Re: [RFC PATCH] memcg: use root_mem_cgroup when css is inherited

[Zhaoyang Huang]

On Thu, Aug 25, 2022 at 2:40 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Thu 25-08-22 08:43:52, Zhaoyang Huang wrote:
> > On Wed, Aug 24, 2022 at 6:27 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > >
> > > On Wed 24-08-22 17:34:42, Zhaoyang Huang wrote:
> [...]
> > > > IMHO, charging the pages which out of explicitly memory
> > > > enabled group to root could solve all of the above constraints with no
> > > > harm.
> > >
> > > This would break the hierarchical property of the controller. So a
> > > strong no no. Consider the following example
> > >
> > >        root
> > >         |
> > >         A
> > > controllers="memory"
> > > memory.max = 1G
> > > subtree_control=""
> > > |      |      |
> > > A1     A2     A3
> > >
> > > althought A1,2,3 do not have their memory controller enabled explicitly
> > > they are still constrained by the A memcg limit. If you just charge to
> > > the root because it doesn't have memory controller enabled explicitly
> > > then you just evade that constrain. I hope you understand why that is a
> > > problem.
> > IMO, A1-A3 should be explicitly enabled via echo "+memory" >
> > A/subtree_control since memory.max has been set.
>
> You seem to be missing the point I've triedy to make here. It is not
> about how the respective subtree should or shouldn't be configured. It
> is about the hierarchical behavior. Configuration at a higher level should be
> enforced under subtree no matter how that subtree decides to
> enabled/disable controllers. Such subtree might have beeb delegated
> and configured differently yet the constrain should be still applied.
> See the point?
>
> What you seem to be proposing is similar to cgroup v1 use_hierarchy
> configuration. It has been decided that this is undesirable very early
> in the cgroup v2 development because it make delegation impossible
> (among other reasons).
Ok, I would like to know how AA3 achieve the goal of competing with A1
and A2 for cpu but keep memory out of control under current policy?
        root
         |
         A
 controllers="memory,cpu"
 memory.max = 1G
 subtree_control="memory,cpu"
 |      |            |
 A1     A2     A3 subtree_control="cpu"
                      |      |
                    AA3   AA4 controllers="cpu"
>
> --
> Michal Hocko
> SUSE Labs