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

Vlastimil Babka
2017-02-17T16:21:25Z

On 02/15/2017 12:56 PM, Xishi Qiu wrote:
> On 2017/2/15 18:47, Vlastimil Babka wrote:
>
>> On 02/14/2017 11:07 AM, Xishi Qiu wrote:
>>> 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@suse.cz>
>>>
>>> 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.
>>
>> What do you mean by "more fixed" here?
>>
>>> So how about use list_move_tail instead of list_move?
>>
>> Hmm, not sure if it can make any difference. We steal because the lists
>> are currently empty (at least for the order we want), so it shouldn't
>> matter if we add to head or tail.
>>
>
> Hi Vlastimil,
>
> Please see the following case, I am not sure if it is right.
>
> MIGRATE_MOVABLE
> order: 0 1 2 3 4 5 6 7 8 9 10
> free num: 1 1 1 1 1 1 1 1 1 1 0 // one page(e.g. page A) was allocated before
>
> MIGRATE_UNMOVABLE
> order: 0 1 2 3 4 5 6 7 8 9 10
> free num: x x x x 0 0 0 0 0 0 0 // we want order=4, so steal from MIGRATE_MOVABLE
>
> We alloc order=4 in MIGRATE_UNMOVABLE, then it will fallback to steal pages from
> MIGRATE_MOVABLE, and we will move free pages form MIGRATE_MOVABLE list to
> MIGRATE_UNMOVABLE list.
>
> List of order 4-9 in MIGRATE_UNMOVABLE is empty, so add head or tail is the same.
> But order 0-3 is not empty, so if we add to the head, we will allocate pages which
> stolen from MIGRATE_MOVABLE first later. So we will have less chance to make a large
> block(order=10) when the one page(page A) free again.

I see. But do we know that page A, and the order-4 page we just allocated, are
both going to be freed soon? It's not a clear win to me, so maybe you can try
implementing it and see if it makes any difference?

> Also we will split order=9 which from MIGRATE_MOVABLE to alloc order=4 in expand(),

Yes, for pageblock order == 9.

> so if we add to the head, we will allocate pages which split from order=9 first later.
> So we will have less chance to make a large block(order=9) when the order=4 page
> free again.

Again that assumes our order-4 allocation is temporary. Is there a significant
chance of this?

>>> __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.
>>
>> Same here. The lists are not empty, but contain probably just the pages
>> from our stolen pageblock. It shouldn't matter how we order them within
>> the same block.
>>
>> So maybe it could make some difference for higher-order allocations, but
>> it's unclear to me. Making e.g. expand() more complex with a flag to
>> tell it the head vs tail add could mean extra overhead in allocator fast
>> path that would offset any gains.
>>
>>> Thanks,
>>> Xishi Qiu
>>>
>>
>>
>> .
>>
>
>
>