Re: [v3 PATCH 07/11] mm: vmscan: add per memcg shrinker nr_deferred

[Yang Shi <shy828301@gmail.com>]

On Wed, Jan 6, 2021 at 3:07 AM Kirill Tkhai <ktkhai@virtuozzo.com> wrote:
>
> On 06.01.2021 01:58, 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@gmail.com>
> > ---
> >  include/linux/memcontrol.h |  7 +++---
> >  mm/vmscan.c                | 49 +++++++++++++++++++++++++-------------
> >  2 files changed, 37 insertions(+), 19 deletions(-)
> >
> > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > index e05bbe8277cc..5599082df623 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 memcg_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 0033659abf9e..72259253e414 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -193,10 +193,12 @@ static void memcg_free_shrinker_info_rcu(struct rcu_head *head)
> >  }
> >
> >  static int memcg_expand_one_shrinker_info(struct mem_cgroup *memcg,
> > -                                       int size, int old_size)
> > +                                       int m_size, int d_size,
> > +                                       int old_m_size, int old_d_size)
> >  {
> >       struct memcg_shrinker_info *new, *old;
> >       int nid;
> > +     int size = m_size + d_size;
> >
> >       for_each_node(nid) {
> >               old = rcu_dereference_protected(
> > @@ -209,9 +211,18 @@ static int memcg_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 *)((unsigned long)new + sizeof(*new));
> > +             new->nr_deferred = (atomic_long_t *)((unsigned long)new +
> > +                                     sizeof(*new) + m_size);
>
> Can't we write this more compact?
>
>                 new->map = (unsigned long *)(new + 1);
>                 new->nr_deferred = (atomic_long_t)(new->map + 1);

Thanks for the suggestion, will incorporate it in v4.

>
> > +
> > +             /* 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((void *)new->nr_deferred, (void *)old->nr_deferred,
> > +                    old_d_size);
>
> Why not
>                 memcpy(new->nr_deferred, old->nr_deferred, old_d_size);
> ?

Will fix in v4.

>
> > +             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, memcg_free_shrinker_info_rcu);
> > @@ -226,9 +237,6 @@ void memcg_free_shrinker_info(struct mem_cgroup *memcg)
> >       struct memcg_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 +250,13 @@ int memcg_alloc_shrinker_info(struct mem_cgroup *memcg)
> >  {
> >       struct memcg_shrinker_info *info;
> >       int nid, size, ret = 0;
> > -
> > -     if (mem_cgroup_is_root(memcg))
> > -             return 0;
> > +     int m_size, d_size = 0;
> >
> >       down_read(&shrinker_rwsem);
> > -     size = DIV_ROUND_UP(shrinker_nr_max, BITS_PER_LONG) * sizeof(unsigned long);
> > +     m_size = DIV_ROUND_UP(shrinker_nr_max, BITS_PER_LONG) * sizeof(unsigned long);
> > +     d_size = shrinker_nr_max * sizeof(atomic_long_t);
> > +     size = m_size + d_size;
> > +
> >       for_each_node(nid) {
> >               info = kvzalloc(sizeof(*info) + size, GFP_KERNEL);
> >               if (!info) {
> > @@ -255,6 +264,9 @@ int memcg_alloc_shrinker_info(struct mem_cgroup *memcg)
> >                       ret = -ENOMEM;
> >                       break;
> >               }
> > +             info->map = (unsigned long *)((unsigned long)info + sizeof(*info));
> > +             info->nr_deferred = (atomic_long_t *)((unsigned long)info +
> > +                                     sizeof(*info) + m_size);
> >               rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_info, info);
> >       }
> >       up_read(&shrinker_rwsem);
> > @@ -265,10 +277,16 @@ int memcg_alloc_shrinker_info(struct mem_cgroup *memcg)
> >  static int memcg_expand_shrinker_info(int new_id)
> >  {
> >       int size, old_size, ret = 0;
> > +     int m_size, d_size = 0;
> > +     int old_m_size, old_d_size = 0;
> >       struct mem_cgroup *memcg;
> >
> > -     size = DIV_ROUND_UP(new_id + 1, BITS_PER_LONG) * sizeof(unsigned long);
> > -     old_size = DIV_ROUND_UP(shrinker_nr_max, BITS_PER_LONG) * sizeof(unsigned long);
> > +     m_size = DIV_ROUND_UP(new_id + 1, BITS_PER_LONG) * sizeof(unsigned long);
> > +     d_size = (new_id + 1) * sizeof(atomic_long_t);
> > +     size = m_size + d_size;
> > +     old_m_size = DIV_ROUND_UP(shrinker_nr_max, BITS_PER_LONG) * sizeof(unsigned long);
> > +     old_d_size = shrinker_nr_max * sizeof(atomic_long_t);
> > +     old_size = old_m_size + old_d_size;
> >       if (size <= old_size)
> >               return 0;
>
> This replication of patch [4/11] looks awkwardly. Please, try to incorporate
> the same changes to nr_deferred as I requested for shrinker_map in [4/11].

Sure. Thanks.

>
> >
> > @@ -277,9 +295,8 @@ static int memcg_expand_shrinker_info(int new_id)
> >
> >       memcg = mem_cgroup_iter(NULL, NULL, NULL);
> >       do {
> > -             if (mem_cgroup_is_root(memcg))
> > -                     continue;
> > -             ret = memcg_expand_one_shrinker_info(memcg, size, old_size);
> > +             ret = memcg_expand_one_shrinker_info(memcg, m_size, d_size,
> > +                                                  old_m_size, old_d_size);
> >               if (ret) {
> >                       mem_cgroup_iter_break(NULL, memcg);
> >                       goto out;
> >
>
>