[PATCH 3/6] rcu/nocb: Optimize kthreads and rdp initialization

[Frederic Weisbecker <frederic@kernel.org>]

Currently cpumask_available() is used to prevent from unwanted
NOCB initialization. However if neither "rcu_nocbs=" nor "nohz_full="
parameters are passed but CONFIG_CPUMASK_OFFSTACK=n, the initialization
path is still taken, running through all sorts of needless operations
and iterations on an empty cpumask.

Fix this with relying on a real initialization state instead. This
also optimize kthreads creation, sparing iteration over all online CPUs
when nocb isn't initialized.

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>
---
 kernel/rcu/tree_nocb.h | 24 +++++++++++++++++-------
 1 file changed, 17 insertions(+), 7 deletions(-)

diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index e1cb06840454..d8ed3ee47a67 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -60,6 +60,9 @@ static inline bool rcu_current_is_nocb_kthread(struct rcu_data *rdp)
  * Parse the boot-time rcu_nocb_mask CPU list from the kernel parameters.
  * If the list is invalid, a warning is emitted and all CPUs are offloaded.
  */
+
+static bool rcu_nocb_is_setup;
+
 static int __init rcu_nocb_setup(char *str)
 {
 	alloc_bootmem_cpumask_var(&rcu_nocb_mask);
@@ -67,6 +70,7 @@ static int __init rcu_nocb_setup(char *str)
 		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);
@@ -1159,13 +1163,17 @@ void __init rcu_init_nohz(void)
 		need_rcu_nocb_mask = true;
 #endif /* #if defined(CONFIG_NO_HZ_FULL) */
 
-	if (!cpumask_available(rcu_nocb_mask) && need_rcu_nocb_mask) {
-		if (!zalloc_cpumask_var(&rcu_nocb_mask, GFP_KERNEL)) {
-			pr_info("rcu_nocb_mask allocation failed, callback offloading disabled.\n");
-			return;
+	if (need_rcu_nocb_mask) {
+		if (!cpumask_available(rcu_nocb_mask)) {
+			if (!zalloc_cpumask_var(&rcu_nocb_mask, GFP_KERNEL)) {
+				pr_info("rcu_nocb_mask allocation failed, callback offloading disabled.\n");
+				return;
+			}
 		}
+		rcu_nocb_is_setup = true;
 	}
-	if (!cpumask_available(rcu_nocb_mask))
+
+	if (!rcu_nocb_is_setup)
 		return;
 
 #if defined(CONFIG_NO_HZ_FULL)
@@ -1267,8 +1275,10 @@ static void __init rcu_spawn_nocb_kthreads(void)
 {
 	int cpu;
 
-	for_each_online_cpu(cpu)
-		rcu_spawn_cpu_nocb_kthread(cpu);
+	if (rcu_nocb_is_setup) {
+		for_each_online_cpu(cpu)
+			rcu_spawn_cpu_nocb_kthread(cpu);
+	}
 }
 
 /* How many CB CPU IDs per GP kthread?  Default of -1 for sqrt(nr_cpu_ids). */
-- 
2.25.1