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>

1 files changed, 41 insertions, 0 deletions

diff --git a/Documentation/RCU/trace.txt b/Documentation/RCU/trace.txt
index 79ce9891a8c7..c776968f4463 100644
--- a/Documentation/RCU/trace.txt
+++ b/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