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

[Yang Shi <shy828301@gmail.com>]

On Mon, Mar 8, 2021 at 11:12 AM Shakeel Butt <shakeelb@google.com> wrote:
>
> 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.

Before the patchset shrinker_info just tracks shrinker_maps which is
not required for root memcg. But the newly added nr_deferred is needed
in root memcg otherwise the nr_deferred work would get lost once the
memcgs are reparented to root.

How's about adding the below paragraph to the commit log:

"To preserve nr_deferred when reparenting memcgs to root, root memcg
needs shrinker_info allocated too."

>
> > +       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
> >