Re: [RFC 11/13] mm, compaction: add the ultimate direct compaction priority

Michal Hocko
2016-05-13T13:38:55Z

On Tue 10-05-16 09:36:01, 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
> - use MIGRATE_SYNC migration mode

I do not think we can do MIGRATE_SYNC because fallback_migrate_page
would trigger pageout and we are in the allocation path and so we
could blow up the stack.

> The new priority should get eventually picked up by should_compact_retry() and
> this should improve success rates for costly allocations using __GFP_RETRY,

s@__GFP_RETRY@__GFP_REPEAT@

> such as hugetlbfs allocations, and reduce some corner-case OOM's for non-costly
> allocations.

My testing has shown that even with the current implementation with
deferring, skip hints and cached positions had (close to) 100% success
rate even with close to OOM conditions.

I am wondering whether this strongest priority should be done only for
!costly high order pages. But we probably want less special cases
between costly and !costly orders.

> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
> include/linux/compaction.h | 1 +
> mm/compaction.c | 15 ++++++++++++---
> 2 files changed, 13 insertions(+), 3 deletions(-)
>
[...]
> @@ -1631,7 +1639,8 @@ 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;
> }

Wouldn't it be better to pull the prio check into compaction_deferred
directly? There are more callers and I am not really sure all of them
would behave consistently.
--
Michal Hocko
SUSE Labs