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 <[email protected]> --- 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
