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

[Vlastimil Babka <vbabka@suse.cz>]

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
>>
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