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

[Baolin Wang]

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?

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

> 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.

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