[tip:sched/core] sched/numa: Change scan period code to match intent

[tip-bot for Rik van Riel]

Commit-ID:  a22b4b012340b988dbe7a58461d6fcc582f34aa0
Gitweb:     http://git.kernel.org/tip/a22b4b012340b988dbe7a58461d6fcc582f34aa0
Author:     Rik van Riel <riel@xxxxxxxxxx>
AuthorDate: Mon, 23 Jun 2014 11:41:35 -0400
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Sat, 5 Jul 2014 11:17:40 +0200

sched/numa: Change scan period code to match intent

Reading through the scan period code and comment, it appears the
intent was to slow down NUMA scanning when a majority of accesses
are on the local node, specifically a local:remote ratio of 3:1.

However, the code actually tests local / (local + remote), and
the actual cut-off point was around 30% local accesses, well before
a task has actually converged on a node.

Changing the threshold to 7 means scanning slows down when a task
has around 70% of its accesses local, which appears to match the
intent of the code more closely.

Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>
Cc: mgorman@xxxxxxx
Cc: chegu_vinod@xxxxxx
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/1403538095-31256-8-git-send-email-riel@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 kernel/sched/fair.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7bb2f46..a140c6a 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1452,12 +1452,12 @@ static void update_numa_active_node_mask(struct numa_group *numa_group)
 /*
  * When adapting the scan rate, the period is divided into NUMA_PERIOD_SLOTS
  * increments. The more local the fault statistics are, the higher the scan
- * period will be for the next scan window. If local/remote ratio is below
- * NUMA_PERIOD_THRESHOLD (where range of ratio is 1..NUMA_PERIOD_SLOTS) the
- * scan period will decrease
+ * period will be for the next scan window. If local/(local+remote) ratio is
+ * below NUMA_PERIOD_THRESHOLD (where range of ratio is 1..NUMA_PERIOD_SLOTS)
+ * the scan period will decrease. Aim for 70% local accesses.
  */
 #define NUMA_PERIOD_SLOTS 10
-#define NUMA_PERIOD_THRESHOLD 3
+#define NUMA_PERIOD_THRESHOLD 7
 
 /*
  * Increase the scan period (slow down scanning) if the majority of
--
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