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SubjectRe: [PATCH v6 05/11] mm, compaction: add the ultimate direct compaction priority
On Wed, Aug 10, 2016 at 11:12:20AM +0200, Vlastimil Babka wrote:
> During reclaim/compaction loop, it's desirable to get a final answer from
> unsuccessful compaction so we can either fail the allocation or invoke the OOM
> killer. However, heuristics such as deferred compaction or pageblock skip bits
> can cause compaction to skip parts or whole zones and lead to premature OOM's,
> failures or excessive reclaim/compaction retries.
>
> To remedy this, we introduce a new direct compaction priority called
> COMPACT_PRIO_SYNC_FULL, which instructs direct compaction to:
>
> - ignore deferred compaction status for a zone
> - ignore pageblock skip hints
> - ignore cached scanner positions and scan the whole zone
>
> The new priority should get eventually picked up by should_compact_retry() and
> this should improve success rates for costly allocations using __GFP_REPEAT,
> such as hugetlbfs allocations, and reduce some corner-case OOM's for non-costly
> allocations.
>
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> Acked-by: Michal Hocko <mhocko@suse.com>
> ---
> include/linux/compaction.h | 3 ++-
> mm/compaction.c | 5 ++++-
> 2 files changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/compaction.h b/include/linux/compaction.h
> index e88c037afe47..a1fba9994728 100644
> --- a/include/linux/compaction.h
> +++ b/include/linux/compaction.h
> @@ -6,8 +6,9 @@
> * Lower value means higher priority, analogically to reclaim priority.
> */
> enum compact_priority {
> + COMPACT_PRIO_SYNC_FULL,
> + MIN_COMPACT_PRIORITY = COMPACT_PRIO_SYNC_FULL,
> COMPACT_PRIO_SYNC_LIGHT,
> - MIN_COMPACT_PRIORITY = COMPACT_PRIO_SYNC_LIGHT,
> DEF_COMPACT_PRIORITY = COMPACT_PRIO_SYNC_LIGHT,
> COMPACT_PRIO_ASYNC,
> INIT_COMPACT_PRIORITY = COMPACT_PRIO_ASYNC
> diff --git a/mm/compaction.c b/mm/compaction.c
> index a144f58f7193..ae4f40afcca1 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1644,6 +1644,8 @@ static enum compact_result compact_zone_order(struct zone *zone, int order,
> .alloc_flags = alloc_flags,
> .classzone_idx = classzone_idx,
> .direct_compaction = true,
> + .whole_zone = (prio == COMPACT_PRIO_SYNC_FULL),
> + .ignore_skip_hint = (prio == COMPACT_PRIO_SYNC_FULL)
> };
> INIT_LIST_HEAD(&cc.freepages);
> INIT_LIST_HEAD(&cc.migratepages);
> @@ -1689,7 +1691,8 @@ enum compact_result try_to_compact_pages(gfp_t gfp_mask, unsigned int order,
> ac->nodemask) {
> enum compact_result status;
>
> - if (compaction_deferred(zone, order)) {
> + if (prio > COMPACT_PRIO_SYNC_FULL
> + && compaction_deferred(zone, order)) {
> rc = max_t(enum compact_result, COMPACT_DEFERRED, rc);
> continue;

Could we provide prio to compaction_deferred() and do the decision in
that that function?

BTW, in kcompactd, compaction_deferred() is checked but
.ignore_skip_hint=true. Is there any reason? If we can remove
compaction_deferred() for kcompactd, we can check .ignore_skip_hint
to determine if defer is needed or not.

Thanks.

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