mm: numa: Use a two-stage filter to restrict pages being migrated for unlikely task<->node relationships
Note: This two-stage filter was taken directly from the sched/numa patch
"sched, numa, mm: Add the scanning page fault machinery" but is
only a partial extraction. As the end result is not necessarily
recognisable, the signed-offs-by had to be removed. Will be added
back if requested.
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@suse.de>
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 4c1c8d8..fd20e28 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -2317,9 +2317,37 @@
}
/* 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: