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

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2024/4/9 14:23, Vlastimil Babka wrote:
> On 4/8/24 4:23 PM, 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.
>>
>> In earlier versions of the patches I had more aggressive capturing,
>> but also did the accounting in add_page_to/del_page_from_freelist(),
>> so capturing only steals what's already accounted to be off list.
>>
>> With the __add/__del and the consolidated account_freepages()
>> optimization, compaction_capture() needs explicit accounting again.
>>
>>>> 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%)
>>
>> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> 
> Acked-by: Vlastimil Babka <vbabka@suse.cz>

Thanks.

>> With fixed indentation:
> 
> Maybe Baolin could resend the finalized 2 fixups in a more ready-to-pick form?

Sure, I've send it out.

And I see Andrew has already folded the first fix 
("mm-page_alloc-fix-freelist-movement-during-block-conversion-fix") into 
the patch set. If a formal fix patch is needed, Andrew, please let me know.