mm: numa: Use a two-stage filter to restrict pages being migrated for unlikely... (e42c8ff2) · Commits · 戴 / test

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>

mm/mempolicy.c

+29 −1

Original line number	Diff line number	Diff line
		@@ -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:

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