Re: [v4 PATCH 07/11] mm: vmscan: add per memcg shrinker nr_deferred

[Yang Shi]

On Mon, Jan 25, 2021 at 1:31 AM Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> wrote:
>
> On 22.01.2021 02:06, Yang Shi 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 ration 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                | 49 +++++++++++++++++++++++++-------------
> >  2 files changed, 36 insertions(+), 20 deletions(-)
> >
> > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > index 62b888b88a5f..e0384367e07d 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[];
> > +     unsigned long *map;
> > +     atomic_long_t *nr_deferred;
> >  };
> >
> >  /*
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 018e1beb24c9..722aa71b13b2 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -192,11 +192,13 @@ static void free_shrinker_info_rcu(struct rcu_head *head)
> >       kvfree(container_of(head, struct shrinker_info, rcu));
> >  }
> >
> > -static int expand_one_shrinker_info(struct mem_cgroup *memcg,
> > -                                int size, int old_size)
> > +static int expand_one_shrinker_info(struct mem_cgroup *memcg, int nr_max,
> > +                                 int m_size, int d_size,
> > +                                 int old_m_size, int old_d_size)
> >  {
> >       struct shrinker_info *new, *old;
> >       int nid;
> > +     int size = m_size + d_size;
> >
> >       for_each_node(nid) {
> >               old = rcu_dereference_protected(
> > @@ -209,9 +211,16 @@ 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->map = (unsigned long *)(new + 1);
> > +             new->nr_deferred = (atomic_long_t *)(new->map +
> > +                                     nr_max / BITS_PER_LONG + 1);
>
> Why not
>
>                 new->nr_deferred = (void *)new->map + m_size;
> ?
>
> > +
> > +             /* map: set all old bits, clear all new bits */
> > +             memset(new->map, (int)0xff, old_m_size);
> > +             memset((void *)new->map + old_m_size, 0, m_size - old_m_size);
> > +             /* nr_deferred: copy old values, clear all new values */
> > +             memcpy(new->nr_deferred, old->nr_deferred, old_d_size);
> > +             memset((void *)new->nr_deferred + old_d_size, 0, d_size - old_d_size);
> >
> >               rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_info, new);
> >               call_rcu(&old->rcu, free_shrinker_info_rcu);
> > @@ -226,9 +235,6 @@ 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 = rcu_dereference_protected(pn->shrinker_info, true);
> > @@ -242,12 +248,13 @@ 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;
> > +     int m_size, d_size = 0;
> >
> >       down_write(&shrinker_rwsem);
> > -     size = (shrinker_nr_max / BITS_PER_LONG + 1) * sizeof(unsigned long);
> > +     m_size = (shrinker_nr_max / BITS_PER_LONG + 1) * sizeof(unsigned long);
> > +     d_size = shrinker_nr_max * sizeof(atomic_long_t);
> > +     size = m_size + d_size;
> > +
> >       for_each_node(nid) {
> >               info = kvzalloc_node(sizeof(*info) + size, GFP_KERNEL, nid);
> >               if (!info) {
> > @@ -255,6 +262,9 @@ int alloc_shrinker_info(struct mem_cgroup *memcg)
> >                       ret = -ENOMEM;
> >                       break;
> >               }
> > +             info->map = (unsigned long *)(info + 1);
> > +             info->nr_deferred = (atomic_long_t *)(info->map +
> > +                                     shrinker_nr_max / BITS_PER_LONG + 1);
>
> Why not:
>                 info->nr_deferred = (void*)info->map + m_size;

Yes, definitely. Will fix in v5.

> ?
>
> >               rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_info, info);
> >       }
> >       up_write(&shrinker_rwsem);
> > @@ -266,10 +276,16 @@ static int expand_shrinker_info(int new_id)
> >  {
> >       int size, old_size, ret = 0;
> >       int new_nr_max = new_id + 1;
> > +     int m_size, d_size = 0;
> > +     int old_m_size, old_d_size = 0;
> >       struct mem_cgroup *memcg;
> >
> > -     size = (new_nr_max / BITS_PER_LONG + 1) * sizeof(unsigned long);
> > -     old_size = (shrinker_nr_max / BITS_PER_LONG + 1) * sizeof(unsigned long);
> > +     m_size = (new_nr_max / BITS_PER_LONG + 1) * sizeof(unsigned long);
> > +     d_size = new_nr_max * sizeof(atomic_long_t);
> > +     size = m_size + d_size;
> > +     old_m_size = (shrinker_nr_max / BITS_PER_LONG + 1) * sizeof(unsigned long);
>
> Could you please pack this twice repeating pattern into some macro? E.g.,
>
> #define NR_MAX_TO_SHR_MAP_SIZE(nr_max)  \
>         ((nr_max / BITS_PER_LONG + 1) * sizeof(unsigned long))

Sure. Will incorporate in v5.

>
> > +     old_d_size = shrinker_nr_max * sizeof(atomic_long_t);
> > +     old_size = old_m_size + old_d_size;
> >       if (size <= old_size)
> >               return 0;
> >
> > @@ -278,9 +294,8 @@ static int expand_shrinker_info(int new_id)
> >
> >       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, new_nr_max, m_size, d_size,
> > +                                            old_m_size, old_d_size);
> >               if (ret) {
> >                       mem_cgroup_iter_break(NULL, memcg);
> >                       goto out;
> >
>
>