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 | 30 +++++++++++++++++++++++++++++-
1 file changed, 29 insertions(+), 1 deletion(-)
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 7acc97b..648423a 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -2453,9 +2453,37 @@ int mpol_misplaced(struct page *page, struct vm_area_struct *vma, unsigned long
}
/* Migrate the page towards the node whose CPU is referencing it */
- if (pol->flags & MPOL_F_MORON)
+ if (pol->flags & MPOL_F_MORON) {
+ int last_nid;
+
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)
ret = polnid;
out:
--
1.7.9.2
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