On Tue, Mar 07, 2017 at 02:15:45PM +0100, Vlastimil Babka wrote:
> The main goal of direct compaction is to form a high-order page for allocation,
> but it should also help against long-term fragmentation when possible. Most
> lower-than-pageblock-order compactions are for non-movable allocations, which
> means that if we compact in a movable pageblock and terminate as soon as we
> create the high-order page, it's unlikely that the fallback heuristics will
> claim the whole block. Instead there might be a single unmovable page in a
> pageblock full of movable pages, and the next unmovable allocation might pick
> another pageblock and increase long-term fragmentation.
>
> To help against such scenarios, this patch changes the termination criteria for
> compaction so that the current pageblock is finished even though the high-order
> page already exists. Note that it might be possible that the high-order page
> formed elsewhere in the zone due to parallel activity, but this patch doesn't
> try to detect that.
>
> This is only done with sync compaction, because async compaction is limited to
> pageblock of the same migratetype, where it cannot result in a migratetype
> fallback. (Async compaction also eagerly skips order-aligned blocks where
> isolation fails, which is against the goal of migrating away as much of the
> pageblock as possible.)
>
> As a result of this patch, long-term memory fragmentation should be reduced.
>
> In testing based on 4.9 kernel with stress-highalloc from mmtests configured
> for order-4 GFP_KERNEL allocations, this patch has reduced the number of
> unmovable allocations falling back to movable pageblocks by 20%. The number
>
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> ---
> mm/compaction.c | 36 ++++++++++++++++++++++++++++++++++--
> mm/internal.h | 1 +
> 2 files changed, 35 insertions(+), 2 deletions(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 2c288e75840d..bc7903130501 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1318,6 +1318,17 @@ static enum compact_result __compact_finished(struct zone *zone,
> if (is_via_compact_memory(cc->order))
> return COMPACT_CONTINUE;
>
> + if (cc->finishing_block) {
> + /*
> + * We have finished the pageblock, but better check again that
> + * we really succeeded.
> + */
> + if (IS_ALIGNED(cc->migrate_pfn, pageblock_nr_pages))
> + cc->finishing_block = false;
> + else
> + return COMPACT_CONTINUE;
> + }
> +
> /* Direct compactor: Is a suitable page free? */
> for (order = cc->order; order < MAX_ORDER; order++) {
> struct free_area *area = &zone->free_area[order];
> @@ -1338,8 +1349,29 @@ static enum compact_result __compact_finished(struct zone *zone,
> * other migratetype buddy lists.
> */
> if (find_suitable_fallback(area, order, migratetype,
> - true, &can_steal) != -1)
> - return COMPACT_SUCCESS;
> + true, &can_steal) != -1) {
> +
> + /* movable pages are OK in any pageblock */
> + if (migratetype == MIGRATE_MOVABLE)
> + return COMPACT_SUCCESS;
> +
> + /*
> + * We are stealing for a non-movable allocation. Make
> + * sure we finish compacting the current pageblock
> + * first so it is as free as possible and we won't
> + * have to steal another one soon. This only applies
> + * to sync compaction, as async compaction operates
> + * on pageblocks of the same migratetype.
> + */
> + if (cc->mode == MIGRATE_ASYNC ||
> + IS_ALIGNED(cc->migrate_pfn,
> + pageblock_nr_pages)) {
> + return COMPACT_SUCCESS;
> + }
If cc->migratetype and cc->migrate_pfn's migratetype is the same, stopping
the compaction here doesn't cause any fragmentation. Do we need to
compact full pageblock in this case?
Thanks.
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