Re: [PATCH v2 0/4] Extend migrate_misplaced_page() to support batch migration

[Baolin Wang]

On 8/24/2023 12:51 PM, Huang, Ying wrote:
> Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> writes:
>
> > On 8/22/2023 10:47 AM, Huang, Ying wrote:
> > > Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> writes:
> > >
> > > > Hi,
> > > >
> > > > Currently, on our ARM servers with NUMA enabled, we found the cross-die latency
> > > > is a little larger that will significantly impact the workload's performance.
> > > > So on ARM servers we will rely on the NUMA balancing to avoid the cross-die
> > > > accessing. And I posted a patchset[1] to support speculative numa fault to
> > > > improve the NUMA balancing's performance according to the principle of data
> > > > locality. Moreover, thanks to Huang Ying's patchset[2], which introduced batch
> > > > migration as a way to reduce the cost of TLB flush, and it will also benefit
> > > > the migration of multiple pages all at once during NUMA balancing.
> > > >
> > > > So we plan to continue to support batch migration in do_numa_page() to improve
> > > > the NUMA balancing's performance, but before adding complicated batch migration
> > > > algorithm for NUMA balancing, some cleanup and preparation work need to do firstly,
> > > > which are done in this patch set. In short, this patchset extends the
> > > > migrate_misplaced_page() interface to support batch migration, and no functional
> > > > changes intended.
> > > >
> > > > In addition, these cleanup can also benefit the compound page's NUMA balancing,
> > > > which was discussed in previous thread[3]. IIUC, for the compound page's NUMA
> > > > balancing, it is possible that partial pages were successfully migrated, so it is
> > > > necessary to return the number of pages that were successfully migrated from
> > > > migrate_misplaced_page().
> > > But I don't find the return number is used except as bool now.
> >
> > As I said above, this is a preparation for batch migration and
> > compound page NUMA balancing in future.
> >
> > In addition, after looking into the THP' NUMA migration, I found this
> > change is necessary for THP migration. Since it is possible that
> > partial subpages were successfully migrated if the THP is split, so
> > below THP numa fault statistics is not always correct:
> >
> > if (page_nid != NUMA_NO_NODE)
> > 	task_numa_fault(last_cpupid, page_nid, HPAGE_PMD_NR,
> > 				flags);
> >
> > I will try to fix this in next version.
>
> IIUC, THP will not be split for NUMA balancing.  Please check the
> nosplit logic in migrate_pages_batch().
>
> 	bool nosplit = (reason == MR_NUMA_MISPLACED);

Yes, I overlooked this. Thanks for reminding.

> > > Per my understanding, I still don't find much value of the changes
> > > except as preparation for batch migration in NUMA balancing.  So I still
> >
> > IMO, only patch 3 is just a preparation for batch migration, but other
> > patches are some cleanups for migrate_misplaced_page(). I can drop the
> > preparation patches in this series and revise the commit message.
> >
> > > think it's better to wait for the whole series.  Where we can check why
> > > these changes are necessary for batch migration.  And I think that you
> > > will provide some number to justify the batch migration, including pros
> > > and cons.
> > > --
> > > Best Regards,
> > > Huang, Ying
> > >
> > > > This series is based on the latest mm-unstable(d226b59b30cc).
> > > >
> > > > [1] https://lore.kernel.org/lkml/cover.1639306956.git.baolin.wang@xxxxxxxxxxxxxxxxx/t/#mc45929849b5d0e29b5fdd9d50425f8e95b8f2563
> > > > [2] https://lore.kernel.org/all/20230213123444.155149-1-ying.huang@xxxxxxxxx/T/#u
> > > > [3] https://lore.kernel.org/all/f8d47176-03a8-99bf-a813-b5942830fd73@xxxxxxx/
> > > >
> > > > Changes from v1:
> > > >    - Move page validation into a new function suggested by Huang Ying.
> > > >    - Change numamigrate_isolate_page() to boolean type.
> > > >    - Update some commit message.
> > > >
> > > > Baolin Wang (4):
> > > >     mm: migrate: factor out migration validation into
> > > >       numa_page_can_migrate()
> > > >     mm: migrate: move the numamigrate_isolate_page() into do_numa_page()
> > > >     mm: migrate: change migrate_misplaced_page() to support multiple pages
> > > >       migration
> > > >     mm: migrate: change to return the number of pages migrated
> > > >       successfully
> > > >
> > > >    include/linux/migrate.h | 15 +++++++---
> > > >    mm/huge_memory.c        | 23 +++++++++++++--
> > > >    mm/internal.h           |  1 +
> > > >    mm/memory.c             | 43 ++++++++++++++++++++++++++-
> > > >    mm/migrate.c            | 64 +++++++++--------------------------------
> > > >    5 files changed, 88 insertions(+), 58 deletions(-)