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    SubjectRe: [RFC PATCH] mm: memcg/slab: Stop reparented obj_cgroups from charging root
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    Hello,

    Michal Koutný <mkoutny@suse.com> writes:

    > On Fri, Oct 16, 2020 at 10:53:08AM -0400, Johannes Weiner <hannes@cmpxchg.org> 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 :-/
    >

    Yes and it also seems

    new_parent.use_hierarch=0 -> new_child.use_hierarchy=0

    and

    new_parent.use_hierarch=0 -> new_child.use_hierarchy=1

    are considered valid on cgroupsV1. The kernel will also allow more
    descendants on new_child.use_hierarchy=0, but sets
    broken_hierarchy=1. However this will not stop the stack trace occuring
    (AFAICT) when the reparenting happens between two descendants.

    >> 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.)
    >
    > Michal


    --
    Thank you,
    Richard.

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