Re: [PATCH v3 1/3] mm, lru_gen: try to prefetch next page when scanning LRU

[Kairui Song <ryncsn@gmail.com>]

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On Fri, Jan 26, 2024 at 8:56 AM Chris Li <chrisl@kernel.org> wrote:
> On Fri, Jan 26, 2024 at 01:51:44AM +0800, Kairui Song wrote:
> > > >  mm/vmscan.c | 30 ++++++++++++++++++++++++++----
> > > >  1 file changed, 26 insertions(+), 4 deletions(-)
> > > >
> > > > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > > > index 4f9c854ce6cc..03631cedb3ab 100644
> > > > --- a/mm/vmscan.c
> > > > +++ b/mm/vmscan.c
> > > > @@ -3681,15 +3681,26 @@ static bool inc_min_seq(struct lruvec
*lruvec, int type, bool can_swap)
> > > >         /* prevent cold/hot inversion if force_scan is true */
> > > >         for (zone = 0; zone < MAX_NR_ZONES; zone++) {
> > > >                 struct list_head *head =
&lrugen->folios[old_gen][type][zone];
> > > > +               struct folio *prev = NULL;
> > > >
> > > > -               while (!list_empty(head)) {
> > > > -                       struct folio *folio = lru_to_folio(head);
> > > > +               if (!list_empty(head))
> > > > +                       prev = lru_to_folio(head);
> > > > +
> > > > +               while (prev) {
> > > > +                       struct folio *folio = prev;
> > > >
> > > >
 VM_WARN_ON_ONCE_FOLIO(folio_test_unevictable(folio), folio);
> > > >
 VM_WARN_ON_ONCE_FOLIO(folio_test_active(folio), folio);
> > > >
 VM_WARN_ON_ONCE_FOLIO(folio_is_file_lru(folio) != type, folio);
> > > >                         VM_WARN_ON_ONCE_FOLIO(folio_zonenum(folio)
!= zone, folio);
> > > >
> > > > +                       if (unlikely(list_is_first(&folio->lru,
head))) {
> > > > +                               prev = NULL;
> > > > +                       } else {
> > > > +                               prev = lru_to_folio(&folio->lru);
> > > > +                               prefetchw(&prev->flags);
> > > > +                       }
> > >
> > > This makes the code flow much harder to follow. Also for architecture
> > > that does not support prefetch, this will be a net loss.
> > >
> > > Can you use refetchw_prev_lru_folio() instead? It will make the code
> > > much easier to follow. It also turns into no-op when prefetch is not
> > > supported.
> > >
> > > Chris
> > >
> >
> > Hi Chris,
> >
> > Thanks for the suggestion.
> >
> > Yes, that's doable, I made it this way because in previous series (V1
> > & V2) I applied the bulk move patch first which needed and introduced
> > the `prev` variable here, so the prefetch logic just used it.
> > For V3 I did a rebase and moved the prefetch commit to be the first
> > one, since it seems to be the most effective one, and just kept the
>
> Maybe something like this? Totally not tested. Feel free to use it any
way you want.
>
> Chris
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 4f9c854ce6cc..2100e786ccc6 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -3684,6 +3684,7 @@ static bool inc_min_seq(struct lruvec *lruvec, int
type, bool can_swap)
>
>                 while (!list_empty(head)) {
>                         struct folio *folio = lru_to_folio(head);
> +                       prefetchw_prev_lru_folio(folio, head, flags);
>
>
 VM_WARN_ON_ONCE_FOLIO(folio_test_unevictable(folio), folio);
>                         VM_WARN_ON_ONCE_FOLIO(folio_test_active(folio),
folio);
> @@ -4346,7 +4347,10 @@ static int scan_folios(struct lruvec *lruvec,
struct scan_control *sc,
>
>                 while (!list_empty(head)) {
>                         struct folio *folio = lru_to_folio(head);
> -                       int delta = folio_nr_pages(folio);
> +                       int delta;
> +
> +                       prefetchw_prev_lru_folio(folio, head, flags);
> +                       delta = folio_nr_pages(folio);
>
>
 VM_WARN_ON_ONCE_FOLIO(folio_test_unevictable(folio), folio);
>                         VM_WARN_ON_ONCE_FOLIO(folio_test_active(folio),
folio);
>

Thanks!

Actually if benefits from 2/3 and 3/3 is trivial compared to the complexity
and not appealing, then let's only keep the prefetch one, which will be
just a one liner change with good result.

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