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

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Message-ID: <ecb88320-9990-49e1-a58a-e8fc85b1da3f@suse.cz>
Date: Mon, 8 Apr 2024 09:38:20 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>,
 Johannes Weiner <hannes@...xchg.org>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: Mel Gorman <mgorman@...hsingularity.net>, Zi Yan <ziy@...dia.com>,
 "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 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).

> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 8330c5c2de6b..2facf844ef84 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -805,8 +805,10 @@ static inline void __free_one_page(struct page *page,
>          while (order < MAX_PAGE_ORDER) {
>                  int buddy_mt = migratetype;
> 
> -               if (compaction_capture(capc, page, order, migratetype))
> +               if (compaction_capture(capc, page, order, migratetype)) {
> +                       account_freepages(zone, -(1 << order), migratetype);
>                          return;
> +               }
> 
>                  buddy = find_buddy_page_pfn(page, pfn, order, &buddy_pfn);
>                  if (!buddy)
> 
> With my fix, the THP percentage looks better:
>                        k6.9-rc2-base          base + patch10 + 2 fixes	+ 
> my fix
> Percentage huge-1        78.18 (   0.00%)       82.83 (   5.94%)
> Percentage huge-3        86.70 (   0.00%)       93.47 (   7.81%)
> Percentage huge-5        90.26 (   0.00%)       94.73 (   4.95%)
> Percentage huge-7        92.34 (   0.00%)       95.22 (   3.12%)
> Percentage huge-12       91.18 (   0.00%)       92.40 (   1.34%)
> Percentage huge-18       89.00 (   0.00%)       85.39 (  -4.06%)
> Percentage huge-24       90.52 (   0.00%)       94.70 (   4.61%)
> Percentage huge-30       94.44 (   0.00%)       97.00 (   2.71%)
> Percentage huge-32       93.09 (   0.00%)       92.87 (  -0.24%)