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

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Message-ID: <1b4de866-df27-46fa-81fa-6818a48d8cc1@redhat.com>
Date:   Tue, 14 Nov 2023 12:35:39 +0100
From:   David Hildenbrand <david@...hat.com>
To:     John Hubbard <jhubbard@...dia.com>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        akpm@...ux-foundation.org
Cc:     ying.huang@...el.com, willy@...radead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] mm: support large folio numa balancing

On 13.11.23 23:15, John Hubbard wrote:
> On 11/13/23 5:01 AM, Baolin Wang wrote:
>>
>>
>> On 11/13/2023 8:10 PM, Kefeng Wang wrote:
>>>
>>>
>>> On 2023/11/13 18:53, 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@...wei.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.
>>>>
>>>>
>>>> I'll send WIP patches for one approach that can improve the situation
>>>> soonish.
> 
> To be honest, I'm still catching up on the approximate vs. exact
> sharers case. It wasn't clear to me why a precise sharers count
> is needed in order to do this. Perhaps the cost of making a wrong
> decision is considered just too high?

Good question, I didn't really look into the impact for the NUMA hinting 
case where we might end up not setting TNF_SHARED although it is shared. 
For other folio_estimate_sharers() users it's more obvious.

As a side note, it could have happened already in corner cases (e.g., 
concurrent page migration of a small folio).

If precision as documented in that commit is really required remains to 
be seen -- just wanted to spell it out.

-- 
Cheers,

David / dhildenb