linux-kernel - [PATCH rcu 3/3] doc: Document rcutree.nocb_nobypass_lim_per

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Message-Id: <20220620221757.3839212-3-paulmck@kernel.org>
Date:   Mon, 20 Jun 2022 15:17:57 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com,
        rostedt@...dmis.org, "Paul E. McKenney" <paulmck@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Uladzislau Rezki <urezki@...il.com>
Subject: [PATCH rcu 3/3] doc: Document rcutree.nocb_nobypass_lim_per_jiffy kernel parameter

This commit provides documentation for the kernel parameter controlling
RCU's handling of callback floods on offloaded (rcu_nocbs) CPUs.
This parameter might be obscure, but it is always there when you need it.

Reported-by: Frederic Weisbecker <frederic@...nel.org>
Reported-by: Uladzislau Rezki <urezki@...il.com>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 Documentation/admin-guide/kernel-parameters.txt | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index bdf431bdbfdc4..a30890141b1a5 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4681,6 +4681,19 @@
 			invoked in the context of an rcuoc kthread, which
 			scheduler will preempt as it does any other task.
 
+	rcutree.nocb_nobypass_lim_per_jiffy= [KNL]
+			On callback-offloaded (rcu_nocbs) CPUs,
+			RCU reduces the lock contention that would
+			otherwise be caused callback floods through
+			use of the ->nocb_bypass list.	However, in the
+			common non-flooded case, RCU queues directly to
+			the main ->cblist in order to avoid the extra
+			overhead of the ->nocb_bypass list and its lock.
+			But if there are too many callbacks queued during
+			a single jiffy, RCU pre-queues the callbacks into
+			the ->nocb_bypass queue.  The definition of "too
+			many" is supplied by this kernel boot parameter.
+
 	rcutree.rcu_nocb_gp_stride= [KNL]
 			Set the number of NOCB callback kthreads in
 			each group, which defaults to the square root
-- 
2.31.1.189.g2e36527f23