rcu: Allow expedited grace periods to be disabled at init

Expedited grace periods can speed up boot, but are undesirable in
aggressive real-time systems.  This commit therefore introduces a
kernel parameter rcupdate.rcu_normal_after_boot that disables
expedited grace periods just before init is spawned.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Documentation/kernel-parameters.txt

@@ -3318,6 +3318,11 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			grace-period latency.  This parameter overrides
 			rcupdate.rcu_expedited.
 
+	rcupdate.rcu_normal_after_boot= [KNL]
+			Once boot has completed (that is, after
+			rcu_end_inkernel_boot() has been invoked), use
+			only normal grace-period primitives.
+
 	rcupdate.rcu_task_stall_timeout= [KNL]
 			Set timeout in jiffies for RCU task stall warning
 			messages.  Disable with a value less than or equal

kernel/rcu/update.c

@@ -63,6 +63,9 @@ MODULE_ALIAS("rcupdate");
 module_param(rcu_expedited, int, 0);
 module_param(rcu_normal, int, 0);
 
+static int rcu_normal_after_boot;
+module_param(rcu_normal_after_boot, int, 0);
+
 #if defined(CONFIG_DEBUG_LOCK_ALLOC) && defined(CONFIG_PREEMPT_COUNT)
 /**
  * rcu_read_lock_sched_held() - might we be in RCU-sched read-side critical section?
@@ -178,6 +181,8 @@ void rcu_end_inkernel_boot(void)
 {
 	if (IS_ENABLED(CONFIG_RCU_EXPEDITE_BOOT))
 		rcu_unexpedite_gp();
+	if (rcu_normal_after_boot)
+		WRITE_ONCE(rcu_normal, 1);
 }
 
 #ifdef CONFIG_PREEMPT_RCU