[PATCH 31/31] mm: numa: Use a two-stage filter to restrict pages being migrated for unlikely task<->node relationships

[Mel Gorman]

While it is desirable that all threads in a process run on its home
node, this is not always possible or necessary. There may be more
threads than exist within the node or the node might over-subscribed
with unrelated processes.

This can cause a situation whereby a page gets migrated off its home
node because the threads clearing pte_numa were running off-node. This
patch uses page->last_nid to build a two-stage filter before pages get
migrated to avoid problems with short or unlikely task<->node
relationships.

Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
---
 mm/mempolicy.c |   27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 076f8f8..89696d7 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -2425,6 +2425,8 @@ int mpol_misplaced(struct page *page, struct vm_area_struct *vma, unsigned long
 
 	/* Migrate pages towards their home node or the referencing node */
 	if (pol->flags & MPOL_F_HOME) {
+		int last_nid;
+
 		/*
 		 * Make a placement decision based on the home node.
 		 * NOTE: Potentially this can result in a remote->remote
@@ -2437,6 +2439,31 @@ int mpol_misplaced(struct page *page, struct vm_area_struct *vma, unsigned long
 			/* No home node, migrate to the referencing node */
 			polnid = numa_node_id();
 		}
+
+		/*
+		 * Multi-stage node selection is used in conjunction
+		 * with a periodic migration fault to build a temporal
+		 * task<->page relation. By using a two-stage filter we
+		 * remove short/unlikely relations.
+		 *
+		 * Using P(p) ~ n_p / n_t as per frequentist
+		 * probability, we can equate a task's usage of a
+		 * particular page (n_p) per total usage of this
+		 * page (n_t) (in a given time-span) to a probability.
+		 *
+		 * Our periodic faults will sample this probability and
+		 * getting the same result twice in a row, given these
+		 * samples are fully independent, is then given by
+		 * P(n)^2, provided our sample period is sufficiently
+		 * short compared to the usage pattern.
+		 *
+		 * This quadric squishes small probabilities, making
+		 * it less likely we act on an unlikely task<->page
+		 * relation.
+		 */
+		last_nid = page_xchg_last_nid(page, polnid);
+		if (last_nid != polnid)
+			goto out;
 	}
 
 	if (curnid != polnid)
-- 
1.7.9.2

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