From nobody Tue Nov 26 20:18:31 2024 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 AD28420F5AA; Wed, 16 Oct 2024 16:19:33 +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=1729095573; cv=none; b=X6o9C53a+5rH6Ua4NaeqjYoPsRC9iaorYphILQFC2FubTgwuQsUEh+72dlZsQwzK6iHDHRIOlaJI068c3UC7rY88lRIXsTklEA5lznqPEdi5Kx80RjqSWm7XhQK53jJvQP/4N/OmvnobT0MJXblM0kpfBtGctAhO1G4yWowbOfc= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1729095573; c=relaxed/simple; bh=b/Ydri4xRhgAwhSosvwIWIB9nRy/kHsOi/HMFdJm+Qk=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version; b=uqiC2Ceh7m+rWBt9Mf8l0eGV9rRvQO5BVJYRr6okVDM8cuOQwqlK1t82+pdvO4IQrj71oxtW1HygpC26w9dYPR9KBiiAV198xzCwyNjgqXpabkGwViOsX/aJH+DyjxS4rfhJoZa23TO81KI39H0C3RcEER+HOWTMkBuFHrKK63s= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=b/+H1O2n; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="b/+H1O2n" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 50B02C4CED0; Wed, 16 Oct 2024 16:19:33 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1729095573; bh=b/Ydri4xRhgAwhSosvwIWIB9nRy/kHsOi/HMFdJm+Qk=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=b/+H1O2nyM7yqxyhnNoIRnVbd3KlUXN5OdWSyBpJdU+tcdrBcP3H8e25dNWN5dxsU WehNWhQrCXxTq7eWm96j8teeXaPSQxmmjHwAj9BrLqf9aXwttXwozD/66Uywfir4HM 2KGdkK5/BGWDYyYcCqUpvsz++LLnFcNg2m9XDr8Z35tdWefkK7zKlk1YXrGThlyKgl Es8UE4/rd6WTfPIea9RaRgNo0oIn0i9CJJc6zD7Gacz3f5Ln6FVdLKthfqPcfw9qTc OGFP/4J9Ak9APcpNAssJfObx1zgiIoUVxb/I6hEm8OYYzDHRoQpSABpCRWvFwSwZF/ dDBda1fD3S0kQ== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id EBDFFCE0F9A; Wed, 16 Oct 2024 09:19:32 -0700 (PDT) From: "Paul E. McKenney" To: frederic@kernel.org, rcu@vger.kernel.org Cc: linux-kernel@vger.kernel.org, kernel-team@meta.com, rostedt@goodmis.org, "Paul E. McKenney" , Joel Fernandes Subject: [PATCH v2 rcu 3/3] rcu: Finer-grained grace-period-end checks in rcu_dump_cpu_stacks() Date: Wed, 16 Oct 2024 09:19:31 -0700 Message-Id: <20241016161931.478592-3-paulmck@kernel.org> X-Mailer: git-send-email 2.40.1 In-Reply-To: <92193018-8624-495e-a685-320119f78db1@paulmck-laptop> References: <92193018-8624-495e-a685-320119f78db1@paulmck-laptop> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" This commit pushes the grace-period-end checks further down into rcu_dump_cpu_stacks(), and also uses lockless checks coupled with finer-grained locking. The result is that the current leaf rcu_node structure's ->lock is acquired only if a stack backtrace might be needed from the current CPU, and is held across only that CPU's backtrace. As a result, if there are no stalled CPUs associated with a given rcu_node structure, then its ->lock will not be acquired at all. On large systems, it is usually (though not always) the case that a small number of CPUs are stalling the current grace period, which means that the ->lock need be acquired only for a small fraction of the rcu_node structures. Signed-off-by: Paul E. McKenney Reviewed-by: Joel Fernandes (Google) --- kernel/rcu/tree_stall.h | 18 +++++++++++------- 1 file changed, 11 insertions(+), 7 deletions(-) diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h index b530844becf85..8994391b95c76 100644 --- a/kernel/rcu/tree_stall.h +++ b/kernel/rcu/tree_stall.h @@ -342,20 +342,24 @@ static void rcu_dump_cpu_stacks(unsigned long gp_seq) struct rcu_node *rnp; =20 rcu_for_each_leaf_node(rnp) { - if (gp_seq !=3D data_race(rcu_state.gp_seq)) { - pr_err("INFO: Stall ended during stack backtracing.\n"); - return; - } printk_deferred_enter(); - raw_spin_lock_irqsave_rcu_node(rnp, flags); - for_each_leaf_node_possible_cpu(rnp, cpu) + for_each_leaf_node_possible_cpu(rnp, cpu) { + if (gp_seq !=3D data_race(rcu_state.gp_seq)) { + printk_deferred_exit(); + pr_err("INFO: Stall ended during stack backtracing.\n"); + return; + } + if (!(data_race(rnp->qsmask) & leaf_node_cpu_bit(rnp, cpu))) + continue; + raw_spin_lock_irqsave_rcu_node(rnp, flags); if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu)) { if (cpu_is_offline(cpu)) pr_err("Offline CPU %d blocking current GP.\n", cpu); else dump_cpu_task(cpu); + raw_spin_unlock_irqrestore_rcu_node(rnp, flags); } - raw_spin_unlock_irqrestore_rcu_node(rnp, flags); + } printk_deferred_exit(); } } --=20 2.40.1