linux-kernel - Re: [v3 PATCH 10/11] mm: memcontrol: reparent nr

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Date:   Mon, 11 Jan 2021 10:43:20 -0800
From:   Yang Shi <shy828301@...il.com>
To:     Kirill Tkhai <ktkhai@...tuozzo.com>
Cc:     Roman Gushchin <guro@...com>, Shakeel Butt <shakeelb@...gle.com>,
        Dave Chinner <david@...morbit.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [v3 PATCH 10/11] mm: memcontrol: reparent nr_deferred when memcg offline

On Wed, Jan 6, 2021 at 3:35 AM Kirill Tkhai <ktkhai@...tuozzo.com> wrote:
>
> On 06.01.2021 01:58, 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@...il.com>
> > ---
> >  include/linux/memcontrol.h |  1 +
> >  mm/memcontrol.c            |  1 +
> >  mm/vmscan.c                | 29 +++++++++++++++++++++++++++++
> >  3 files changed, 31 insertions(+)
> >
> > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > index 5599082df623..d1e52e916cc2 100644
> > --- a/include/linux/memcontrol.h
> > +++ b/include/linux/memcontrol.h
> > @@ -1586,6 +1586,7 @@ extern int memcg_alloc_shrinker_info(struct mem_cgroup *memcg);
> >  extern void memcg_free_shrinker_info(struct mem_cgroup *memcg);
> >  extern void memcg_set_shrinker_bit(struct mem_cgroup *memcg,
> >                                  int nid, int shrinker_id);
> > +extern void memcg_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 126f1fd550c8..19e555675582 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -5284,6 +5284,7 @@ static void mem_cgroup_css_offline(struct cgroup_subsys_state *css)
> >       page_counter_set_low(&memcg->memory, 0);
> >
> >       memcg_offline_kmem(memcg);
> > +     memcg_reparent_shrinker_deferred(memcg);
> >       wb_memcg_offline(memcg);
> >
> >       drain_all_stock(memcg);
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index d9795fb0f1c5..71056057d26d 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -396,6 +396,35 @@ static long set_nr_deferred_memcg(long nr, int nid, struct shrinker *shrinker,
> >       return atomic_long_add_return(nr, &info->nr_deferred[shrinker->id]);
> >  }
> >
> > +void memcg_reparent_shrinker_deferred(struct mem_cgroup *memcg)
> > +{
> > +     int i, nid;
> > +     long nr;
> > +     struct mem_cgroup *parent;
> > +     struct memcg_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 = rcu_dereference_protected(
> > +                                     memcg->nodeinfo[nid]->shrinker_info,
> > +                                     true);
> > +             parent_info = rcu_dereference_protected(
> > +                                     parent->nodeinfo[nid]->shrinker_info,
> > +                                     true);
>
> Simple assignment can't take such lots of space, we have to do something with that.
>
> Number of these
>
>         rcu_dereference_protected(memcg->nodeinfo[nid]->shrinker_info, true)
>
> became too big, and we can't allow every of them takes 3 lines.
>
> We should introduce a short helper to dereferrence this, so we will be able to give
> out attention to really difficult logic instead of wasting it on parsing this.
>
>                 child_info = memcg_shrinker_info(memcg, nid);
> or
>                 child_info = memcg_shrinker_info_protected(memcg, nid);
>
> Both of them fit in single line.
>
> struct memcg_shrinker_info *memcg_shrinker_info_protected(
>                                         struct mem_cgroup *memcg, int nid)
> {
>         return rcu_dereference_protected(memcg->nodeinfo[nid]->shrinker_info,
>                                          lockdep_assert_held(&shrinker_rwsem));
> }

Thanks for the suggestion, it makes sense to me. Will incorporate it in v4.

>
>
> > +             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]);
>
> Why new line is here? In case of you merge it up, it will be even shorter then previous line.

Just keep in 80 lines. We could relax it.

>
> > +             }
> > +     }
> > +     up_read(&shrinker_rwsem);
> > +}
> > +
> >  static bool cgroup_reclaim(struct scan_control *sc)
> >  {
> >       return sc->target_mem_cgroup;
> >
>
>