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

[Zi Yan <ziy@nvidia.com>]

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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@linux.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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