kthread: Add pointer to vmstat-avoidance patch

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

Documentation/kernel-per-CPU-kthreads.txt

@@ -181,12 +181,17 @@ To reduce its OS jitter, do any of the following:
 		make sure that this is safe on your particular system.
 	d.	It is not possible to entirely get rid of OS jitter
 		from vmstat_update() on CONFIG_SMP=y systems, but you
-		can decrease its frequency by writing a large value to
-		/proc/sys/vm/stat_interval.  The default value is HZ,
-		for an interval of one second.  Of course, larger values
-		will make your virtual-memory statistics update more
-		slowly.  Of course, you can also run your workload at
-		a real-time priority, thus preempting vmstat_update().
+		can decrease its frequency by writing a large value
+		to /proc/sys/vm/stat_interval.	The default value is
+		HZ, for an interval of one second.  Of course, larger
+		values will make your virtual-memory statistics update
+		more slowly.  Of course, you can also run your workload
+		at a real-time priority, thus preempting vmstat_update(),
+		but if your workload is CPU-bound, this is a bad idea.
+		However, there is an RFC patch from Christoph Lameter
+		(based on an earlier one from Gilad Ben-Yossef) that
+		reduces or even eliminates vmstat overhead for some
+		workloads at https://lkml.org/lkml/2013/9/4/379.
 	e.	If running on high-end powerpc servers, build with
 		CONFIG_PPC_RTAS_DAEMON=n.  This prevents the RTAS
 		daemon from running on each CPU every second or so.