Re: [RFC PATCH 2/3] mm: more aggressive page stealing for UNMOVABLE allocations

[Minchan Kim]

On Thu, Dec 04, 2014 at 06:12:57PM +0100, Vlastimil Babka wrote:
> When allocation falls back to stealing free pages of another migratetype,
> it can decide to steal extra pages, or even the whole pageblock in order to
> reduce fragmentation, which could happen if further allocation fallbacks
> pick a different pageblock. In try_to_steal_freepages(), one of the situations
> where extra pages are stolen happens when we are trying to allocate a
> MIGRATE_RECLAIMABLE page.
> 
> However, MIGRATE_UNMOVABLE allocations are not treated the same way, although
> spreading such allocation over multiple fallback pageblocks is arguably even
> worse than it is for RECLAIMABLE allocations. To minimize fragmentation, we
> should minimize the number of such fallbacks, and thus steal as much as is
> possible from each fallback pageblock.

Fair enough.

> 
> This patch thus adds a check for MIGRATE_UNMOVABLE to the decision to steal
> extra free pages. When evaluating with stress-highalloc from mmtests, this has
> reduced the number of MIGRATE_UNMOVABLE fallbacks to roughly 1/6. The number
> of these fallbacks stealing from MIGRATE_MOVABLE block is reduced to 1/3.
> 
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
Acked-by: Minchan Kim <minchan@xxxxxxxxxx>

Nit:

Please fix comment on try_to_steal_freepages.
We don't bias MIGRATE_RECLAIMABLE any more so remove it. Instead,
put some words about the policy and why.

Thanks.

> ---
>  mm/page_alloc.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 548b072..a14249c 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1098,6 +1098,7 @@ static int try_to_steal_freepages(struct zone *zone, struct page *page,
>  
>  	if (current_order >= pageblock_order / 2 ||
>  	    start_type == MIGRATE_RECLAIMABLE ||
> +	    start_type == MIGRATE_UNMOVABLE ||
>  	    page_group_by_mobility_disabled) {
>  		int pages;
>  
> -- 
> 2.1.2
> 
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-- 
Kind regards,
Minchan Kim
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