documentation: Update based on on-demand vmstat workers

Now that the on-demand vmstat workers commit is in mainline, it is
possible to eliminate vmstat_update()-induced OS jitter.  This commit
updates the documentation accordingly.

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

1 files changed, 10 insertions, 8 deletions

diff --git a/Documentation/kernel-per-CPU-kthreads.txt b/Documentation/kernel-per-CPU-kthreads.txt
index f3cd299fcc41..81fe051c4447 100644
--- a/Documentation/kernel-per-CPU-kthreads.txt
+++ b/Documentation/kernel-per-CPU-kthreads.txt
@@ -190,14 +190,16 @@ To reduce its OS jitter, do any of the following:
                 on each CPU, including cs_dbs_timer() and od_dbs_timer().
                 WARNING:  Please check your CPU specifications to
                 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(),
+        d.      As of v3.18, Christoph Lameter's on-demand vmstat workers
+                commit prevents OS jitter due to vmstat_update() on
+                CONFIG_SMP=y systems.  Before v3.18, is not possible
+                to entirely get rid of the OS jitter, 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(),
                 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