Re: [patch -mm 3/4] mm, memcg: replace memory.oom_group with policy tunable

Michal Hocko
2018-01-24T08:20:41Z

On Tue 23-01-18 14:22:07, David Rientjes wrote:
> On Tue, 23 Jan 2018, Michal Hocko wrote:
>
> > > It can't, because the current patchset locks the system into a single
> > > selection criteria that is unnecessary and the mount option would become a
> > > no-op after the policy per subtree becomes configurable by the user as
> > > part of the hierarchy itself.
> >
> > This is simply not true! OOM victim selection has changed in the
> > past and will be always a subject to changes in future. Current
> > implementation doesn't provide any externally controlable selection
> > policy and therefore the default can be assumed. Whatever that default
> > means now or in future. The only contract added here is the kill full
> > memcg if selected and that can be implemented on _any_ selection policy.
> >
>
> The current implementation of memory.oom_group is based on top of a
> selection implementation that is broken in three ways I have listed for
> months:

This doesn't lead to anywhere. You are not presenting any new arguments
and you are ignoring feedback you have received so far. We have tried
really hard. Considering different _independent_ people presented more or
less consistent view on these points I think you should deeply
reconsider how you take that feedback.

> - allows users to intentionally/unintentionally evade the oom killer,
> requires not locking the selection implementation for the entire
> system, requires subtree control to prevent, makes a mount option
> obsolete, and breaks existing users who would use the implementation
> based on 4.16 if this were merged,
>
> - unfairly compares the root mem cgroup vs leaf mem cgroup such that
> users must structure their hierarchy only for 4.16 in such a way
> that _all_ processes are under hierarchical control and have no
> power to create sub cgroups because of the point above and
> completely breaks any user of oom_score_adj in a completely
> undocumented and unspecified way, such that fixing that breakage
> would also break any existing users who would use the implementation
> based on 4.16 if this were merged, and
>
> - does not allow userspace to protect important cgroups, which can be
> built on top.

For the last time. This all can be done on top of the proposed solution
without breaking the proposed user API. I am really _convinced_ that you
underestimate how complex it is to provide a sane selection policy API
and it will take _months_ to settle on something. Existing OOM APIs are
a sad story and I definitly do not want to repeat same mistakes from the
past.
--
Michal Hocko
SUSE Labs