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

[Huang, Ying]

Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> writes:

> On 11/14/2023 9:12 AM, Huang, Ying wrote:
>> David Hildenbrand <david@xxxxxxxxxx> writes:
>> 
>>> 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.
>> Even 4k folio may be shared by multiple processes/threads.  So, numa
>> balancing uses a multi-stage node selection algorithm (mostly
>> implemented in should_numa_migrate_memory()) to identify shared folios.
>> I think that the algorithm needs to be adjusted for PTE mapped large
>> folio for shared folios.
>
> Not sure I get you here. In should_numa_migrate_memory(), it will use
> last CPU id, last PID and group numa faults to determine if this page
> can be migrated to the target node. So for large folio, a precise
> folio sharers check can make the numa faults of a group more accurate,
> which is enough for should_numa_migrate_memory() to make a decision?

A large folio that is mapped by multiple process may be accessed by one
remote NUMA node, so we still want to migrate it.  A large folio that is
mapped by one process but accessed by multiple threads on multiple NUMA
node may be not migrated.

> Could you provide a more detailed description of the algorithm you
> would like to change for large folio? Thanks.

I haven't thought about that thoroughly.  So, please evaluate the
algorithm by yourself.

For example, the 2 sub-pages of a shared PTE-mapped large folio may be
accessed together by a task.  This may cause the folio be migrated
wrongly.  One possible solution is to restore all other PTE mappings of
the large folio in do_numa_page() as the first step.  This resembles the
PMD-mapped THP behavior.

>> And, as a performance improvement patch, some performance data needs to
>
> Do you have some benchmark recommendation? I know the the autonuma can
> not support large folio now.

There are autonuma-benchmark, and specjbb is used by someone before.

>> be provided.  And, the effect of shared folio detection needs to be
>> tested too

--
Best Regards,
Huang, Ying