Re: [PATCH v6 2/6] memcontrol: allows mem_cgroup_iter() to check for onlineness

[Nhat Pham <nphamcs@gmail.com>]

On Wed, Nov 29, 2023 at 1:18 AM Michal Hocko <mhocko@suse.com> wrote:
>
> On Tue 28-11-23 08:53:56, Nhat Pham wrote:
> > On Tue, Nov 28, 2023 at 1:38 AM Michal Hocko <mhocko@suse.com> wrote:
> > >
> > > On Mon 27-11-23 11:36:59, Nhat Pham wrote:
> > > > The new zswap writeback scheme requires an online-only memcg hierarchy
> > > > traversal. Add a new parameter to mem_cgroup_iter() to check for
> > > > onlineness before returning.
> > >
> > > Why is this needed?
> >
> > For context, in patch 3 of this series, Domenico and I are adding
> > cgroup-aware LRU to zswap, so that we can perform workload-specific
> > zswap writeback. When the reclaim happens due to the global zswap
> > limit being hit, a cgroup is selected by the mem_cgroup_iter(), and
> > the last one selected is saved in the zswap pool (so that the
> > iteration can follow from there next time the limit is hit).
> >
> > However, one problem with this scheme is we will be pinning the
> > reference to that saved memcg until the next global reclaim attempt,
> > which could prevent it from being killed for quite some time after it
> > has been offlined. Johannes, Yosry, and I discussed a couple of
> > approaches for a while, and decided to add a callback that would
> > release the reference held by the zswap pool when the memcg is
> > offlined, and the zswap pool will obtain the reference to the next
> > online memcg in the traversal (or at least one that has not had the
> > zswap-memcg-release-callback run on it yet).
>
> This should be a part of the changelog along with an explanation why
> this cannot be handled on the caller level? You have a pin on the memcg,
> you can check it is online and scratch it if not, right? Why do we need
> to make a rather convoluted iterator interface more complex when most
> users simply do not require that?

Ah that's a good point. Hmm then I'll just do an extra online check in
the zswap reclaim callsite - cleaner and less invasive.

Thanks for the suggestion!
>
> --
> Michal Hocko
> SUSE Labs