Date:   Wed, 25 Jan 2023 14:13:00 -0700
From:   Yu Zhao <yuzhao@...gle.com>
To:     Nhat Pham <nphamcs@...il.com>
Cc:     akpm@...ux-foundation.org, hannes@...xchg.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, bfoster@...hat.com,
        willy@...radead.org, linux-api@...r.kernel.org,
        kernel-team@...a.com
Subject: Re: [PATCH v7 1/3] workingset: refactor LRU refault to expose refault
 recency check

On Mon, Jan 23, 2023 at 7:11 PM Nhat Pham <nphamcs@...il.com> wrote:
>
> In preparation for computing recently evicted pages in cachestat,
> refactor workingset_refault and lru_gen_refault to expose a helper
> function that would test if an evicted page is recently evicted.
>
> Signed-off-by: Nhat Pham <nphamcs@...il.com>
> ---
>  include/linux/swap.h |   1 +
>  mm/workingset.c      | 142 +++++++++++++++++++++++++++++--------------
>  2 files changed, 97 insertions(+), 46 deletions(-)
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index a18cf4b7c724..dae6f6f955eb 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -361,6 +361,7 @@ static inline void folio_set_swap_entry(struct folio *folio, swp_entry_t entry)
>  }
>
>  /* linux/mm/workingset.c */
> +bool workingset_test_recent(void *shadow, bool file, bool *workingset);
>  void workingset_age_nonresident(struct lruvec *lruvec, unsigned long nr_pages);
>  void *workingset_eviction(struct folio *folio, struct mem_cgroup *target_memcg);
>  void workingset_refault(struct folio *folio, void *shadow);
> diff --git a/mm/workingset.c b/mm/workingset.c
> index 79585d55c45d..2f2d94867366 100644
> --- a/mm/workingset.c
> +++ b/mm/workingset.c
> @@ -244,6 +244,30 @@ static void *lru_gen_eviction(struct folio *folio)
>         return pack_shadow(mem_cgroup_id(memcg), pgdat, token, refs);
>  }
>
> +/*
> + * Test if the folio is recently evicted.
> + *
> + * As a side effect, also populates the references with
> + * values unpacked from the shadow of the evicted folio.
> + */
> +static bool lru_gen_test_recent(void *shadow, bool file, int *memcgid,
> +               struct pglist_data **pgdat, unsigned long *token, bool *workingset)
> +{
> +       struct mem_cgroup *eviction_memcg;
> +       struct lruvec *lruvec;
> +       struct lru_gen_struct *lrugen;
> +       unsigned long min_seq;
> +
> +       unpack_shadow(shadow, memcgid, pgdat, token, workingset);
> +       eviction_memcg = mem_cgroup_from_id(*memcgid);
> +
> +       lruvec = mem_cgroup_lruvec(eviction_memcg, *pgdat);
> +       lrugen = &lruvec->lrugen;
> +
> +       min_seq = READ_ONCE(lrugen->min_seq[file]);
> +       return (*token >> LRU_REFS_WIDTH) == (min_seq & (EVICTION_MASK >> LRU_REFS_WIDTH));
> +}
> +
>  static void lru_gen_refault(struct folio *folio, void *shadow)
>  {
>         int hist, tier, refs;
> @@ -258,23 +282,22 @@ static void lru_gen_refault(struct folio *folio, void *shadow)
>         int type = folio_is_file_lru(folio);
>         int delta = folio_nr_pages(folio);
>
> -       unpack_shadow(shadow, &memcg_id, &pgdat, &token, &workingset);
> -
> -       if (pgdat != folio_pgdat(folio))
> -               return;
> -
>         rcu_read_lock();
>
> +       if (!lru_gen_test_recent(shadow, type, &memcg_id, &pgdat, &token,
> +                       &workingset))
> +               goto unlock;
> +
>         memcg = folio_memcg_rcu(folio);
>         if (memcg_id != mem_cgroup_id(memcg))
>                 goto unlock;
>
> +       if (pgdat != folio_pgdat(folio))
> +               return;
> +
>         lruvec = mem_cgroup_lruvec(memcg, pgdat);
>         lrugen = &lruvec->lrugen;
> -
>         min_seq = READ_ONCE(lrugen->min_seq[type]);
> -       if ((token >> LRU_REFS_WIDTH) != (min_seq & (EVICTION_MASK >> LRU_REFS_WIDTH)))
> -               goto unlock;
>
>         hist = lru_hist_from_seq(min_seq);
>         /* see the comment in folio_lru_refs() */
> @@ -306,6 +329,12 @@ static void *lru_gen_eviction(struct folio *folio)
>         return NULL;
>  }
>
> +static bool lru_gen_test_recent(void *shadow, bool file, int *memcgid,
> +               struct pglist_data **pgdat, unsigned long *token, bool *workingset)
> +{
> +       return false;
> +}
> +
>  static void lru_gen_refault(struct folio *folio, void *shadow)
>  {
>  }
> @@ -373,40 +402,31 @@ void *workingset_eviction(struct folio *folio, struct mem_cgroup *target_memcg)
>                                 folio_test_workingset(folio));
>  }
>
> -/**
> - * workingset_refault - Evaluate the refault of a previously evicted folio.
> - * @folio: The freshly allocated replacement folio.
> - * @shadow: Shadow entry of the evicted folio.
> +/*
> + * Test if the folio is recently evicted by checking if
> + * refault distance of shadow exceeds workingset size.
>   *
> - * Calculates and evaluates the refault distance of the previously
> - * evicted folio in the context of the node and the memcg whose memory
> - * pressure caused the eviction.
> + * As a side effect, populate workingset with the value
> + * unpacked from shadow.
>   */
> -void workingset_refault(struct folio *folio, void *shadow)
> +bool workingset_test_recent(void *shadow, bool file, bool *workingset)
>  {
> -       bool file = folio_is_file_lru(folio);
>         struct mem_cgroup *eviction_memcg;
>         struct lruvec *eviction_lruvec;
>         unsigned long refault_distance;
>         unsigned long workingset_size;
> -       struct pglist_data *pgdat;
> -       struct mem_cgroup *memcg;
> -       unsigned long eviction;
> -       struct lruvec *lruvec;
>         unsigned long refault;
> -       bool workingset;
>         int memcgid;
> -       long nr;
> +       struct pglist_data *pgdat;
> +       unsigned long eviction;
>
> -       if (lru_gen_enabled()) {
> -               lru_gen_refault(folio, shadow);
> -               return;
> -       }
> +       if (lru_gen_enabled())
> +               lru_gen_test_recent(shadow, file, &memcgid, &pgdat, &eviction,
> +                       workingset);

Missing "return", which was correctly handled in your v2, btw.

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