Re: [PATCH v2 04/10] mm, page_alloc: count movable pages when stealing from pageblock

[Xishi Qiu]

On 2017/2/11 1:23, Vlastimil Babka wrote:

> When stealing pages from pageblock of a different migratetype, we count how
> many free pages were stolen, and change the pageblock's migratetype if more
> than half of the pageblock was free. This might be too conservative, as there
> might be other pages that are not free, but were allocated with the same
> migratetype as our allocation requested.
> 
> While we cannot determine the migratetype of allocated pages precisely (at
> least without the page_owner functionality enabled), we can count pages that
> compaction would try to isolate for migration - those are either on LRU or
> __PageMovable(). The rest can be assumed to be MIGRATE_RECLAIMABLE or
> MIGRATE_UNMOVABLE, which we cannot easily distinguish. This counting can be
> done as part of free page stealing with little additional overhead.
> 
> The page stealing code is changed so that it considers free pages plus pages
> of the "good" migratetype for the decision whether to change pageblock's
> migratetype.
> 
> The result should be more accurate migratetype of pageblocks wrt the actual
> pages in the pageblocks, when stealing from semi-occupied pageblocks. This
> should help the efficiency of page grouping by mobility.
> 
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>

Hi Vlastimil,

How about these two changes?

1. If we steal some free pages, we will add these page at the head of start_migratetype
list, it will cause more fixed, because these pages will be allocated more easily.
So how about use list_move_tail instead of list_move?

__rmqueue_fallback
	steal_suitable_fallback
		move_freepages_block
			move_freepages
				list_move

2. When doing expand() - list_add(), usually the list is empty, but in the
following case, the list is not empty, because we did move_freepages_block()
before.

__rmqueue_fallback
	steal_suitable_fallback
		move_freepages_block  // move to the list of start_migratetype
	expand  // split the largest order
		list_add  // add to the list of start_migratetype

So how about use list_add_tail instead of list_add? Then we can merge the large
block again as soon as the page freed.

Thanks,
Xishi Qiu