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

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Date:   Wed, 15 Feb 2017 19:56:40 +0800
From:   Xishi Qiu <qiuxishi@...wei.com>
To:     Vlastimil Babka <vbabka@...e.cz>
CC:     <linux-mm@...ck.org>, Johannes Weiner <hannes@...xchg.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        David Rientjes <rientjes@...gle.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        <linux-kernel@...r.kernel.org>, <kernel-team@...com>
Subject: Re: [PATCH v2 04/10] mm, page_alloc: count movable pages when stealing
 from pageblock

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@...e.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.

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

>> __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
>>
> 
> 
> .
>