Re: [RFC PATCH v2] mm: support multi-size THP numa balancing

[Baolin Wang]

On 2024/3/18 17:48, David Hildenbrand wrote:
> On 18.03.24 10:42, Baolin Wang wrote:
> > On 2024/3/18 14:16, Huang, Ying wrote:
> > > Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> writes:
> > >
> > > > Now the anonymous page allocation already supports multi-size THP 
> > > > (mTHP),
> > > > but the numa balancing still prohibits mTHP migration even though it 
> > > > is an
> > > > exclusive mapping, which is unreasonable. Thus let's support the 
> > > > exclusive
> > > > mTHP numa balancing firstly.
> > > >
> > > > Allow scanning mTHP:
> > > > Commit 859d4adc3415 ("mm: numa: do not trap faults on shared data 
> > > > section
> > > > pages") skips shared CoW pages' NUMA page migration to avoid shared 
> > > > data
> > > > segment migration. In addition, commit 80d47f5de5e3 ("mm: don't try to
> > > > NUMA-migrate COW pages that have other uses") change to use 
> > > > page_count()
> > > > to avoid GUP pages migration, that will also skip the mTHP numa 
> > > > scaning.
> > > > Theoretically, we can use folio_maybe_dma_pinned() to detect the GUP
> > > > issue, although there is still a GUP race, the issue seems to have been
> > > > resolved by commit 80d47f5de5e3. Meanwhile, use the 
> > > > folio_estimated_sharers()
> > > > to skip shared CoW pages though this is not a precise sharers count. To
> > > > check if the folio is shared, ideally we want to make sure every 
> > > > page is
> > > > mapped to the same process, but doing that seems expensive and using
> > > > the estimated mapcount seems can work when running autonuma benchmark.
> > > >
> > > > Allow migrating mTHP:
> > > > As mentioned in the previous thread[1], large folios are more 
> > > > susceptible
> > > > to false sharing issues, leading to pages ping-pong back and forth 
> > > > during
> > > > numa balancing, which is currently hard to resolve. Therefore, as a 
> > > > start to
> > > > support mTHP numa balancing, only exclusive mappings are allowed to 
> > > > perform
> > > > numa migration to avoid the false sharing issues with large folios. 
> > > > Similarly,
> > > > use the estimated mapcount to skip shared mappings, which seems can 
> > > > work
> > > > in most cases (?), and we've used folio_estimated_sharers() to skip 
> > > > shared
> > > > mappings in migrate_misplaced_folio() for numa balancing, seems no real
> > > > complaints.
> > >
> > > IIUC, folio_estimated_sharers() cannot identify multi-thread
> > > applications.  If some mTHP is shared by multiple threads in one
> >
> > Right.
>
> Wasn't this "false sharing" previously raised/described by Mel in this 
> context?

Yes, I got confused with the process's false sharing.

> > > process, how to deal with that?
> >
> > IMHO, seems the should_numa_migrate_memory() already did something to 
> > help?
> >
> > ......
> >     if (!cpupid_pid_unset(last_cpupid) &&
> >                 cpupid_to_nid(last_cpupid) != dst_nid)
> >         return false;
> >
> >     /* Always allow migrate on private faults */
> >     if (cpupid_match_pid(p, last_cpupid))
> >         return true;
> > ......
> >
> > If the node of the CPU that accessed the mTHP last time is different
> > from this time, which means there is some contention for that mTHP among
> > threads. So it will not allow migration.
> >
> > If the contention for the mTHP among threads is light or the accessing
> > is relatively stable, then we can allow migration?
> >
> > > For example, I think that we should avoid to migrate on the first fault
> > > for mTHP in should_numa_migrate_memory().
> > >
> > > More thoughts?  Can we add a field in struct folio for mTHP to count
> > > hint page faults from the same node?
> >
> > IIUC, we do not need add a new field for folio, seems we can reuse
> > ->_flags_2a field. But how to use it? If there are multiple consecutive
> > NUMA faults from the same node, then allow migration?
>
> _flags_2a cannot be used. You could place something after _deferred_list 

Could you be more explicit? I didn't see _flags_2 currently being used, 
did I miss something?

> IIRC. But only for folios with order>1.

Yes, order 1 folio may use the same strategy with order 0, but need some 
evaluation.

> But I also wonder how one could achieve that using a new field.