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

[Baolin Wang]

(sorry for late reply)

On 2024/3/19 15:26, Huang, Ying wrote:
> Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> writes:
>
> > 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.
> >
> > > 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.
>
> Yes.  The two-stage filter in should_numa_migrate_memory() helps at some
> degree.
>
> But the situation is somewhat different after your change.  Previously,
> in one round of NUMA balancing page table scanning, the number of the
> hint page fault for one process and one folio is 1.  After your change,
> the number may become folio_nr_pages().  So we need to evaluate the

Yes, this follows the same strategy as THP.

> original algorithm in the new situation and revise.  For example, use a
> N-stage filter for mTHP.

Yes, let me try if N-stage filter for mTHP can helpful.

> Anyway, the NUMA balancing algorithm adjustment needs to be based on
> test results.
>
> Another possibility is to emulate the original behavior as much as
> possible to try to reuse the original algorithm.  For example, we can
> restore all PTE maps upon the first hint page fault of a folio.  Then,
> the behavior is almost same as the original PMD mapped THP.  Personally,
> I prefer to use this as the first step.  Then, try to adjust the
> algorithm to take advantage of more information available.

OK, sounds reasonable, I will try.

> > 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().
>
> I am referring to the following code in should_numa_migrate_memory().
>
> 	/*
> 	 * Allow first faults or private faults to migrate immediately early in
> 	 * the lifetime of a task. The magic number 4 is based on waiting for
> 	 * two full passes of the "multi-stage node selection" test that is
> 	 * executed below.
> 	 */
> 	if ((p->numa_preferred_nid == NUMA_NO_NODE || p->numa_scan_seq <= 4) &&
> 	    (cpupid_pid_unset(last_cpupid) || cpupid_match_pid(p, last_cpupid)))
> 		return true;
>
> But, after thought about this again, I realized that the original PMD
> mapped THP may be migrated on the first fault sometimes.  So, this isn't
> a new problem.  We may "optimize" it.  But it needn't to be part of this
> series.

Make sense. Thanks for your input.