rcu: Add RCU_FAST_NO_HZ tracing for idle exit

Traces of rcu_prep_idle events can be confusing because
rcu_cleanup_after_idle() does no tracing.  This commit therefore adds
this tracing.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

include/trace/events/rcu.h

@@ -292,6 +292,7 @@ TRACE_EVENT(rcu_dyntick,
  *	"More callbacks": Still more callbacks, try again to clear them out.
  *	"Callbacks drained": All callbacks processed, off to dyntick idle!
  *	"Timer": Timer fired to cause CPU to continue processing callbacks.
+ *	"Cleanup after idle": Idle exited, timer canceled.
  */
 TRACE_EVENT(rcu_prep_idle,
 

kernel/rcutree_plugin.h

@@ -2085,6 +2085,7 @@ static void rcu_prepare_for_idle_init(int cpu)
 static void rcu_cleanup_after_idle(int cpu)
 {
 	hrtimer_cancel(&per_cpu(rcu_idle_gp_timer, cpu));
+	trace_rcu_prep_idle("Cleanup after idle");
 }
 
 /*