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Date: Wed, 18 Oct 2023 10:00:52 -0400
From: Zi Yan <ziy@...dia.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: akpm@...ux-foundation.org, mgorman@...hsingularity.net,
hughd@...gle.com, vbabka@...e.cz, ying.huang@...el.com,
linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: migrate: record the mlocked page status to remove
unnecessary lru drain
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().
>
> 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@...ux.alibaba.com>
> ---
> 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.
> 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,
Yan, Zi
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