rcu: Enable rcu_normal_after_boot unconditionally for RT

Expedited RCU grace periods send IPIs to all non-idle CPUs, and thus can
disrupt time-critical code in real-time applications.  However, there
is a portion of boot-time processing (presumably before any real-time
applications have started) where expedited RCU grace periods are the only
option.  And so it is that experience with the -rt patchset indicates that
PREEMPT_RT systems should always set the rcupdate.rcu_normal_after_boot
kernel boot parameter.

This commit therefore makes the post-boot application environment safe
for real-time applications by making PREEMPT_RT systems disable the
rcupdate.rcu_normal_after_boot kernel boot parameter and acting as
if this parameter had been set.  This means that post-boot calls to
synchronize_rcu_expedited() will be treated as if they were instead
calls to synchronize_rcu(), thus preventing the IPIs, and thus avoiding
disrupting real-time applications.

Suggested-by: Luiz Capitulino <lcapitulino@redhat.com>
Acked-by: Paul E. McKenney <paulmck@linux.ibm.com>
Signed-off-by: Julia Cartwright <julia@ni.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
[ paulmck: Update kernel-parameters.txt accordingly. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Documentation/admin-guide/kernel-parameters.txt

@@ -4474,6 +4474,13 @@
 			only normal grace-period primitives.  No effect
 			on CONFIG_TINY_RCU kernels.
 
+			But note that CONFIG_PREEMPT_RT=y kernels enables
+			this kernel boot parameter, forcibly setting
+			it to the value one, that is, converting any
+			post-boot attempt at an expedited RCU grace
+			period to instead use normal non-expedited
+			grace-period processing.
+
 	rcupdate.rcu_task_ipi_delay= [KNL]
 			Set time in jiffies during which RCU tasks will
 			avoid sending IPIs, starting with the beginning

kernel/rcu/update.c

@@ -56,8 +56,10 @@
 #ifndef CONFIG_TINY_RCU
 module_param(rcu_expedited, int, 0);
 module_param(rcu_normal, int, 0);
-static int rcu_normal_after_boot;
+static int rcu_normal_after_boot = IS_ENABLED(CONFIG_PREEMPT_RT);
+#ifndef CONFIG_PREEMPT_RT
 module_param(rcu_normal_after_boot, int, 0);
+#endif
 #endif /* #ifndef CONFIG_TINY_RCU */
 
 #ifdef CONFIG_DEBUG_LOCK_ALLOC