Re: [RFC PATCH] mm: memcg/slab: Stop reparented obj_cgroups from charging root

[Roman Gushchin]

On Fri, Oct 16, 2020 at 07:15:02PM +0200, Michal Koutny wrote:
> On Fri, Oct 16, 2020 at 10:53:08AM -0400, Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
> > The central try_charge() function charges recursively all the way up
> > to and including the root.
> Except for use_hiearchy=0 (which is the case here as Richard
> wrote). The reparenting is hence somewhat incompatible with
> new_parent.use_hiearchy=0 :-/
> 
> > We should clean this up one way or another: either charge the root or
> > don't, but do it consistently.
> I agree this'd be good to unify. One upside of excluding root memcg from
> charging is that users are spared from the charging overhead when memcg
> tree is not created.  (Actually, I thought that was the reason for this
> exception.)

Yeah, I'm completely on the same page. Moving a process to the root memory
cgroup is currently a good way to estimate the memory cgroup overhead.

How about the patch below, which consistently avoids charging the root
memory cgroup? It seems like it doesn't add too many checks.

Thanks!

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