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From:   Yosry Ahmed <yosryahmed@google.com>
Date:   Wed, 27 Sep 2023 13:57:17 -0700
Message-ID: <CAJD7tkbHsmM9tVTwv2Ve7Ekj_bgcZxVgpXKG8upNtZjAkimeZw@mail.gmail.com>
Subject: Re: [PATCH v2 1/2] zswap: make shrinking memcg-aware
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Nhat Pham <nphamcs@gmail.com>, akpm@linux-foundation.org,
        cerasuolodomenico@gmail.com, sjenning@redhat.com,
        ddstreet@ieee.org, vitaly.wool@konsulko.com, mhocko@kernel.org,
        roman.gushchin@linux.dev, shakeelb@google.com,
        muchun.song@linux.dev, linux-mm@kvack.org, kernel-team@meta.com,
        linux-kernel@vger.kernel.org, cgroups@vger.kernel.org,
        Chris Li <chrisl@kernel.org>
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On Wed, Sep 27, 2023 at 1:51=E2=80=AFPM Johannes Weiner <hannes@cmpxchg.org=
> wrote:
>
> On Mon, Sep 25, 2023 at 01:17:04PM -0700, Yosry Ahmed wrote:
> > On Tue, Sep 19, 2023 at 10:14=E2=80=AFAM Nhat Pham <nphamcs@gmail.com> =
wrote:
> > > +                       is_empty =3D false;
> > > +       }
> > > +       zswap_pool_put(pool);
> > > +
> > > +       if (is_empty)
> > > +               return -EINVAL;
> > > +       if (shrunk)
> > > +               return 0;
> > > +       return -EAGAIN;
> > >  }
> > >
> > >  static void shrink_worker(struct work_struct *w)
> > >  {
> > >         struct zswap_pool *pool =3D container_of(w, typeof(*pool),
> > >                                                 shrink_work);
> > > -       int ret, failures =3D 0;
> > > +       int ret, failures =3D 0, memcg_selection_failures =3D 0;
> > >
> > > +       /* global reclaim will select cgroup in a round-robin fashion=
. */
> > >         do {
> > > -               ret =3D zswap_reclaim_entry(pool);
> > > +               /* previous next_shrink has become a zombie - restart=
 from the top */
> >
> > Do we skip zombies because all zswap entries are reparented with the ob=
jcg?
> >
> > If yes, why do we restart from the top instead of just skipping them?
> > memcgs after a zombie will not be reachable now IIUC.
> >
> > Also, why explicitly check for zombies instead of having
> > shrink_memcg() just skip memcgs with no zswap entries? The logic is
> > slightly complicated.
>
> I think this might actually be a leftover from the initial plan to do
> partial walks without holding on to a reference to the last scanned
> css. Similar to mem_cgroup_iter() does with the reclaim cookie - if a
> dead cgroup is encountered and we lose the tree position, restart.
>
> But now the code actually holds a reference, so I agree the zombie
> thing should just be removed.

It might be nice to keep in shrink_memcg() as an optimization and for
fairness. IIUC, if a memcg is zombified the list_lrus will be
reparented, so we will scan the parent's list_lrus again, which can be
unfair to that parent. It can also slow things down if we have a large
number of zombies, as their number is virtually unbounded.