Re: [PATCH v3 4/8] mm, page_alloc: count movable pages when stealing from pageblock

[Joonsoo Kim]

On Tue, Mar 07, 2017 at 02:15:41PM +0100, 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.

I think that too conservative is good for movable case. In my experiments,
fragmentation spreads out when unmovable/reclaimable pageblock is
changed to movable pageblock prematurely ('prematurely' means that
allocated unmovable pages remains). As you said below, movable allocations
falling back to other pageblocks don't causes permanent fragmentation.
Therefore, we don't need to be less conservative for movable
allocation. So, how about following change to keep the criteria for
movable allocation conservative even with this counting improvement?

threshold = (1 << (pageblock_order - 1));
if (start_type == MIGRATE_MOVABLE)
        threshold += (1 << (pageblock_order - 2));

if (free_pages + alike_pages >= threshold)
        ...

Thanks.