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

[Michal Hocko <mhocko@suse.com>]

On Wed 24-08-22 17:34:42, Zhaoyang Huang wrote:
> On Wed, Aug 24, 2022 at 3:50 PM Michal Hocko <mhocko@suse.com> wrote:
> >
> > On Wed 24-08-22 10:23:14, Zhaoyang Huang wrote:
> > > On Tue, Aug 23, 2022 at 7:51 PM Michal Hocko <mhocko@suse.com> 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.
-- 
Michal Hocko
SUSE Labs