Re: [RFC PATCH 3/3] mm: always steal split buddies in fallback allocations

[Joonsoo Kim <iamjoonsoo.kim@lge.com>]

On Thu, Dec 04, 2014 at 06:12:58PM +0100, Vlastimil Babka wrote:
> When allocation falls back to another migratetype, it will steal a page with
> highest available order, and (depending on this order and desired migratetype),
> it might also steal the rest of free pages from the same pageblock.
> 
> Given the preference of highest available order, it is likely that it will be
> higher than the desired order, and result in the stolen buddy page being split.
> The remaining pages after split are currently stolen only when the rest of the
> free pages are stolen. This can however lead to situations where for MOVABLE
> allocations we split e.g. order-4 fallback UNMOVABLE page, but steal only
> order-0 page. Then on the next MOVABLE allocation (which may be batched to
> fill the pcplists) we split another order-3 or higher page, etc. By stealing
> all pages that we have split, we can avoid further stealing.
> 
> This patch therefore adjust the page stealing so that buddy pages created by
> split are always stolen. This has effect only on MOVABLE allocations, as
> RECLAIMABLE and UNMOVABLE allocations already always do that in addition to
> stealing the rest of free pages from the pageblock.

In fact, CMA also has same problem and this patch skips to fix it.
If movable allocation steals the page on CMA reserved area, remained split
freepages are always linked to original CMA buddy list. And then, next
fallback allocation repeately selects most highorder freepage on CMA
area and split it.

IMO, It'd be better to re-consider whole fragmentation avoidance logic.

Thanks.

> 
> Note that commit 7118af076f6 ("mm: mmzone: MIGRATE_CMA migration type added")
> has already performed this change (unintentinally), but was reverted by commit
> 0cbef29a7821 ("mm: __rmqueue_fallback() should respect pageblock type").
> Neither included evaluation. My evaluation with stress-highalloc from mmtests
> shows about 2.5x reduction of page stealing events for MOVABLE allocations,
> without affecting the page stealing events for other allocation migratetypes.
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---
>  mm/page_alloc.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index a14249c..82096a6 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1108,11 +1108,9 @@ static int try_to_steal_freepages(struct zone *zone, struct page *page,
>  		if (pages >= (1 << (pageblock_order-1)) ||
>  				page_group_by_mobility_disabled)
>  			set_pageblock_migratetype(page, start_type);
> -
> -		return start_type;
>  	}
>  
> -	return fallback_type;
> +	return start_type;
>  }
>  
>  /* Remove an element from the buddy allocator from the fallback list */
> -- 
> 2.1.2
> 
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