Re: [PATCH v2 1/2] zswap: make shrinking memcg-aware

[Yosry Ahmed <yosryahmed@google.com>]

On Wed, Sep 27, 2023 at 1:51 PM 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 AM Nhat Pham <nphamcs@gmail.com> wrote:
> > > +                       is_empty = 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 = container_of(w, typeof(*pool),
> > >                                                 shrink_work);
> > > -       int ret, failures = 0;
> > > +       int ret, failures = 0, memcg_selection_failures = 0;
> > >
> > > +       /* global reclaim will select cgroup in a round-robin fashion. */
> > >         do {
> > > -               ret = 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 objcg?
> >
> > 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.