Messages in this thread | | | Date | Tue, 14 Nov 2023 12:35:39 +0100 | Subject | Re: [RFC PATCH] mm: support large folio numa balancing | From | David Hildenbrand <> |
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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
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