausmOe/aMP4TSQxNUVFNwxNs2ckDELdQKhNjH/ CErRcPqTJwySnYPVb6xOKP/PZChPa2TyKzUSeQflGz7KY72yCemhOCK96hJQzj3ISrycPY IapCln2HhUSIqzoTOUaFX1IdtrIgP1031dyTuRqh6caRMTVTtCsFPaTFKoEA6A== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1703347789; h=from:from:sender:sender:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=bT6h0tGdiOTZ3K5/IhOutd8TItu/QUf8/c+iv/bvYKQ=; b=a7otgDy55+CoGiuCBRYNXxxCob1f647k060+fa6cX7/qUc7T6D66aS6AGURJAOFbzxkLF4 Ig6u4KMGHaxbRYCg== From: "tip-bot2 for Pierre Gondois" Sender: tip-bot2@linutronix.de Reply-to: linux-kernel@vger.kernel.org To: linux-tip-commits@vger.kernel.org Subject: [tip: sched/core] sched/fair: Use all little CPUs for CPU-bound workloads Cc: Vincent Guittot , Pierre Gondois , Ingo Molnar , Dietmar Eggemann , Qais Yousef , x86@kernel.org, linux-kernel@vger.kernel.org In-Reply-To: <20231206090043.634697-1-pierre.gondois@arm.com> References: <20231206090043.634697-1-pierre.gondois@arm.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Message-ID: <170334778858.398.14876709177297398482.tip-bot2@tip-bot2> Robot-ID: Robot-Unsubscribe: Contact to get blacklisted from these emails Precedence: bulk Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable The following commit has been merged into the sched/core branch of tip: Commit-ID: 3af7524b14198f5159a86692d57a9f28ec9375ce Gitweb: https://git.kernel.org/tip/3af7524b14198f5159a86692d57a9f28e= c9375ce Author: Pierre Gondois AuthorDate: Wed, 06 Dec 2023 10:00:43 +01:00 Committer: Ingo Molnar CommitterDate: Sat, 23 Dec 2023 16:06:36 +01:00 sched/fair: Use all little CPUs for CPU-bound workloads 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 <=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 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 --- 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 823dd76..1d561b5 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -9071,7 +9071,7 @@ 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;