[PATCH RFC] rcu/nocb: Provide default all-CPUs mask for RCU_NOCB_CPU=y

[Joel Fernandes <joel@joelfernandes.org>]

On systems with CONFIG_RCU_NOCB_CPU=y, there is no default mask provided
which ends up not offloading any CPU. This patch removes yet another
dependency from the bootloader having to know about RCU, about how many
CPUs the system has, and about how to provide the mask. Basically, I
think we should stop pretending that the user knows what they are doing :).
In other words, if NO_CB_CPU is enabled, lets make use of it.

My goal is to make RCU as zero-config as possible with sane defaults. If
user wants to provide rcu_nocbs= or nohz_full= options, then those will
take precedence and this patch will have no effect.

I tested providing rcu_nocbs= option, ensuring that is preferred over this.

Signed-off-by: Joel Fernandes <joel@joelfernandes.org>
---
 kernel/rcu/tree_nocb.h | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index eeafb546a7a0..607fbf843467 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1165,12 +1165,25 @@ EXPORT_SYMBOL_GPL(rcu_nocb_cpu_offload);
 void __init rcu_init_nohz(void)
 {
 	int cpu;
-	bool need_rcu_nocb_mask = false;
+	bool need_rcu_nocb_mask = false, set_nocb_mask_all = false;
 	struct rcu_data *rdp;
 
+	/*
+	 * In case rcu_nocbs= was not passed on the kernel command line,
+	 * provide a sane default by offloading all CPUs. This provides a
+	 * sane default for rcu_nocbs and prevents users overlooking these
+	 * details.
+	 */
+	if (!rcu_nocb_is_setup) {
+		need_rcu_nocb_mask = true;
+		set_nocb_mask_all = true;
+	}
+
 #if defined(CONFIG_NO_HZ_FULL)
-	if (tick_nohz_full_running && cpumask_weight(tick_nohz_full_mask))
+	if (tick_nohz_full_running && cpumask_weight(tick_nohz_full_mask)) {
 		need_rcu_nocb_mask = true;
+		set_nocb_mask_all = false; /* NO_HZ_FULL provides its own mask. */
+	}
 #endif /* #if defined(CONFIG_NO_HZ_FULL) */
 
 	if (need_rcu_nocb_mask) {
@@ -1191,6 +1204,9 @@ void __init rcu_init_nohz(void)
 		cpumask_or(rcu_nocb_mask, rcu_nocb_mask, tick_nohz_full_mask);
 #endif /* #if defined(CONFIG_NO_HZ_FULL) */
 
+	if (set_nocb_mask_all)
+		cpumask_setall(rcu_nocb_mask);
+
 	if (!cpumask_subset(rcu_nocb_mask, cpu_possible_mask)) {
 		pr_info("\tNote: kernel parameter 'rcu_nocbs=', 'nohz_full', or 'isolcpus=' contains nonexistent CPUs.\n");
 		cpumask_and(rcu_nocb_mask, cpu_possible_mask,
-- 
2.35.1.1178.g4f1659d476-goog