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

[Joonsoo 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.

I'm not sure that this change is good. If we steal order 0 pages,
this may be good. But, sometimes, we try to steal high order page
and, in this case, there would be many order 0 freepages and blindly
stealing freepages in that pageblock make the system more fragmented.

MIGRATE_RECLAIMABLE is different case than MIGRATE_UNMOVABLE, because
it can be reclaimed so excessive migratetype movement doesn't result
in permanent fragmentation.

What I'd like to do to prevent fragmentation is
1) check whether we can steal all or almost freepages and change
migratetype of pageblock.
2) If above condition isn't met, deny allocation and invoke compaction.

Maybe knob to control behaviour would be needed.
How about it?

Thanks.

> 
> 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>
> ---
>  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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