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

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Date:   Thu, 24 Aug 2023 14:26:25 +0800
From:   Baolin Wang <baolin.wang@...ux.alibaba.com>
To:     "Huang, Ying" <ying.huang@...el.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



On 8/24/2023 12:51 PM, Huang, Ying wrote:
> 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);

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@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(-)