lkml.org 
[lkml]   [2023]   [Nov]   [14]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
SubjectRe: [RFC PATCH] mm: support large folio numa balancing
From
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@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.
>>>>
>>>>
>>>> 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

\
 
 \ /
  Last update: 2023-11-20 13:59    [W:0.055 / U:0.084 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site