linux-kernel - Re: [PATCH v2 0/4] Extend migrate_misplaced

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Date:   Thu, 24 Aug 2023 12:51:59 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc:     <akpm@...ux-foundation.org>, <mgorman@...hsingularity.net>,
        <shy828301@...il.com>, <david@...hat.com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 0/4] Extend migrate_misplaced_page() to support batch
 migration

Baolin Wang <baolin.wang@...ux.alibaba.com> writes:

> On 8/22/2023 10:47 AM, Huang, Ying wrote:
>> Baolin Wang <baolin.wang@...ux.alibaba.com> 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);

--
Best Regards,
Huang, Ying

>> 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@linux.alibaba.com/t/#mc45929849b5d0e29b5fdd9d50425f8e95b8f2563
>>> [2] https://lore.kernel.org/all/20230213123444.155149-1-ying.huang@intel.com/T/#u
>>> [3] https://lore.kernel.org/all/f8d47176-03a8-99bf-a813-b5942830fd73@arm.com/
>>>
>>> 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(-)