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

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Date:	Fri, 13 May 2016 15:38:51 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Rik van Riel <riel@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Mel Gorman <mgorman@...hsingularity.net>,
	Johannes Weiner <hannes@...xchg.org>,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC 11/13] mm, compaction: add the ultimate direct compaction
 priority

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@...FP_RETRY@...FP_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@...e.cz>

Acked-by: Michal Hocko <mhocko@...e.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