Re: [v5 PATCH 10/11] mm: memcontrol: reparent nr_deferred when memcg offline

[Yang Shi <shy828301@gmail.com>]

On Fri, Jan 29, 2021 at 7:52 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> On 1/28/21 12:33 AM, Yang Shi wrote:
> > Now shrinker's nr_deferred is per memcg for memcg aware shrinkers, add to parent's
> > corresponding nr_deferred when memcg offline.
> >
> > Signed-off-by: Yang Shi <shy828301@gmail.com>
>
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
>
> A question somewhat outside of the scope of the series. Should we shrink before
> reparenting on memcg offline? Would it make more sense than assume the kmemcg
> objects that are still cached are used also by others?

TBH, I'm not sure. I think it depends on workload. For example, the
build server may prefer to keep the objects cached since the samce
objects may be reused by multiple build jobs.

>
> > ---
> >  include/linux/memcontrol.h |  1 +
> >  mm/memcontrol.c            |  1 +
> >  mm/vmscan.c                | 31 +++++++++++++++++++++++++++++++
> >  3 files changed, 33 insertions(+)
> >
> > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > index e0384367e07d..fe1375f08881 100644
> > --- a/include/linux/memcontrol.h
> > +++ b/include/linux/memcontrol.h
> > @@ -1586,6 +1586,7 @@ extern int alloc_shrinker_info(struct mem_cgroup *memcg);
> >  extern void free_shrinker_info(struct mem_cgroup *memcg);
> >  extern void set_shrinker_bit(struct mem_cgroup *memcg,
> >                            int nid, int shrinker_id);
> > +extern void reparent_shrinker_deferred(struct mem_cgroup *memcg);
> >  #else
> >  #define mem_cgroup_sockets_enabled 0
> >  static inline void mem_cgroup_sk_alloc(struct sock *sk) { };
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index f64ad0d044d9..21f36b73f36a 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -5282,6 +5282,7 @@ static void mem_cgroup_css_offline(struct cgroup_subsys_state *css)
> >       page_counter_set_low(&memcg->memory, 0);
> >
> >       memcg_offline_kmem(memcg);
> > +     reparent_shrinker_deferred(memcg);
> >       wb_memcg_offline(memcg);
> >
> >       drain_all_stock(memcg);
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 0373d7619d7b..55ad91a26ba3 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -386,6 +386,37 @@ static long set_nr_deferred_memcg(long nr, int nid, struct shrinker *shrinker,
> >       return atomic_long_add_return(nr, &info->nr_deferred[shrinker->id]);
> >  }
> >
> > +static struct shrinker_info *shrinker_info_protected(struct mem_cgroup *memcg,
> > +                                                  int nid)
> > +{
> > +     return rcu_dereference_protected(memcg->nodeinfo[nid]->shrinker_info,
> > +                                      lockdep_is_held(&shrinker_rwsem));
> > +}
> > +
> > +void reparent_shrinker_deferred(struct mem_cgroup *memcg)
> > +{
> > +     int i, nid;
> > +     long nr;
> > +     struct mem_cgroup *parent;
> > +     struct shrinker_info *child_info, *parent_info;
> > +
> > +     parent = parent_mem_cgroup(memcg);
> > +     if (!parent)
> > +             parent = root_mem_cgroup;
> > +
> > +     /* Prevent from concurrent shrinker_info expand */
> > +     down_read(&shrinker_rwsem);
> > +     for_each_node(nid) {
> > +             child_info = shrinker_info_protected(memcg, nid);
> > +             parent_info = shrinker_info_protected(parent, nid);
> > +             for (i = 0; i < shrinker_nr_max; i++) {
> > +                     nr = atomic_long_read(&child_info->nr_deferred[i]);
> > +                     atomic_long_add(nr, &parent_info->nr_deferred[i]);
> > +             }
> > +     }
> > +     up_read(&shrinker_rwsem);
> > +}
> > +
> >  static bool cgroup_reclaim(struct scan_control *sc)
> >  {
> >       return sc->target_mem_cgroup;
> >
>