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

[Zhaoyang Huang]

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:
> > On Wed, Aug 24, 2022 at 3:50 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > >
> > > On Wed 24-08-22 10:23:14, Zhaoyang Huang wrote:
> > > > On Tue, Aug 23, 2022 at 7:51 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > > [...]
> > > > > One way to achieve that would be shaping the hierarchy the following way
> > > > >             root
> > > > >         /         \
> > > > > no_memcg[1]      memcg[2]
> > > > > ||||||||         |||||
> > > > > app_cgroups     app_cgroups
> > > > >
> > > > > with
> > > > > no_memcg.subtree_control = ""
> > > > > memcg.subtree_control = memory
> > > > >
> > > > > no?
> > > > According to my understanding, No as there will be no no_memcg. All
> > > > children groups under root would have its cgroup.controllers = memory
> > > > as long as root has memory enabled.
> > >
> > > Correct
> > >
> > > > Under this circumstance, all
> > > > descendants group under 'no_memcg' will charge memory to its parent
> > > > group.
> > >
> > > Correct. And why is that a problem? I thought you main concern was a per
> > > application LRUs. With the above configuration all app_cgroups which do
> > > not require an explicit memory control will share the same (no_memcg)
> > > LRU and they will be aged together.
> > I can't agree since this indicates the processes want memory free
> > depending on a specific hierarchy which could have been determined by
> > other subsys.
>
> I really fail to understand your requirements.
>
> > 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. How should AA3
achieve the goal of compete with AA4,A1,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