[PATCH 5/6] rcu/nocb: Allow empty "rcu_nocbs" kernel parameter

[Frederic Weisbecker <frederic@kernel.org>]

If a user wants to boot without any CPU in offloaded mode initially but
with the possibility to offload them later using cpusets, provide a way
to simply pass an empty "rcu_nocbs" kernel parameter which will enforce
the creation of dormant nocb kthreads.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Uladzislau Rezki <urezki@gmail.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Joel Fernandes <joel@joelfernandes.org>
---
 .../admin-guide/kernel-parameters.txt         | 26 ++++++++++++-------
 kernel/rcu/tree_nocb.h                        | 10 ++++---
 2 files changed, 23 insertions(+), 13 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index cd860dc7c60b..6ff1a5f06383 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4351,20 +4351,28 @@
 				Disable the Correctable Errors Collector,
 				see CONFIG_RAS_CEC help text.
 
-	rcu_nocbs=	[KNL]
-			The argument is a cpu list, as described above.
+	rcu_nocbs[=cpu-list]
+			[KNL] The optional argument is a cpu list,
+			as described above.
 
-			In kernels built with CONFIG_RCU_NOCB_CPU=y, set
-			the specified list of CPUs to be no-callback CPUs.
-			Invocation of these CPUs' RCU callbacks will be
-			offloaded to "rcuox/N" kthreads created for that
-			purpose, where "x" is "p" for RCU-preempt, and
-			"s" for RCU-sched, and "N" is the CPU number.
-			This reduces OS jitter on the offloaded CPUs,
+			In kernels built with CONFIG_RCU_NOCB_CPU=y, enable the
+			no-callback CPU mode. Invocation of such CPUs' RCU
+			callbacks will be offloaded to "rcuox/N" kthreads
+			created	for that purpose, where "x" is "p" for
+			RCU-preempt, and "s" for RCU-sched, and "N" is the CPU
+			number.	This reduces OS jitter on the offloaded CPUs,
 			which can be useful for HPC and real-time
 			workloads.  It can also improve energy efficiency
 			for asymmetric multiprocessors.
 
+			If a cpulist is passed as an argument, the specified
+			list of	CPUs is set to no-callback mode from boot.
+
+			If otherwise the '=' sign and the cpulist arguments are
+			omitted, no CPU will be set to no-callback mode from
+			boot but cpuset will allow for toggling that mode at
+			runtime.
+
 	rcu_nocb_poll	[KNL]
 			Rather than requiring that offloaded CPUs
 			(specified by rcu_nocbs= above) explicitly
diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 9d37916278d4..d915780d40c8 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -66,14 +66,16 @@ static bool rcu_nocb_is_setup;
 static int __init rcu_nocb_setup(char *str)
 {
 	alloc_bootmem_cpumask_var(&rcu_nocb_mask);
-	if (cpulist_parse(str, rcu_nocb_mask)) {
-		pr_warn("rcu_nocbs= bad CPU range, all CPUs set\n");
-		cpumask_setall(rcu_nocb_mask);
+	if (*str == '=') {
+		if (cpulist_parse(++str, rcu_nocb_mask)) {
+			pr_warn("rcu_nocbs= bad CPU range, all CPUs set\n");
+			cpumask_setall(rcu_nocb_mask);
+		}
 	}
 	rcu_nocb_is_setup = true;
 	return 1;
 }
-__setup("rcu_nocbs=", rcu_nocb_setup);
+__setup("rcu_nocbs", rcu_nocb_setup);
 
 static int __init parse_rcu_nocb_poll(char *arg)
 {
-- 
2.25.1