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@kernel.org>
Reported-by: Uladzislau Rezki <urezki@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>

1 files changed, 13 insertions, 0 deletions

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index bdf431bdbfdc..40a05df05580 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 by 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