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SubjectRe: [PATCH v2] mm: migrate: record the mlocked page status to remove unnecessary lru drain
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On 10/20/2023 12:48 PM, Hugh Dickins wrote:
> On Fri, 20 Oct 2023, 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.
>>
>> So we can record the source pages' mlocked status in migrate_folio_unmap(),
>> and only drain the lru list when the mlocked status is set in migrate_folio_move().
>> In addition, the page was already isolated from lru when migrating, so checking
>> the mlocked status is stable by folio_test_mlocked() in migrate_folio_unmap().
>>
>> After this patch, I can see the hotpot of the lru_add_drain() is gone:
>> - 9.41% migrate_pages_batch
>> - 6.15% migrate_folio_move
>> - 3.64% move_to_new_folio
>> + 1.80% migrate_folio_extra
>> + 1.70% buffer_migrate_folio
>> + 1.41% rmap_walk
>> + 0.62% folio_add_lru
>> + 3.07% migrate_folio_unmap
>>
>> Meanwhile, the compaction latency shows some improvements when running
>> thpscale:
>> base patched
>> Amean fault-both-1 1131.22 ( 0.00%) 1112.55 * 1.65%*
>> Amean fault-both-3 2489.75 ( 0.00%) 2324.15 * 6.65%*
>> Amean fault-both-5 3257.37 ( 0.00%) 3183.18 * 2.28%*
>> Amean fault-both-7 4257.99 ( 0.00%) 4079.04 * 4.20%*
>> Amean fault-both-12 6614.02 ( 0.00%) 6075.60 * 8.14%*
>> Amean fault-both-18 10607.78 ( 0.00%) 8978.86 * 15.36%*
>> Amean fault-both-24 14911.65 ( 0.00%) 11619.55 * 22.08%*
>> Amean fault-both-30 14954.67 ( 0.00%) 14925.66 * 0.19%*
>> Amean fault-both-32 16654.87 ( 0.00%) 15580.31 * 6.45%*
>>
>
> Seems a sensible change with good results (I'll conceal how little of
> the stats I understand, I expect everyone else understands them: in my
> naivety, I'm mainly curious why rmap_walk's 1.23% didn't get a + on it).

TBH, I also don't know why the rmap_walk didn't get a + on it, let me
check it again.

>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>> ---
>> Chages from v1:
>> - Use separate flags in __migrate_folio_record() to avoid to pack flags
>> in each call site per Ying.
>> ---
>> mm/migrate.c | 47 +++++++++++++++++++++++++++++++++++------------
>> 1 file changed, 35 insertions(+), 12 deletions(-)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index 125194f5af0f..fac96139dbba 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -1027,22 +1027,39 @@ union migration_ptr {
>> struct anon_vma *anon_vma;
>> struct address_space *mapping;
>> };
>> +
>> +enum {
>> + PAGE_WAS_MAPPED = 1 << 0,
>> + PAGE_WAS_MLOCKED = 1 << 1,
>> +};
>> +
>
> I was whispering to myself "I bet someone will suggest BIT()";
> and indeed that someone has turned out to be Huang, Ying.

Sure.

>
>> static void __migrate_folio_record(struct folio *dst,
>> - unsigned long page_was_mapped,
>> + unsigned int page_was_mapped,
>> + unsigned int page_was_mlocked,
>> struct anon_vma *anon_vma)
>> {
>> union migration_ptr ptr = { .anon_vma = anon_vma };
>> + unsigned long page_flags = 0;
>
> Huang, Ying preferred a different name, me too: old_page_state?

OK, sounds better to me.

>
>> +
>> + if (page_was_mapped)
>> + page_flags |= PAGE_WAS_MAPPED;
>> + if (page_was_mlocked)
>> + page_flags |= PAGE_WAS_MLOCKED;
>
> What's annoying me about the patch is all this mix of page_was_mapped and
> page_was_mlocked variables, then the old_page_state bits. Can't it be
> done with PAGE_WAS_ bits in old_page_state throughout, without any
> page_was_mapped and page_was_mlocked variables?

Yes, good point. Let me try it. Thanks for your comments.

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