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

From: Huang, Ying
Date: Thu Oct 19 2023 - 02:11:46 EST

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.

>>
>> 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 we
>> check 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%*
>>
>> Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
>> ---
>> mm/migrate.c | 50 ++++++++++++++++++++++++++++++++++++++------------
>> 1 file changed, 38 insertions(+), 12 deletions(-)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index 4caf405b6504..32c96f89710f 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -1027,22 +1027,32 @@ union migration_ptr {
>> struct anon_vma *anon_vma;
>> struct address_space *mapping;
>> };
>> +
>> +enum {
>> + PAGE_WAS_MAPPED = 1 << 0,
>> + PAGE_WAS_MLOCKED = 1 << 1,
>> +};
>> +
>> static void __migrate_folio_record(struct folio *dst,
>> - unsigned long page_was_mapped,
>> + unsigned long page_flags,
>> struct anon_vma *anon_vma)
>> {
>> union migration_ptr ptr = { .anon_vma = anon_vma };
>> dst->mapping = ptr.mapping;
>> - dst->private = (void *)page_was_mapped;
>> + dst->private = (void *)page_flags;
>> }
>>
>> static void __migrate_folio_extract(struct folio *dst,
>> int *page_was_mappedp,
>> + int *page_was_mlocked,
>> struct anon_vma **anon_vmap)
>> {
>> union migration_ptr ptr = { .mapping = dst->mapping };
>> + unsigned long page_flags = (unsigned long)dst->private;
>> +
>> *anon_vmap = ptr.anon_vma;
>> - *page_was_mappedp = (unsigned long)dst->private;
>> + *page_was_mappedp = page_flags & PAGE_WAS_MAPPED ? 1 : 0;
>> + *page_was_mlocked = page_flags & PAGE_WAS_MLOCKED ? 1 : 0;
>> dst->mapping = NULL;
>> dst->private = NULL;
>> }
>> @@ -1103,7 +1113,7 @@ static int migrate_folio_unmap(new_folio_t get_new_folio,
>> {
>> struct folio *dst;
>> int rc = -EAGAIN;
>> - int page_was_mapped = 0;
>> + int page_was_mapped = 0, page_was_mlocked = 0;
>> struct anon_vma *anon_vma = NULL;
>> bool is_lru = !__folio_test_movable(src);
>> bool locked = false;
>> @@ -1157,6 +1167,7 @@ static int migrate_folio_unmap(new_folio_t get_new_folio,
>> folio_lock(src);
>> }
>> locked = true;
>> + page_was_mlocked = folio_test_mlocked(src);
>>
>> if (folio_test_writeback(src)) {
>> /*
>> @@ -1206,7 +1217,7 @@ static int migrate_folio_unmap(new_folio_t get_new_folio,
>> dst_locked = true;
>>
>> if (unlikely(!is_lru)) {
>> - __migrate_folio_record(dst, page_was_mapped, anon_vma);
>> + __migrate_folio_record(dst, 0, anon_vma);
>> return MIGRATEPAGE_UNMAP;
>> }
>>
>> @@ -1236,7 +1247,13 @@ static int migrate_folio_unmap(new_folio_t get_new_folio,
>> }
>>
>> if (!folio_mapped(src)) {
>> - __migrate_folio_record(dst, page_was_mapped, anon_vma);
>> + unsigned int page_flags = 0;
>> +
>> + if (page_was_mapped)
>> + page_flags |= PAGE_WAS_MAPPED;
>> + if (page_was_mlocked)
>> + page_flags |= PAGE_WAS_MLOCKED;
>> + __migrate_folio_record(dst, page_flags, anon_vma);
>> return MIGRATEPAGE_UNMAP;
>> }
>>
>> @@ -1261,12 +1278,13 @@ static int migrate_folio_move(free_folio_t put_new_folio, unsigned long private,
>> struct list_head *ret)
>> {
>> int rc;
>> - int page_was_mapped = 0;
>> + int page_was_mapped = 0, page_was_mlocked = 0;
>> struct anon_vma *anon_vma = NULL;
>> bool is_lru = !__folio_test_movable(src);
>> struct list_head *prev;
>>
>> - __migrate_folio_extract(dst, &page_was_mapped, &anon_vma);
>> + __migrate_folio_extract(dst, &page_was_mapped,
>> + &page_was_mlocked, &anon_vma);
>
> It is better to read out the flag, then check page_was_mapped and page_was_mlocked
> to avoid future __migrate_folio_extract() interface churns.

IHMO, in contrast, it's better to use separate flags in
__migrate_folio_record() too to avoid to pack flags in each call site.

>> prev = dst->lru.prev;
>> list_del(&dst->lru);
>>
>> @@ -1287,7 +1305,7 @@ static int migrate_folio_move(free_folio_t put_new_folio, unsigned long private,
>> * isolated from the unevictable LRU: but this case is the easiest.
>> */
>> folio_add_lru(dst);
>> - if (page_was_mapped)
>> + if (page_was_mlocked)
>> lru_add_drain();
>
> Like I said at the top, this would be if (page_was_mapped || page_was_mlocked).
>
>>
>> if (page_was_mapped)
>> @@ -1321,8 +1339,15 @@ static int migrate_folio_move(free_folio_t put_new_folio, unsigned long private,
>> * right list unless we want to retry.
>> */
>> if (rc == -EAGAIN) {
>> + unsigned int page_flags = 0;
>> +
>> + if (page_was_mapped)
>> + page_flags |= PAGE_WAS_MAPPED;
>> + if (page_was_mlocked)
>> + page_flags |= PAGE_WAS_MLOCKED;
>> +
>> list_add(&dst->lru, prev);
>> - __migrate_folio_record(dst, page_was_mapped, anon_vma);
>> + __migrate_folio_record(dst, page_flags, anon_vma);
>> return rc;
>> }
>>
>> @@ -1799,10 +1824,11 @@ static int migrate_pages_batch(struct list_head *from,
>> dst = list_first_entry(&dst_folios, struct folio, lru);
>> dst2 = list_next_entry(dst, lru);
>> list_for_each_entry_safe(folio, folio2, &unmap_folios, lru) {
>> - int page_was_mapped = 0;
>> + int page_was_mapped = 0, page_was_mlocked = 0;
>> struct anon_vma *anon_vma = NULL;
>>
>> - __migrate_folio_extract(dst, &page_was_mapped, &anon_vma);
>> + __migrate_folio_extract(dst, &page_was_mapped,
>> + &page_was_mlocked, &anon_vma);
>> migrate_folio_undo_src(folio, page_was_mapped, anon_vma,
>> true, ret_folios);
>> list_del(&dst->lru);
>> --
>> 2.39.3

--
Best Regards,
Huang, Ying