rcu: Add documentation for the new rcuexp debugfs trace file

This commit adds the documentation of the rcuexp debugfs trace file
that records statistics for expedited grace periods.
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

@@ -41,6 +41,8 @@ also rcu/rcu_preempt) the following files will be present:
 
 rcudata:
 	Displays fields in struct rcu_data.
+rcuexp:
+	Displays statistics for expedited grace periods.
 rcugp:
 	Displays grace-period counters.
 rcuhier:
@@ -233,6 +235,45 @@ o	"ktl" is the low-order 16 bits (in hexadecimal) of the count of
 	This field is displayed only for CONFIG_RCU_BOOST kernels.
 
 
+The output of "cat rcu/rcu_preempt/rcuexp" looks as follows:
+
+s=21872 d=21872 w=0 tf=0 wd1=0 wd2=0 n=0 sc=21872 dt=21872 dl=0 dx=21872
+
+These fields are as follows:
+
+o	"s" is the starting sequence number.
+
+o	"d" is the ending sequence number.  When the starting and ending
+	numbers differ, there is an expedited grace period in progress.
+
+o	"w" is the number of times that the sequence numbers have been
+	in danger of wrapping.
+
+o	"tf" is the number of times that contention has resulted in a
+	failure to begin an expedited grace period.
+
+o	"wd1" and "wd2" are the number of times that an attempt to
+	start an expedited grace period found that someone else had
+	completed an expedited grace period that satisfies the
+	attempted request.  "Our work is done."
+
+o	"n" is number of times that contention was so great that
+	the request was demoted from an expedited grace period to
+	a normal grace period.
+
+o	"sc" is the number of times that the attempt to start a
+	new expedited grace period succeeded.
+
+o	"dt" is the number of times that we attempted to update
+	the "d" counter.
+
+o	"dl" is the number of times that we failed to update the "d"
+	counter.
+
+o	"dx" is the number of times that we succeeded in updating
+	the "d" counter.
+
+
 The output of "cat rcu/rcu_preempt/rcugp" looks as follows:
 
 completed=31249  gpnum=31250  age=1  max=18