From nobody Wed Nov 27 14:34:18 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 140B81A76D5; Wed, 9 Oct 2024 18:00:46 +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=1728496847; cv=none; b=QhpoVUi5hwecunHmO1lhP+DJBVwvlI+GAGepFNOklODPxCw5RMRlt4/QIB+SrIMpvwWYOsmO1zvxX0q/8PXE/NjisFGOy5ftKE/8K5c0dir3KYFjspM+2TNv708I+lect5u2otwUNfRudkWqKyR+utVh9VbHPMH+iJWNRkJZCKo= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1728496847; c=relaxed/simple; bh=ChclQjdk1wWpYrsOxGeef20vXWr7m8b4FD5ACzrOUE0=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version; b=WhEYlHBMkqgzH9hYygxu05uWJhHyv+KG+Dn3vTcYJH14Ug7v9+KQ8+ZBAKZk64zVnGOji0W7T4heL8QrG1WJlN+KcXlCS4Ut6qJzHvLqv4yZBJWFjkqVr2ohcsEyYHhyz3aIdzt9ueV5pvhX05HK6f+IafPFwAwh7GVMU+sOS+E= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=UloGKvEj; 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="UloGKvEj" Received: by smtp.kernel.org (Postfix) with ESMTPSA id AB5F7C4CECC; Wed, 9 Oct 2024 18:00:46 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1728496846; bh=ChclQjdk1wWpYrsOxGeef20vXWr7m8b4FD5ACzrOUE0=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=UloGKvEjq5TAcI62jhgjmcZ0N9cY+N11CprmxYp48Lp9TbqchMs5ZPJjdTgLbuEZ4 VrjqgeK1hYHexzCx62kSUX/mhiE5OJzH99/N+DTLKYv4aYm9/5dwshq/QjSKgcPtqu +DQGkHSb9/YqyNW7iai6ZUDCN0BAXPVvvAoemfd2784Ys3d0XO0LXX0uf1KutzH8WE F+vvl0AMKQlPap7w71QoVcU7lIB+jCORGB94VHToTUhcKMX1+tjHBN52ASJxhSQu1w cQAwxJ1hla7QZy7hNJfP64yNgVKtS4hMGU0xJUi4Vb4nc2GiOlsJbjIVKlUOS0oGi+ zKQQmKsvdIF2w== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 693FECE08E5; Wed, 9 Oct 2024 11:00:46 -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" , Olivier Langlois Subject: [PATCH rcu 1/7] rcu: Add rcuog kthreads to RCU_NOCB_CPU help text Date: Wed, 9 Oct 2024 11:00:39 -0700 Message-Id: <20241009180045.777721-1-paulmck@kernel.org> X-Mailer: git-send-email 2.40.1 In-Reply-To: <2dc6de07-374d-44da-82e9-fa9d9c516b46@paulmck-laptop> References: <2dc6de07-374d-44da-82e9-fa9d9c516b46@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" The RCU_NOCB_CPU help text currently fails to mention rcuog kthreads, so this commit adds this information. Reported-by: Olivier Langlois Signed-off-by: Paul E. McKenney --- kernel/rcu/Kconfig | 28 ++++++++++++++++++---------- 1 file changed, 18 insertions(+), 10 deletions(-) diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig index 3e079de0f5b43..b9b6bc55185db 100644 --- a/kernel/rcu/Kconfig +++ b/kernel/rcu/Kconfig @@ -249,16 +249,24 @@ config RCU_NOCB_CPU workloads will incur significant increases in context-switch rates. =20 - This option offloads callback invocation from the set of CPUs - specified at boot time by the rcu_nocbs parameter. For each - such CPU, a kthread ("rcuox/N") will be created to invoke - callbacks, where the "N" is the CPU being offloaded, and where - the "x" is "p" for RCU-preempt (PREEMPTION kernels) and "s" for - RCU-sched (!PREEMPTION kernels). Nothing prevents this kthread - from running on the specified CPUs, but (1) the kthreads may be - preempted between each callback, and (2) affinity or cgroups can - be used to force the kthreads to run on whatever set of CPUs is - desired. + This option offloads callback invocation from the set of + CPUs specified at boot time by the rcu_nocbs parameter. + For each such CPU, a kthread ("rcuox/N") will be created to + invoke callbacks, where the "N" is the CPU being offloaded, + and where the "x" is "p" for RCU-preempt (PREEMPTION kernels) + and "s" for RCU-sched (!PREEMPTION kernels). This option + also creates another kthread for each sqrt(nr_cpu_ids) CPUs + ("rcuog/N", where N is the first CPU in that group to come + online), which handles grace periods for its group. Nothing + prevents these kthreads from running on the specified CPUs, + but (1) the kthreads may be preempted between each callback, + and (2) affinity or cgroups can be used to force the kthreads + to run on whatever set of CPUs is desired. + + The sqrt(nr_cpu_ids) grouping may be overridden using the + rcutree.rcu_nocb_gp_stride kernel boot parameter. This can + be especially helpful for smaller numbers of CPUs, where + sqrt(nr_cpu_ids) can be a bit of a blunt instrument. =20 Say Y here if you need reduced OS jitter, despite added overhead. Say N here if you are unsure. --=20 2.40.1