Re: [PATCH 2/3] mm: vmscan: detect file thrashing at the reclaim root

[Shakeel Butt <shakeelb@google.com>]

On Thu, Nov 7, 2019 at 12:53 PM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> We use refault information to determine whether the cache workingset
> is stable or transitioning, and dynamically adjust the inactive:active
> file LRU ratio so as to maximize protection from one-off cache during
> stable periods, and minimize IO during transitions.
>
> With cgroups and their nested LRU lists, we currently don't do this
> correctly. While recursive cgroup reclaim establishes a relative LRU
> order among the pages of all involved cgroups, refaults only affect
> the local LRU order in the cgroup in which they are occuring. As a
> result, cache transitions can take longer in a cgrouped system as the
> active pages of sibling cgroups aren't challenged when they should be.
>
> [ Right now, this is somewhat theoretical, because the siblings, under
>   continued regular reclaim pressure, should eventually run out of
>   inactive pages - and since inactive:active *size* balancing is also
>   done on a cgroup-local level, we will challenge the active pages
>   eventually in most cases. But the next patch will move that relative
>   size enforcement to the reclaim root as well, and then this patch
>   here will be necessary to propagate refault pressure to siblings. ]
>
> This patch moves refault detection to the root of reclaim. Instead of
> remembering the cgroup owner of an evicted page, remember the cgroup
> that caused the reclaim to happen. When refaults later occur, they'll
> correctly influence the cross-cgroup LRU order that reclaim follows.

Can you please explain how "they'll correctly influence"? I see that
if the refaulted page was evicted due to pressure in some ancestor,
then that's ancestor's refault distance and active file size will be
used to decide to activate the refaulted page but how that is
influencing cross-cgroup LRUs?

>
> I.e. if global reclaim kicked out pages in some subgroup A/B/C, the
> refault of those pages will challenge the global LRU order, and not
> just the local order down inside C.
>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> ---
>  include/linux/memcontrol.h |  5 +++
>  include/linux/swap.h       |  2 +-
>  mm/vmscan.c                | 32 ++++++++---------
>  mm/workingset.c            | 72 +++++++++++++++++++++++++++++---------
>  4 files changed, 77 insertions(+), 34 deletions(-)
>
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 5b86287fa069..a7a0a1a5c8d5 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -901,6 +901,11 @@ static inline struct lruvec *mem_cgroup_page_lruvec(struct page *page,
>         return &pgdat->__lruvec;
>  }
>
> +static inline struct mem_cgroup *parent_mem_cgroup(struct mem_cgroup *memcg)
> +{
> +       return NULL;
> +}
> +
>  static inline bool mm_match_cgroup(struct mm_struct *mm,
>                 struct mem_cgroup *memcg)
>  {
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 063c0c1e112b..1e99f7ac1d7e 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -307,7 +307,7 @@ struct vma_swap_readahead {
>  };
>
>  /* linux/mm/workingset.c */
> -void *workingset_eviction(struct page *page);
> +void *workingset_eviction(struct page *page, struct mem_cgroup *target_memcg);
>  void workingset_refault(struct page *page, void *shadow);
>  void workingset_activation(struct page *page);
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index e8dd601e1fad..527617ee9b73 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -853,7 +853,7 @@ static pageout_t pageout(struct page *page, struct address_space *mapping)
>   * gets returned with a refcount of 0.
>   */
>  static int __remove_mapping(struct address_space *mapping, struct page *page,
> -                           bool reclaimed)
> +                           bool reclaimed, struct mem_cgroup *target_memcg)
>  {
>         unsigned long flags;
>         int refcount;
> @@ -925,7 +925,7 @@ static int __remove_mapping(struct address_space *mapping, struct page *page,
>                  */
>                 if (reclaimed && page_is_file_cache(page) &&
>                     !mapping_exiting(mapping) && !dax_mapping(mapping))
> -                       shadow = workingset_eviction(page);
> +                       shadow = workingset_eviction(page, target_memcg);
>                 __delete_from_page_cache(page, shadow);
>                 xa_unlock_irqrestore(&mapping->i_pages, flags);
>
> @@ -948,7 +948,7 @@ static int __remove_mapping(struct address_space *mapping, struct page *page,
>   */
>  int remove_mapping(struct address_space *mapping, struct page *page)
>  {
> -       if (__remove_mapping(mapping, page, false)) {
> +       if (__remove_mapping(mapping, page, false, NULL)) {
>                 /*
>                  * Unfreezing the refcount with 1 rather than 2 effectively
>                  * drops the pagecache ref for us without requiring another
> @@ -1426,7 +1426,8 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>
>                         count_vm_event(PGLAZYFREED);
>                         count_memcg_page_event(page, PGLAZYFREED);
> -               } else if (!mapping || !__remove_mapping(mapping, page, true))
> +               } else if (!mapping || !__remove_mapping(mapping, page, true,
> +                                                        sc->target_mem_cgroup))
>                         goto keep_locked;
>
>                 unlock_page(page);
> @@ -2189,6 +2190,7 @@ static bool inactive_list_is_low(struct lruvec *lruvec, bool file,
>         enum lru_list inactive_lru = file * LRU_FILE;
>         unsigned long inactive, active;
>         unsigned long inactive_ratio;
> +       struct lruvec *target_lruvec;
>         unsigned long refaults;
>         unsigned long gb;
>
> @@ -2200,8 +2202,9 @@ static bool inactive_list_is_low(struct lruvec *lruvec, bool file,
>          * is being established. Disable active list protection to get
>          * rid of the stale workingset quickly.
>          */
> -       refaults = lruvec_page_state_local(lruvec, WORKINGSET_ACTIVATE);
> -       if (file && lruvec->refaults != refaults) {
> +       target_lruvec = mem_cgroup_lruvec(sc->target_mem_cgroup, pgdat);
> +       refaults = lruvec_page_state(target_lruvec, WORKINGSET_ACTIVATE);
> +       if (file && target_lruvec->refaults != refaults) {
>                 inactive_ratio = 0;
>         } else {
>                 gb = (inactive + active) >> (30 - PAGE_SHIFT);
> @@ -2973,19 +2976,14 @@ static void shrink_zones(struct zonelist *zonelist, struct scan_control *sc)
>         sc->gfp_mask = orig_mask;
>  }
>
> -static void snapshot_refaults(struct mem_cgroup *root_memcg, pg_data_t *pgdat)
> +static void snapshot_refaults(struct mem_cgroup *target_memcg, pg_data_t *pgdat)
>  {
> -       struct mem_cgroup *memcg;
> -
> -       memcg = mem_cgroup_iter(root_memcg, NULL, NULL);
> -       do {
> -               unsigned long refaults;
> -               struct lruvec *lruvec;
> +       struct lruvec *target_lruvec;
> +       unsigned long refaults;
>
> -               lruvec = mem_cgroup_lruvec(memcg, pgdat);
> -               refaults = lruvec_page_state_local(lruvec, WORKINGSET_ACTIVATE);
> -               lruvec->refaults = refaults;
> -       } while ((memcg = mem_cgroup_iter(root_memcg, memcg, NULL)));
> +       target_lruvec = mem_cgroup_lruvec(target_memcg, pgdat);
> +       refaults = lruvec_page_state(target_lruvec, WORKINGSET_ACTIVATE);
> +       target_lruvec->refaults = refaults;
>  }
>
>  /*
> diff --git a/mm/workingset.c b/mm/workingset.c
> index e8212123c1c3..f0885d9f41cd 100644
> --- a/mm/workingset.c
> +++ b/mm/workingset.c
> @@ -213,28 +213,53 @@ static void unpack_shadow(void *shadow, int *memcgidp, pg_data_t **pgdat,
>         *workingsetp = workingset;
>  }
>
> +static void advance_inactive_age(struct mem_cgroup *memcg, pg_data_t *pgdat)
> +{
> +       /*
> +        * Reclaiming a cgroup means reclaiming all its children in a
> +        * round-robin fashion. That means that each cgroup has an LRU
> +        * order that is composed of the LRU orders of its child
> +        * cgroups; and every page has an LRU position not just in the
> +        * cgroup that owns it, but in all of that group's ancestors.
> +        *
> +        * So when the physical inactive list of a leaf cgroup ages,
> +        * the virtual inactive lists of all its parents, including
> +        * the root cgroup's, age as well.
> +        */
> +       do {
> +               struct lruvec *lruvec;
> +
> +               lruvec = mem_cgroup_lruvec(memcg, pgdat);
> +               atomic_long_inc(&lruvec->inactive_age);
> +       } while (memcg && (memcg = parent_mem_cgroup(memcg)));
> +}
> +
>  /**
>   * workingset_eviction - note the eviction of a page from memory
> + * @target_memcg: the cgroup that is causing the reclaim
>   * @page: the page being evicted
>   *
>   * Returns a shadow entry to be stored in @page->mapping->i_pages in place
>   * of the evicted @page so that a later refault can be detected.
>   */
> -void *workingset_eviction(struct page *page)
> +void *workingset_eviction(struct page *page, struct mem_cgroup *target_memcg)
>  {
>         struct pglist_data *pgdat = page_pgdat(page);
> -       struct mem_cgroup *memcg = page_memcg(page);
> -       int memcgid = mem_cgroup_id(memcg);
>         unsigned long eviction;
>         struct lruvec *lruvec;
> +       int memcgid;
>
>         /* Page is fully exclusive and pins page->mem_cgroup */
>         VM_BUG_ON_PAGE(PageLRU(page), page);
>         VM_BUG_ON_PAGE(page_count(page), page);
>         VM_BUG_ON_PAGE(!PageLocked(page), page);
>
> -       lruvec = mem_cgroup_lruvec(memcg, pgdat);
> -       eviction = atomic_long_inc_return(&lruvec->inactive_age);
> +       advance_inactive_age(page_memcg(page), pgdat);
> +
> +       lruvec = mem_cgroup_lruvec(target_memcg, pgdat);
> +       /* XXX: target_memcg can be NULL, go through lruvec */
> +       memcgid = mem_cgroup_id(lruvec_memcg(lruvec));
> +       eviction = atomic_long_read(&lruvec->inactive_age);
>         return pack_shadow(memcgid, pgdat, eviction, PageWorkingset(page));
>  }
>
> @@ -244,10 +269,13 @@ void *workingset_eviction(struct page *page)
>   * @shadow: shadow entry of the evicted page
>   *
>   * Calculates and evaluates the refault distance of the previously
> - * evicted page in the context of the node it was allocated in.
> + * evicted page in the context of the node and the memcg whose memory
> + * pressure caused the eviction.
>   */
>  void workingset_refault(struct page *page, void *shadow)
>  {
> +       struct mem_cgroup *eviction_memcg;
> +       struct lruvec *eviction_lruvec;
>         unsigned long refault_distance;
>         struct pglist_data *pgdat;
>         unsigned long active_file;
> @@ -277,12 +305,12 @@ void workingset_refault(struct page *page, void *shadow)
>          * would be better if the root_mem_cgroup existed in all
>          * configurations instead.
>          */
> -       memcg = mem_cgroup_from_id(memcgid);
> -       if (!mem_cgroup_disabled() && !memcg)
> +       eviction_memcg = mem_cgroup_from_id(memcgid);
> +       if (!mem_cgroup_disabled() && !eviction_memcg)
>                 goto out;
> -       lruvec = mem_cgroup_lruvec(memcg, pgdat);
> -       refault = atomic_long_read(&lruvec->inactive_age);
> -       active_file = lruvec_lru_size(lruvec, LRU_ACTIVE_FILE, MAX_NR_ZONES);
> +       eviction_lruvec = mem_cgroup_lruvec(eviction_memcg, pgdat);
> +       refault = atomic_long_read(&eviction_lruvec->inactive_age);
> +       active_file = lruvec_page_state(eviction_lruvec, NR_ACTIVE_FILE);
>
>         /*
>          * Calculate the refault distance
> @@ -302,6 +330,17 @@ void workingset_refault(struct page *page, void *shadow)
>          */
>         refault_distance = (refault - eviction) & EVICTION_MASK;
>
> +       /*
> +        * The activation decision for this page is made at the level
> +        * where the eviction occurred, as that is where the LRU order
> +        * during page reclaim is being determined.
> +        *
> +        * However, the cgroup that will own the page is the one that
> +        * is actually experiencing the refault event.
> +        */
> +       memcg = get_mem_cgroup_from_mm(current->mm);

Why not page_memcg(page)? page is locked.


> +       lruvec = mem_cgroup_lruvec(memcg, pgdat);
> +
>         inc_lruvec_state(lruvec, WORKINGSET_REFAULT);
>
>         /*
> @@ -310,10 +349,10 @@ void workingset_refault(struct page *page, void *shadow)
>          * the memory was available to the page cache.
>          */
>         if (refault_distance > active_file)
> -               goto out;
> +               goto out_memcg;
>
>         SetPageActive(page);
> -       atomic_long_inc(&lruvec->inactive_age);
> +       advance_inactive_age(memcg, pgdat);
>         inc_lruvec_state(lruvec, WORKINGSET_ACTIVATE);
>
>         /* Page was active prior to eviction */
> @@ -321,6 +360,9 @@ void workingset_refault(struct page *page, void *shadow)
>                 SetPageWorkingset(page);
>                 inc_lruvec_state(lruvec, WORKINGSET_RESTORE);
>         }
> +
> +out_memcg:
> +       mem_cgroup_put(memcg);
>  out:
>         rcu_read_unlock();
>  }
> @@ -332,7 +374,6 @@ void workingset_refault(struct page *page, void *shadow)
>  void workingset_activation(struct page *page)
>  {
>         struct mem_cgroup *memcg;
> -       struct lruvec *lruvec;
>
>         rcu_read_lock();
>         /*
> @@ -345,8 +386,7 @@ void workingset_activation(struct page *page)
>         memcg = page_memcg_rcu(page);
>         if (!mem_cgroup_disabled() && !memcg)
>                 goto out;
> -       lruvec = mem_cgroup_lruvec(memcg, page_pgdat(page));
> -       atomic_long_inc(&lruvec->inactive_age);
> +       advance_inactive_age(memcg, page_pgdat(page));
>  out:
>         rcu_read_unlock();
>  }
> --
> 2.24.0
>