Re: [RFC PATCH] mm: support large folio numa balancing

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

On 11/13/2023 10:49 PM, David Hildenbrand wrote:
> On 13.11.23 13:59, Baolin Wang wrote:
>>
>>
>> On 11/13/2023 6:53 PM, David Hildenbrand wrote:
>>> On 13.11.23 11:45, Baolin Wang wrote:
>>>> Currently, the file pages already support large folio, and 
>>>> supporting for
>>>> anonymous pages is also under discussion[1]. Moreover, the numa 
>>>> balancing
>>>> code are converted to use a folio by previous thread[2], and the
>>>> migrate_pages
>>>> function also already supports the large folio migration.
>>>>
>>>> So now I did not see any reason to continue restricting NUMA balancing
>>>> for
>>>> large folio.
>>>
>>> I recall John wanted to look into that. CCing him.
>>>
>>> I'll note that the "head page mapcount" heuristic to detect sharers will
>>> now strike on the PTE path and make us believe that a large folios is
>>> exclusive, although it isn't.
>>>
>>> As spelled out in the commit you are referencing:
>>>
>>> commit 6695cf68b15c215d33b8add64c33e01e3cbe236c
>>> Author: Kefeng Wang <wangkefeng.wang@huawei.com>
>>> Date:   Thu Sep 21 15:44:14 2023 +0800
>>>
>>>       mm: memory: use a folio in do_numa_page()
>>>       Numa balancing only try to migrate non-compound page in
>>> do_numa_page(),
>>>       use a folio in it to save several compound_head calls, note we use
>>>       folio_estimated_sharers(), it is enough to check the folio sharers
>>> since
>>>       only normal page is handled, if large folio numa balancing is
>>> supported, a
>>>       precise folio sharers check would be used, no functional change
>>> intended.
>>
>> Thanks for pointing out the part I missed.
>>
>> I saw the migrate_pages() syscall is also using
>> folio_estimated_sharers() to check if the folio is shared, and I wonder
>> it will bring about any significant issues?
> 
> It's now used all over the place, in some places for making manual 
> decisions (e.g., MADV_PAGEOUT works although it shouldn't) and more and 
> more automatic places (e.g., the system ends up migrating a folio 
> although it shouldn't). The nasty thing about it is that it doesn't give 
> you "certainly exclusive" vs. "maybe shared" but "maybe exclusive" vs. 
> "certainly shared".
> 
> IIUC, the side effect could be that we migrate folios because we assume 
> they are exclusive even though they are actually shared. Right now, it's 
> sufficient to not have the first page of the folio mapped anymore for 
> that to happen.

Yes.

> Anyhow, it's worth mentioning that in the commit message as long as we 
> have no better solution for that. For many cases it might be just 
> tolerable.

Agree. The 'maybe shared' folio may affect the numa group statistics, 
which is used to accumulate the numa faults in one group to choose a 
prefered node for the tasks. For this case, it may be tolerable too, but 
I have no performance numbers now. Let me think about it.

>>> I'll send WIP patches for one approach that can improve the situation
>>> soonish.
>>
>> Great. Look forward to seeing this:)
> 
> I'm still trying to evaluate the performance hit of the additional 
> tracking ... turns out there is no such thing as free food ;)

Make sense.