Re: [v3 PATCH 09/11] mm: vmscan: don't need allocate shrinker->nr_deferred for memcg aware shrinkers

[Yang Shi <shy828301@gmail.com>]

On Wed, Jan 6, 2021 at 3:16 AM Kirill Tkhai <ktkhai@virtuozzo.com> wrote:
>
> On 06.01.2021 01:58, Yang Shi wrote:
> > Now nr_deferred is available on per memcg level for memcg aware shrinkers, so don't need
> > allocate shrinker->nr_deferred for such shrinkers anymore.
> >
> > The prealloc_memcg_shrinker() would return -ENOSYS if !CONFIG_MEMCG or memcg is disabled
> > by kernel command line, then shrinker's SHRINKER_MEMCG_AWARE flag would be cleared.
> > This makes the implementation of this patch simpler.
> >
> > Signed-off-by: Yang Shi <shy828301@gmail.com>
> > ---
> >  mm/vmscan.c | 33 ++++++++++++++++++---------------
> >  1 file changed, 18 insertions(+), 15 deletions(-)
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index f20ed8e928c2..d9795fb0f1c5 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -340,6 +340,9 @@ static int prealloc_memcg_shrinker(struct shrinker *shrinker)
> >  {
> >       int id, ret = -ENOMEM;
> >
> > +     if (mem_cgroup_disabled())
> > +             return -ENOSYS;
> > +
> >       down_write(&shrinker_rwsem);
> >       /* This may call shrinker, so it must use down_read_trylock() */
> >       id = idr_alloc(&shrinker_idr, SHRINKER_REGISTERING, 0, 0, GFP_KERNEL);
> > @@ -424,7 +427,7 @@ static bool writeback_throttling_sane(struct scan_control *sc)
> >  #else
> >  static int prealloc_memcg_shrinker(struct shrinker *shrinker)
> >  {
> > -     return 0;
> > +     return -ENOSYS;
> >  }
> >
> >  static void unregister_memcg_shrinker(struct shrinker *shrinker)
> > @@ -535,8 +538,20 @@ unsigned long lruvec_lru_size(struct lruvec *lruvec, enum lru_list lru, int zone
> >   */
> >  int prealloc_shrinker(struct shrinker *shrinker)
> >  {
> > -     unsigned int size = sizeof(*shrinker->nr_deferred);
> > +     unsigned int size;
> > +     int err;
> > +
> > +     if (shrinker->flags & SHRINKER_MEMCG_AWARE) {
> > +             err = prealloc_memcg_shrinker(shrinker);
> > +             if (!err)
> > +                     return 0;
> > +             if (err != -ENOSYS)
> > +                     return err;
> > +
> > +             shrinker->flags &= ~SHRINKER_MEMCG_AWARE;
>
> This looks very confusing.
>
> In case of you want to disable preallocation branch for !MEMCG case,
> you should firstly consider something like the below:

Not only !CONFIG_MEMCG, but also "cgroup_disable=memory" case.

>
> #ifdef CONFIG_MEMCG
> #define SHRINKER_MEMCG_AWARE    (1 << 2)
> #else
> #define SHRINKER_MEMCG_AWARE    0
> #endif

This could handle !CONFIG_MEMCG case, but can't deal with
"cgroup_disable=memory" case. We could consider check
mem_cgroup_disabled() when initializing shrinker, but this may result
in touching fs codes like below:

--- a/fs/super.c
+++ b/fs/super.c
@@ -266,7 +266,9 @@ static struct super_block *alloc_super(struct
file_system_type *type, int flags,
        s->s_shrink.scan_objects = super_cache_scan;
        s->s_shrink.count_objects = super_cache_count;
        s->s_shrink.batch = 1024;
-       s->s_shrink.flags = SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE;
+       s->s_shrink.flags = SHRINKER_NUMA_AWARE;
+       if (!mem_cgroup_disabled())
+               s->s_shrink.flags |= SHRINKER_MEMCG_AWARE;
        if (prealloc_shrinker(&s->s_shrink))
                goto fail;
        if (list_lru_init_memcg(&s->s_dentry_lru, &s->s_shrink))


>
> > +     }
> >
> > +     size = sizeof(*shrinker->nr_deferred);
> >       if (shrinker->flags & SHRINKER_NUMA_AWARE)
> >               size *= nr_node_ids;
> >
> > @@ -544,26 +559,14 @@ int prealloc_shrinker(struct shrinker *shrinker)
> >       if (!shrinker->nr_deferred)
> >               return -ENOMEM;
> >
> > -     if (shrinker->flags & SHRINKER_MEMCG_AWARE) {
> > -             if (prealloc_memcg_shrinker(shrinker))
> > -                     goto free_deferred;
> > -     }
> >
> >       return 0;
> > -
> > -free_deferred:
> > -     kfree(shrinker->nr_deferred);
> > -     shrinker->nr_deferred = NULL;
> > -     return -ENOMEM;
> >  }
> >
> >  void free_prealloced_shrinker(struct shrinker *shrinker)
> >  {
> > -     if (!shrinker->nr_deferred)
> > -             return;
> > -
> >       if (shrinker->flags & SHRINKER_MEMCG_AWARE)
> > -             unregister_memcg_shrinker(shrinker);
> > +             return unregister_memcg_shrinker(shrinker);
> >
> >       kfree(shrinker->nr_deferred);
> >       shrinker->nr_deferred = NULL;
> >
>
>