Re: [PATCH] mm: migrate: record the mlocked page status to remove unnecessary lru drain

[Yin, Fengwei]

On 10/19/2023 4:51 PM, Baolin Wang wrote:
> 
> 
> On 10/19/2023 4:22 PM, Yin Fengwei wrote:
>> Hi Baolin,
>>
>> On 10/19/23 15:25, Baolin Wang wrote:
>>>
>>>
>>> On 10/19/2023 2:09 PM, Huang, Ying wrote:
>>>> Zi Yan <ziy@xxxxxxxxxx> writes:
>>>>
>>>>> On 18 Oct 2023, at 9:04, Baolin Wang wrote:
>>>>>
>>>>>> When doing compaction, I found the lru_add_drain() is an obvious hotspot
>>>>>> when migrating pages. The distribution of this hotspot is as follows:
>>>>>>      - 18.75% compact_zone
>>>>>>         - 17.39% migrate_pages
>>>>>>            - 13.79% migrate_pages_batch
>>>>>>               - 11.66% migrate_folio_move
>>>>>>                  - 7.02% lru_add_drain
>>>>>>                     + 7.02% lru_add_drain_cpu
>>>>>>                  + 3.00% move_to_new_folio
>>>>>>                    1.23% rmap_walk
>>>>>>               + 1.92% migrate_folio_unmap
>>>>>>            + 3.20% migrate_pages_sync
>>>>>>         + 0.90% isolate_migratepages
>>>>>>
>>>>>> The lru_add_drain() was added by commit c3096e6782b7 ("mm/migrate:
>>>>>> __unmap_and_move() push good newpage to LRU") to drain the newpage to LRU
>>>>>> immediately, to help to build up the correct newpage->mlock_count in
>>>>>> remove_migration_ptes() for mlocked pages. However, if there are no mlocked
>>>>>> pages are migrating, then we can avoid this lru drain operation, especailly
>>>>>> for the heavy concurrent scenarios.
>>>>>
>>>>> lru_add_drain() is also used to drain pages out of folio_batch. Pages in folio_batch
>>>>> have an additional pin to prevent migration. See folio_get(folio); in folio_add_lru().
>>>>
>>>> lru_add_drain() is called after the page reference count checking in
>>>> move_to_new_folio().  So, I don't this is an issue.
>>>
>>> Agree. The purpose of adding lru_add_drain() is to address the 'mlock_count' issue for mlocked pages. Please see commit c3096e6782b7 and related comments. Moreover I haven't seen an increase in the number of page migration failures due to page reference count checking after this patch.
>>
>> I agree with your. My understanding also is that the lru_add_drain() is only needed
>> for mlocked folio to correct mlock_count. Like to hear the confirmation from Huge.
>>
>>
>> But I have question: why do we need use page_was_mlocked instead of check
>> folio_test_mlocked(src)? Does page migration clear the mlock flag? Thanks.
> 
> Yes, please see the call trace: try_to_migrate_one() ---> page_remove_rmap() ---> munlock_vma_folio().

Yes. This will clear mlock bit.

What about set dst folio mlocked if source is before try_to_migrate_one()? And
then check whether dst folio is mlocked after? And need clear mlocked if migration
fails. I suppose the change is minor. Just a thought. Thanks.


Regards
Yin, Fengwei