mm: numa: Use a two-stage filter to restrict pages being migrated for unlikely task...

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 4c1c8d83ac6afa97c2399c1c5bd786c27486402c..fd20e28fd2adcfdd540ef7fcdfefb319d3c0d72b 100644 (file)
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -2317,9 +2317,37 @@ 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: