diff --git a/sys/conf/NOTES b/sys/conf/NOTES
index 33a0e7983c2a..1914f54d1f5e 100644
--- a/sys/conf/NOTES
+++ b/sys/conf/NOTES
@@ -134,6 +134,28 @@ options 	WITNESS
 options 	WITNESS_DDB
 options 	WITNESS_SKIPSPIN
 
+#
+# MUTEX_PROFILING - Profiling mutual exclusion locks (mutexes).  This
+# records four numbers for each acquisition point (identified by
+# source file name and line number): longest time held, total time held,
+# number of non-recursive acquisitions, and average time held. Measurements
+# are made and stored in nanoseconds (using nanotime(9)), but are presented
+# in microseconds, which should be sufficient for the locks which actually
+# want this (those that are held long and / or often).  The MUTEX_PROFILING
+# option has the following sysctl namespace for controlling and viewing its
+# operation:
+#
+#  debug.mutex.prof.enable - enable / disable profiling
+#  debug.mutex.prof.acquisitions - number of mutex acquisitions held
+#  debug.mutex.prof.records - number of acquisition points recorded
+#  debug.mutex.prof.maxrecords - max number of acquisition points
+#  debug.mutex.prof.rejected - number of rejections (due to full table)
+#  debug.mutex.prof.hashsize - hash size
+#  debug.mutex.prof.collisions - number of hash collisions
+#  debug.mutex.prof.stats - profiling statistics
+#
+options		MUTEX_PROFILING
+
 
 #####################################################################
 # COMPATIBILITY OPTIONS