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[PATCH v3 13/17] mm, compaction: use correct watermark when checking allocation success

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  [PATCH v3 13/17] mm, compaction: use correct watermark when checking allocation success Vlastimil Babka <vbabka@suse.cz> - 2016-06-24 12:00 +0200
    Re: [PATCH v3 13/17] mm, compaction: use correct watermark when  checking allocation success Joonsoo Kim <iamjoonsoo.kim@lge.com> - 2016-07-06 07:50 +0200

#1430539 — [PATCH v3 13/17] mm, compaction: use correct watermark when checking allocation success

The __compact_finished() function uses low watermark in a check that has to
pass if the direct compaction is to finish and allocation should succeed. This
is too pessimistic, as the allocation will typically use min watermark. It may
happen that during compaction, we drop below the low watermark (due to parallel
activity), but still form the target high-order page. By checking against low
watermark, we might needlessly continue compaction.

Similarly, __compaction_suitable() uses low watermark in a check whether
allocation can succeed without compaction. Again, this is unnecessarily
pessimistic.

After this patch, these check will use direct compactor's alloc_flags to
determine the watermark, which is effectively the min watermark.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Michal Hocko <mhocko@suse.com>
---
 mm/compaction.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index 76897850c3c2..371760a85085 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1320,7 +1320,7 @@ static enum compact_result __compact_finished(struct zone *zone, struct compact_
 		return COMPACT_CONTINUE;
 
 	/* Compaction run is not finished if the watermark is not met */
-	watermark = low_wmark_pages(zone);
+	watermark = zone->watermark[cc->alloc_flags & ALLOC_WMARK_MASK];
 
 	if (!zone_watermark_ok(zone, cc->order, watermark, cc->classzone_idx,
 							cc->alloc_flags))
@@ -1385,7 +1385,7 @@ static enum compact_result __compaction_suitable(struct zone *zone, int order,
 	if (is_via_compact_memory(order))
 		return COMPACT_CONTINUE;
 
-	watermark = low_wmark_pages(zone);
+	watermark = zone->watermark[alloc_flags & ALLOC_WMARK_MASK];
 	/*
 	 * If watermarks for high-order allocation are already met, there
 	 * should be no need for compaction at all.
@@ -1399,7 +1399,7 @@ static enum compact_result __compaction_suitable(struct zone *zone, int order,
 	 * This is because during migration, copies of pages need to be
 	 * allocated and for a short time, the footprint is higher
 	 */
-	watermark += (2UL << order);
+	watermark = low_wmark_pages(zone) + (2UL << order);
 	if (!__zone_watermark_ok(zone, 0, watermark, classzone_idx,
 				 alloc_flags, wmark_target))
 		return COMPACT_SKIPPED;
-- 
2.8.4

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#1437434 — Re: [PATCH v3 13/17] mm, compaction: use correct watermark when checking allocation success

On Fri, Jun 24, 2016 at 11:54:33AM +0200, Vlastimil Babka wrote:
> The __compact_finished() function uses low watermark in a check that has to
> pass if the direct compaction is to finish and allocation should succeed. This
> is too pessimistic, as the allocation will typically use min watermark. It may
> happen that during compaction, we drop below the low watermark (due to parallel
> activity), but still form the target high-order page. By checking against low
> watermark, we might needlessly continue compaction.
> 
> Similarly, __compaction_suitable() uses low watermark in a check whether
> allocation can succeed without compaction. Again, this is unnecessarily
> pessimistic.
> 
> After this patch, these check will use direct compactor's alloc_flags to
> determine the watermark, which is effectively the min watermark.
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> Acked-by: Michal Hocko <mhocko@suse.com>
> ---
>  mm/compaction.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 76897850c3c2..371760a85085 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1320,7 +1320,7 @@ static enum compact_result __compact_finished(struct zone *zone, struct compact_
>  		return COMPACT_CONTINUE;
>  
>  	/* Compaction run is not finished if the watermark is not met */
> -	watermark = low_wmark_pages(zone);
> +	watermark = zone->watermark[cc->alloc_flags & ALLOC_WMARK_MASK];

finish condition is changed. We have two more watermark checks in
try_to_compact_pages() and kcompactd_do_work() and they should be
changed too.

Thanks.
>  
>  	if (!zone_watermark_ok(zone, cc->order, watermark, cc->classzone_idx,
>  							cc->alloc_flags))
> @@ -1385,7 +1385,7 @@ static enum compact_result __compaction_suitable(struct zone *zone, int order,
>  	if (is_via_compact_memory(order))
>  		return COMPACT_CONTINUE;
>  
> -	watermark = low_wmark_pages(zone);
> +	watermark = zone->watermark[alloc_flags & ALLOC_WMARK_MASK];
>  	/*
>  	 * If watermarks for high-order allocation are already met, there
>  	 * should be no need for compaction at all.
> @@ -1399,7 +1399,7 @@ static enum compact_result __compaction_suitable(struct zone *zone, int order,
>  	 * This is because during migration, copies of pages need to be
>  	 * allocated and for a short time, the footprint is higher
>  	 */
> -	watermark += (2UL << order);
> +	watermark = low_wmark_pages(zone) + (2UL << order);
>  	if (!__zone_watermark_ok(zone, 0, watermark, classzone_idx,
>  				 alloc_flags, wmark_target))
>  		return COMPACT_SKIPPED;
> -- 
> 2.8.4

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