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

[Shakeel Butt]

On Mon, Mar 8, 2021 at 12:30 PM Yang Shi <shy828301@xxxxxxxxx> wrote:
>
> On Mon, Mar 8, 2021 at 11:12 AM Shakeel Butt <shakeelb@xxxxxxxxxx> wrote:
> >
> > On Tue, Feb 16, 2021 at 4:13 PM Yang Shi <shy828301@xxxxxxxxx> 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@xxxxxxxxx>
> > > ---
> > >  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."
>

LGTM and you can add:

Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>