Messages in this thread | | | Date | Mon, 08 Dec 2014 11:30:58 +0100 | From | Vlastimil Babka <> | Subject | Re: [RFC PATCH 3/3] mm: always steal split buddies in fallback allocations |
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On 12/08/2014 08:36 AM, Joonsoo Kim wrote: > 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.
Hm yeah, for CMA it would make more sense to steal page of the lowest available order, not highest.
> 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 >> >> -- >> To unsubscribe, send a message with 'unsubscribe linux-mm' in >> the body to majordomo@kvack.org. For more info on Linux MM, >> see: http://www.linux-mm.org/ . >> Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
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