[PATCH] mm:vmscan:replace zone_watermark_ok with zone_balanced for determining if kswapd will call compaction

[PATCH] mm:vmscan:replace zone_watermark_ok with zone_balanced for determining if kswapd will call compaction

[Chen Yucong]

According to the commit messages of "mm: vmscan: fix endless loop in kswapd balancing"
and "mm: vmscan: decide whether to compact the pgdat based on reclaim progress", minor
change is required to the following snippet.

        /*
         * If any zone is currently balanced then kswapd will
         * not call compaction as it is expected that the
         * necessary pages are already available.
         */
        if (pgdat_needs_compaction &&
                zone_watermark_ok(zone, order,
                                        low_wmark_pages(zone),
                                        *classzone_idx, 0))
                pgdat_needs_compaction = false;

zone_watermark_ok() should be replaced by zone_balanced() in the above snippet. That's
because zone_balanced() is more suitable for the context.

Signed-off-by: Chen Yucong <slaoub@gmail.com>
---
 mm/vmscan.c |    5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index a8ffe4e..e1004ad 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3157,9 +3157,8 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
 			 * necessary pages are already available.
 			 */
 			if (pgdat_needs_compaction &&
-					zone_watermark_ok(zone, order,
-						low_wmark_pages(zone),
-						*classzone_idx, 0))
+					zone_balanced(zone, order, 0,
+						*classzone_idx))
 				pgdat_needs_compaction = false;
 		}
 
-- 
1.7.10.4

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Re: [PATCH] mm:vmscan:replace zone_watermark_ok with zone_balanced for determining if kswapd will call compaction

[Mel Gorman]

On Sun, Jun 22, 2014 at 04:51:00PM +0800, Chen Yucong wrote:
> According to the commit messages of "mm: vmscan: fix endless loop in kswapd balancing"
> and "mm: vmscan: decide whether to compact the pgdat based on reclaim progress", minor
> change is required to the following snippet.
> 
>         /*
>          * If any zone is currently balanced then kswapd will
>          * not call compaction as it is expected that the
>          * necessary pages are already available.
>          */
>         if (pgdat_needs_compaction &&
>                 zone_watermark_ok(zone, order,
>                                         low_wmark_pages(zone),
>                                         *classzone_idx, 0))
>                 pgdat_needs_compaction = false;
> 
> zone_watermark_ok() should be replaced by zone_balanced() in the above snippet. That's
> because zone_balanced() is more suitable for the context.
> 

What bug does this fix?

The intent here is to prevent kswapd compacting a node if an allocation
request within that node would succeed against the low watermark.
Your change alters that to check against hte high watermark + balance gap
without explaining why kswapd should compact until the high watermark is
reached.

-- 
Mel Gorman
SUSE Labs

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