Messages in this thread | | | Date | Fri, 20 Oct 2023 10:09:35 +0800 | Subject | Re: [PATCH] mm: migrate: record the mlocked page status to remove unnecessary lru drain | From | Baolin Wang <> |
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On 10/19/2023 8:07 PM, Yin, Fengwei wrote: > > > 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@nvidia.com> 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.
IMO, this will break the mlock related statistics in mlock_folio() when the remove_migration_pte() rebuilds the mlock status and mlock count.
Another concern I can see is that, during the page migration, a concurrent munlock() can be called to clean the VM_LOCKED flags for the VMAs, so the remove_migration_pte() should not rebuild the mlock status and mlock count. But the dst folio's mlcoked status is still remained, which is wrong.
So your suggested apporach seems not easy, and I think my patch is simple with re-using existing __migrate_folio_record() and __migrate_folio_extract() :)
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