rcu: add grace-period age and more kthread state to tracing

This commit adds the age in jiffies of the current grace period along
with the duration in jiffies of the longest grace period since boot
to the rcu/rcugp debugfs file.  It also adds an additional "O" state
to kthread tracing to differentiate between the kthread waiting due to
having nothing to do on the one hand and waiting due to being on the
wrong CPU on the other hand.

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

Documentation/RCU/trace.txt

@@ -159,8 +159,8 @@ o	"qs" gives an indication of the state of the callback queue
 	the corresponding character is replaced by ".".
 
 o	"kt" is the per-CPU kernel-thread state.  The digit preceding
-	the slash is zero if there is no work pending and 1 otherwise.
-	The character after the slash is as follows:
+	the first slash is zero if there is no work pending and 1
+	otherwise.  The character between the slashes is as follows:
 
 	"S"	The kernel thread is stopped, in other words, all
 		CPUs corresponding to this rcu_node structure are
@@ -171,10 +171,18 @@ o	"kt" is the per-CPU kernel-thread state.  The digit preceding
 	"W"	The kernel thread is waiting because there is no work
 		for it to do.
 
+	"O"	The kernel thread is waiting because it has been
+		forced off of its designated CPU or because its
+		->cpus_allowed mask permits it to run on other than
+		its designated CPU.
+
 	"Y"	The kernel thread is yielding to avoid hogging CPU.
 
 	"?"	Unknown value, indicates a bug.
 
+	The number after the final slash is the CPU that the kthread
+	is actually running on.
+
 o	"b" is the batch limit for this CPU.  If more than this number
 	of RCU callbacks is ready to invoke, then the remainder will
 	be deferred.