[PATCH 36/52] sched: Add hysteresis to p->numa_shared

[Ingo Molnar]

Make p->numa_shared flip/flop less around unstable equilibriums,
instead require a significant move in either direction to trigger
'dominantly shared accesses' versus 'dominantly private accesses'
NUMA status.

Suggested-by: Rik van Riel <riel@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 kernel/sched/fair.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 46d23c7..f3fb508 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1111,7 +1111,20 @@ static void task_numa_placement(struct task_struct *p)
 	 * we might want to consider a different equation below to reduce
 	 * the impact of a little private memory accesses.
 	 */
-	shared = (total[0] >= total[1] / 2);
+	shared = p->numa_shared;
+
+	if (shared < 0) {
+		shared = (total[0] >= total[1]);
+	} else if (shared == 0) {
+		/* If it was private before, make it harder to become shared: */
+		if (total[0] >= total[1]*2)
+			shared = 1;
+	} else if (shared == 1 ) {
+		 /* If it was shared before, make it harder to become private: */
+		if (total[0]*2 <= total[1])
+			shared = 0;
+	}
+
 	if (shared)
 		p->ideal_cpu = sched_update_ideal_cpu_shared(p);
 	else
-- 
1.7.11.7

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