Re: [v5 PATCH 07/11] mm: vmscan: add per memcg shrinker nr_deferred

[Yang Shi <shy828301@gmail.com>]

On Fri, Jan 29, 2021 at 5:00 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> On 1/28/21 12:33 AM, 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                | 48 +++++++++++++++++++++++++-------------
> >  2 files changed, 36 insertions(+), 19 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 256896d157d4..20be0db291fe 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -187,16 +187,21 @@ static DECLARE_RWSEM(shrinker_rwsem);
> >  #ifdef CONFIG_MEMCG
> >  static int shrinker_nr_max;
> >
> > +#define NR_MAX_TO_SHR_MAP_SIZE(nr_max)       \
> > +     ((nr_max / BITS_PER_LONG + 1) * sizeof(unsigned long))
>
> Could have been part of patch 4 already. And yeah, using DIV_ROUND_UP(), as
> being hidden in a macro makes the "shorter statement" benefit disappear :)
>
> > +
> >  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)
> > +                                 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 +214,15 @@ 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 = (void *)new->map + m_size;
>
> This better be aligned to sizeof(atomic_long_t). Can we be sure about that?

Good point. No, if unsigned long is 32 bit on some 64 bit machines.

> Also it's all quite ugly and complex. Is it worth it? What about just leaving
> map as it is and allocating a nr_deferred array separately, i.e.:
>
>   struct shrinker_info {
>         struct rcu_head rcu;
>         atomic_long_t *nr_deferred; // allocated separately
>         unsigned long map[];
>   };

So, you mean we allocate shrinker info with map array in the first
step, then allocate nr_deferred? It is ok, but I'm afraid the error
handling may make the code not that clean as what you expect since we
have to call kvmalloc() twice. And we still need to do all the
initialization and copy work. So, eventually we just replace the
pointer assignment to error handling. I'm not quite sure if it is
worth it. The nested error handling might be more error prone.

>
> > +             /* 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 +237,6 @@ 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 +250,13 @@ 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 = NR_MAX_TO_SHR_MAP_SIZE(shrinker_nr_max);
> > +     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 +264,8 @@ int alloc_shrinker_info(struct mem_cgroup *memcg)
> >                       ret = -ENOMEM;
> >                       break;
> >               }
> > +             info->map = (unsigned long *)(info + 1);
> > +             info->nr_deferred = (void *)info->map + m_size;
> >               rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_info, info);
> >       }
> >       up_write(&shrinker_rwsem);
> > @@ -266,10 +277,16 @@ 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 = NR_MAX_TO_SHR_MAP_SIZE(new_nr_max);
> > +     d_size = new_nr_max * sizeof(atomic_long_t);
> > +     size = m_size + d_size;
> > +     old_m_size = NR_MAX_TO_SHR_MAP_SIZE(shrinker_nr_max);
> > +     old_d_size = shrinker_nr_max * sizeof(atomic_long_t);
> > +     old_size = old_m_size + old_d_size;
> >       if (size <= old_size)
> >               goto out;
> >
> > @@ -278,9 +295,8 @@ 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, m_size, d_size,
> > +                                            old_m_size, old_d_size);
> >               if (ret) {
> >                       mem_cgroup_iter_break(NULL, memcg);
> >                       goto out;
> >
>