[PATCH 2/2] doc: Clarify rate-monotonic statistics

[Sebastian Huber]

---
 doc/user/rtmon.t | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/doc/user/rtmon.t b/doc/user/rtmon.t
index 7786e76..92f6b0b 100644
--- a/doc/user/rtmon.t
+++ b/doc/user/rtmon.t
@@ -52,10 +52,10 @@ A clock tick is required to support the functionality provided by this manager.
 
 @subsection Period Statistics
 
-This manager maintains a set of statistics on each period.  These
-statistics are reset implictly at period creation time and may be
-reset or obtained at any time by the application.  The following
-is a list of the information kept:
+This manager maintains a set of statistics on each period object.  These
+statistics are reset implictly at period creation time and may be reset or
+obtained at any time by the application.  The following is a list of the
+information kept:
 
 @itemize @bullet
 @item @code{owner}
@@ -93,6 +93,16 @@ during executions of the periodic loop.
 
 @end itemize
 
+Each period is divided into two consecutive phases.  The period starts with the
+active phase of the task and is followed by the inactive phase of the task.  In
+the inactive phase the task is blocked and waits for the start of the next
+period.  The inactive phase is skipped in case of a period miss.  The wall time
+includes the time during the active phase of the task on which the task is not
+executing on a processor.  The task may be blocked waiting for a resource or a
+higher priority task executes instead.  In case the wall time exceeds the
+period time, then this is a period miss.  So the gap between the maximum wall
+time and the period time is the safety margin between a period miss or success.
+
 The period statistics information is inexpensive to maintain
 and can provide very useful insights into the execution
 characteristics of a periodic task loop.  But it is just information.
-- 
1.8.4.5