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

[Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>]

On 10/19/2023 2:09 PM, Huang, Ying wrote:
> 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.

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.

> > > 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.

Either way is okay for me. And avoiding to pack flags in each call site 
seems more reasonable to me.

> > >   	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).

I don't think so. Like I said above, we can drain lru list only if 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