Re: [v8 PATCH 09/13] mm: vmscan: add per memcg shrinker nr_deferred

[Shakeel Butt <shakeelb@google.com>]

On Tue, Feb 16, 2021 at 4:13 PM Yang Shi <shy828301@gmail.com> wrote:
>
> Currently the number of deferred objects are per shrinker, but some slabs, for example,
> vfs inode/dentry cache are per memcg, this would result in poor isolation among memcgs.
>
> The deferred objects typically are generated by __GFP_NOFS allocations, one memcg with
> excessive __GFP_NOFS allocations may blow up deferred objects, then other innocent memcgs
> may suffer from over shrink, excessive reclaim latency, etc.
>
> For example, two workloads run in memcgA and memcgB respectively, workload in B is vfs
> heavy workload.  Workload in A generates excessive deferred objects, then B's vfs cache
> might be hit heavily (drop half of caches) by B's limit reclaim or global reclaim.
>
> We observed this hit in our production environment which was running vfs heavy workload
> shown as the below tracing log:
>
> <...>-409454 [016] .... 28286961.747146: mm_shrink_slab_start: super_cache_scan+0x0/0x1a0 ffff9a83046f3458:
> nid: 1 objects to shrink 3641681686040 gfp_flags GFP_HIGHUSER_MOVABLE|__GFP_ZERO pgs_scanned 1 lru_pgs 15721
> cache items 246404277 delta 31345 total_scan 123202138
> <...>-409454 [022] .... 28287105.928018: mm_shrink_slab_end: super_cache_scan+0x0/0x1a0 ffff9a83046f3458:
> nid: 1 unused scan count 3641681686040 new scan count 3641798379189 total_scan 602
> last shrinker return val 123186855
>
> The vfs cache and page cache ratio was 10:1 on this machine, and half of caches were dropped.
> This also resulted in significant amount of page caches were dropped due to inodes eviction.
>
> Make nr_deferred per memcg for memcg aware shrinkers would solve the unfairness and bring
> better isolation.
>
> When memcg is not enabled (!CONFIG_MEMCG or memcg disabled), the shrinker's nr_deferred
> would be used.  And non memcg aware shrinkers use shrinker's nr_deferred all the time.
>
> Signed-off-by: Yang Shi <shy828301@gmail.com>
> ---
>  include/linux/memcontrol.h |  7 +++--
>  mm/vmscan.c                | 60 ++++++++++++++++++++++++++------------
>  2 files changed, 46 insertions(+), 21 deletions(-)
>
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 4c9253896e25..c457fc7bc631 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -93,12 +93,13 @@ struct lruvec_stat {
>  };
>
>  /*
> - * Bitmap of shrinker::id corresponding to memcg-aware shrinkers,
> - * which have elements charged to this memcg.
> + * Bitmap and deferred work of shrinker::id corresponding to memcg-aware
> + * shrinkers, which have elements charged to this memcg.
>   */
>  struct shrinker_info {
>         struct rcu_head rcu;
> -       unsigned long map[];
> +       atomic_long_t *nr_deferred;
> +       unsigned long *map;
>  };
>
>  /*
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index a1047ea60ecf..fcb399e18fc3 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -187,11 +187,17 @@ static DECLARE_RWSEM(shrinker_rwsem);
>  #ifdef CONFIG_MEMCG
>  static int shrinker_nr_max;
>
> +/* The shrinker_info is expanded in a batch of BITS_PER_LONG */
>  static inline int shrinker_map_size(int nr_items)
>  {
>         return (DIV_ROUND_UP(nr_items, BITS_PER_LONG) * sizeof(unsigned long));
>  }
>
> +static inline int shrinker_defer_size(int nr_items)
> +{
> +       return (round_up(nr_items, BITS_PER_LONG) * sizeof(atomic_long_t));
> +}
> +
>  static struct shrinker_info *shrinker_info_protected(struct mem_cgroup *memcg,
>                                                      int nid)
>  {
> @@ -200,10 +206,12 @@ static struct shrinker_info *shrinker_info_protected(struct mem_cgroup *memcg,
>  }
>
>  static int expand_one_shrinker_info(struct mem_cgroup *memcg,
> -                                   int size, int old_size)
> +                                   int map_size, int defer_size,
> +                                   int old_map_size, int old_defer_size)
>  {
>         struct shrinker_info *new, *old;
>         int nid;
> +       int size = map_size + defer_size;
>
>         for_each_node(nid) {
>                 old = shrinker_info_protected(memcg, nid);
> @@ -215,9 +223,16 @@ static int expand_one_shrinker_info(struct mem_cgroup *memcg,
>                 if (!new)
>                         return -ENOMEM;
>
> -               /* Set all old bits, clear all new bits */
> -               memset(new->map, (int)0xff, old_size);
> -               memset((void *)new->map + old_size, 0, size - old_size);
> +               new->nr_deferred = (atomic_long_t *)(new + 1);
> +               new->map = (void *)new->nr_deferred + defer_size;
> +
> +               /* map: set all old bits, clear all new bits */
> +               memset(new->map, (int)0xff, old_map_size);
> +               memset((void *)new->map + old_map_size, 0, map_size - old_map_size);
> +               /* nr_deferred: copy old values, clear all new values */
> +               memcpy(new->nr_deferred, old->nr_deferred, old_defer_size);
> +               memset((void *)new->nr_deferred + old_defer_size, 0,
> +                      defer_size - old_defer_size);
>
>                 rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_info, new);
>                 kvfree_rcu(old);
> @@ -232,9 +247,6 @@ void free_shrinker_info(struct mem_cgroup *memcg)
>         struct shrinker_info *info;
>         int nid;
>
> -       if (mem_cgroup_is_root(memcg))
> -               return;
> -
>         for_each_node(nid) {
>                 pn = mem_cgroup_nodeinfo(memcg, nid);
>                 info = shrinker_info_protected(memcg, nid);
> @@ -247,12 +259,12 @@ int alloc_shrinker_info(struct mem_cgroup *memcg)
>  {
>         struct shrinker_info *info;
>         int nid, size, ret = 0;
> -
> -       if (mem_cgroup_is_root(memcg))
> -               return 0;

Can you please comment on the consequences on allowing to allocate
shrinker_info for root memcg? Why didn't we do that before but now it
is fine (or maybe required)? Please add the explanation in the commit
message.

> +       int map_size, defer_size = 0;
>
>         down_write(&shrinker_rwsem);
> -       size = shrinker_map_size(shrinker_nr_max);
> +       map_size = shrinker_map_size(shrinker_nr_max);
> +       defer_size = shrinker_defer_size(shrinker_nr_max);
> +       size = map_size + defer_size;
>         for_each_node(nid) {
>                 info = kvzalloc_node(sizeof(*info) + size, GFP_KERNEL, nid);
>                 if (!info) {
> @@ -260,6 +272,8 @@ int alloc_shrinker_info(struct mem_cgroup *memcg)
>                         ret = -ENOMEM;
>                         break;
>                 }
> +               info->nr_deferred = (atomic_long_t *)(info + 1);
> +               info->map = (void *)info->nr_deferred + defer_size;
>                 rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_info, info);
>         }
>         up_write(&shrinker_rwsem);
> @@ -267,15 +281,21 @@ int alloc_shrinker_info(struct mem_cgroup *memcg)
>         return ret;
>  }
>
> +static inline bool need_expand(int nr_max)
> +{
> +       return round_up(nr_max, BITS_PER_LONG) >
> +              round_up(shrinker_nr_max, BITS_PER_LONG);
> +}
> +
>  static int expand_shrinker_info(int new_id)
>  {
> -       int size, old_size, ret = 0;
> +       int ret = 0;
>         int new_nr_max = new_id + 1;
> +       int map_size, defer_size = 0;
> +       int old_map_size, old_defer_size = 0;
>         struct mem_cgroup *memcg;
>
> -       size = shrinker_map_size(new_nr_max);
> -       old_size = shrinker_map_size(shrinker_nr_max);
> -       if (size <= old_size)
> +       if (!need_expand(new_nr_max))
>                 goto out;
>
>         if (!root_mem_cgroup)
> @@ -283,11 +303,15 @@ static int expand_shrinker_info(int new_id)
>
>         lockdep_assert_held(&shrinker_rwsem);
>
> +       map_size = shrinker_map_size(new_nr_max);
> +       defer_size = shrinker_defer_size(new_nr_max);
> +       old_map_size = shrinker_map_size(shrinker_nr_max);
> +       old_defer_size = shrinker_defer_size(shrinker_nr_max);
> +
>         memcg = mem_cgroup_iter(NULL, NULL, NULL);
>         do {
> -               if (mem_cgroup_is_root(memcg))
> -                       continue;
> -               ret = expand_one_shrinker_info(memcg, size, old_size);
> +               ret = expand_one_shrinker_info(memcg, map_size, defer_size,
> +                                              old_map_size, old_defer_size);
>                 if (ret) {
>                         mem_cgroup_iter_break(NULL, memcg);
>                         goto out;
> --
> 2.26.2
>