Re: [PATCH v4 3/4] mm: memcg: let non-unified root stats flushes help unified flushes

[Yosry Ahmed]

On Tue, Sep 5, 2023 at 7:10 AM Michal Koutný <mkoutny@xxxxxxxx> wrote:
>
> On Mon, Sep 04, 2023 at 05:41:10PM +0200, Michal Hocko <mhocko@xxxxxxxx> wrote:
> > So it also creates an undocumented but userspace visible behavior.
> > Something that userspace might start depending on, right?
>
> Yes but -
> - depending on undocumented behavior is a mistake,
> - breaking the dependency would manifest (in the case I imagine) as a
>   performance regression (and if there are some users, the future can
>   allow them configuring periodic kernel flush to compensate for that).

I think I am missing something. This change basically makes userspace
readers (for the root memcg) help out unified flushers, which are
in-kernel readers (e.g. reclaim) -- not the other way around.

How would that create a userspace visible behavior that a dependency
can be formed on? Users expecting reclaim to be faster right after
reading root stats? I would guess that would be too flaky to cause a
behavior that people can depend on tbh.