rcu: Update RCU_FAST_NO_HZ tracing for lazy callbacks

In the current code, a short dyntick-idle interval (where there is
at least one non-lazy callback on the CPU) and a long dyntick-idle
interval (where there are only lazy callbacks on the CPU) are traced
identically, which can be less than helpful.  This commit therefore
emits different event traces in these two cases.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Tested-by: Pascal Chapperon <pascal.chapperon@wanadoo.fr>

include/trace/events/rcu.h

@@ -289,6 +289,7 @@ TRACE_EVENT(rcu_dyntick,
  *	"In holdoff": Nothing to do, holding off after unsuccessful attempt.
  *	"Begin holdoff": Attempt failed, don't retry until next jiffy.
  *	"Dyntick with callbacks": Entering dyntick-idle despite callbacks.
+ *	"Dyntick with lazy callbacks": Entering dyntick-idle w/lazy callbacks.
  *	"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.

kernel/rcutree_plugin.h

@@ -2165,15 +2165,17 @@ static void rcu_prepare_for_idle(int cpu)
 		   !rcu_pending(cpu) &&
 		   !local_softirq_pending()) {
 		/* Can we go dyntick-idle despite still having callbacks? */
-		trace_rcu_prep_idle("Dyntick with callbacks");
 		per_cpu(rcu_dyntick_drain, cpu) = 0;
 		per_cpu(rcu_dyntick_holdoff, cpu) = jiffies;
-		if (rcu_cpu_has_nonlazy_callbacks(cpu))
+		if (rcu_cpu_has_nonlazy_callbacks(cpu)) {
+			trace_rcu_prep_idle("Dyntick with callbacks");
 			per_cpu(rcu_idle_gp_timer_expires, cpu) =
 					   jiffies + RCU_IDLE_GP_DELAY;
-		else
+		} else {
 			per_cpu(rcu_idle_gp_timer_expires, cpu) =
 					   jiffies + RCU_IDLE_LAZY_GP_DELAY;
+			trace_rcu_prep_idle("Dyntick with lazy callbacks");
+		}
 		tp = &per_cpu(rcu_idle_gp_timer, cpu);
 		mod_timer_pinned(tp, per_cpu(rcu_idle_gp_timer_expires, cpu));
 		per_cpu(rcu_nonlazy_posted_snap, cpu) =