rcu: Add tracing data to support queueing models

The current tracing data is not sufficient to deduce the average time
that a callback spends waiting for a grace period to end.  Add three
per-CPU counters recording the number of callbacks invoked (ci), the
number of callbacks orphaned (co), and the number of callbacks adopted
(ca).  Given the existing callback queue length (ql), the average wait
time in absence of CPU hotplug operations is ql/ci.  The units of wait
time will be in terms of the duration over which ci was measured.

In the presence of CPU hotplug operations, there is room for argument,
but ql/(ci-co+ca) won't steer you too far wrong.

Also fixes a typo called out by Lucas De Marchi <lucas.de.marchi@gmail.com>.

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

1 files changed, 12 insertions, 1 deletions

diff --git a/Documentation/RCU/trace.txt b/Documentation/RCU/trace.txt
index efd8cc95c06b..a851118775d8 100644
--- a/Documentation/RCU/trace.txt
+++ b/Documentation/RCU/trace.txt
@@ -125,6 +125,17 @@ 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.
 
+o       "ci" is the number of RCU callbacks that have been invoked for
+        this CPU.  Note that ci+ql is the number of callbacks that have
+        been registered in absence of CPU-hotplug activity.
+
+o       "co" is the number of RCU callbacks that have been orphaned due to
+        this CPU going offline.
+
+o       "ca" is the number of RCU callbacks that have been adopted due to
+        other CPUs going offline.  Note that ci+co-ca+ql is the number of
+        RCU callbacks registered on this CPU.
+
 There is also an rcu/rcudata.csv file with the same information in
 comma-separated-variable spreadsheet format.
 
@@ -180,7 +191,7 @@ o	"s" is the "signaled" state that drives force_quiescent_state()'s
 
 o       "jfq" is the number of jiffies remaining for this grace period
         before force_quiescent_state() is invoked to help push things
-        along.  Note that CPUs in dyntick-idle mode thoughout the grace
+        along.  Note that CPUs in dyntick-idle mode throughout the grace
         period will not report on their own, but rather must be check by
         some other CPU via force_quiescent_state().