linux-kernel - Re: [PATCH 10/10] mm: page_alloc: consolidate free page accounting

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Date: Wed, 10 Apr 2024 16:49:42 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Zi Yan <ziy@...dia.com>
Cc: Johannes Weiner <hannes@...xchg.org>, Vlastimil Babka <vbabka@...e.cz>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Mel Gorman <mgorman@...hsingularity.net>, "Huang, Ying"
 <ying.huang@...el.com>, David Hildenbrand <david@...hat.com>,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 10/10] mm: page_alloc: consolidate free page accounting



On 2024/4/9 22:46, Zi Yan wrote:
> On 9 Apr 2024, at 5:31, Baolin Wang wrote:
> 
>> On 2024/4/8 22:23, Johannes Weiner wrote:
>>> On Mon, Apr 08, 2024 at 09:38:20AM +0200, Vlastimil Babka wrote:
>>>> On 4/7/24 12:19 PM, Baolin Wang wrote:
>>>>> On 2024/3/21 02:02, Johannes Weiner wrote:
>>>>>>    +	account_freepages(page, zone, 1 << order, migratetype);
>>>>>> +
>>>>>>     	while (order < MAX_PAGE_ORDER) {
>>>>>> -		if (compaction_capture(capc, page, order, migratetype)) {
>>>>>> -			__mod_zone_freepage_state(zone, -(1 << order),
>>>>>> -								migratetype);
>>>>>> +		int buddy_mt = migratetype;
>>>>>> +
>>>>>> +		if (compaction_capture(capc, page, order, migratetype))
>>>>>>     			return;
>>>>>> -		}
>>>>>
>>>>> IIUC, if the released page is captured by compaction, then the
>>>>> statistics for free pages should be correspondingly decreased,
>>>>> otherwise, there will be a slight regression for my thpcompact benchmark.
>>>>>
>>>>> thpcompact Percentage Faults Huge
>>>>>                              k6.9-rc2-base        base + patch10 + 2 fixes	
>>>>> Percentage huge-1        78.18 (   0.00%)       71.92 (  -8.01%)
>>>>> Percentage huge-3        86.70 (   0.00%)       86.07 (  -0.73%)
>>>>> Percentage huge-5        90.26 (   0.00%)       78.02 ( -13.57%)
>>>>> Percentage huge-7        92.34 (   0.00%)       78.67 ( -14.81%)
>>>>> Percentage huge-12       91.18 (   0.00%)       81.04 ( -11.12%)
>>>>> Percentage huge-18       89.00 (   0.00%)       79.57 ( -10.60%)
>>>>> Percentage huge-24       90.52 (   0.00%)       80.07 ( -11.54%)
>>>>> Percentage huge-30       94.44 (   0.00%)       96.28 (   1.95%)
>>>>> Percentage huge-32       93.09 (   0.00%)       99.39 (   6.77%)
>>>>>
>>>>> I add below fix based on your fix 2, then the thpcompact Percentage
>>>>> looks good. How do you think for the fix?
>>>>
>>>> Yeah another well spotted, thanks. "slight regression" is an understatement,
>>>> this affects not just a "statistics" but very important counter
>>>> NR_FREE_PAGES which IIUC would eventually become larger than reality, make
>>>> the watermark checks false positive and result in depleted reserves etc etc.
>>>> Actually wondering why we're not seeing -next failures already (or maybe I
>>>> just haven't noticed).
>>>
>>> Good catch indeed.
>>>
>>> Trying to understand why I didn't notice this during testing, and I
>>> think it's because I had order-10 pageblocks in my config. There is
>>> this in compaction_capture():
>>>
>>> 	if (order < pageblock_order && migratetype == MIGRATE_MOVABLE)
>>> 		return false;
>>>
>>> Most compaction is for order-9 THPs on movable blocks, so I didn't get
>>> much capturing in practice in order for that leak to be noticable.
>>
>> This makes me wonder why not use 'cc->migratetype' for migratetype comparison, so that low-order (like mTHP) compaction can directly get the released pages, which could avoid some compaction scans without mixing the migratetype?
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 2facf844ef84..7a64020f8222 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -622,7 +622,7 @@ compaction_capture(struct capture_control *capc, struct page *page,
>>           * and vice-versa but no more than normal fallback logic which can
>>           * have trouble finding a high-order free page.
>>           */
>> -       if (order < pageblock_order && migratetype == MIGRATE_MOVABLE)
>> +       if (order < pageblock_order && capc->cc->migratetype != migratetype)
>>                  return false;
>>
>>          capc->page = page;
> 
> It is worth trying, since at the original patch time mTHP was not present and
> not capturing any MIGRATE_MOVABLE makes sense. But with your change, the capture
> will lose the opportunity of letting an unmovable request use a reclaimable
> pageblock and vice-versa, like the comment says. Please change the comment
> as well and we should monitor potential unmovable and reclaimable regression.

Yes, but I think this case is easy to solve. Anyway let me try to do 
some measurement for mTHP.