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

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Message-ID: <CAJD7tkaHVtMiMYFocNiABuyhPcqt77gei0UeaDq4J7V-=tMFYA@mail.gmail.com>
Date:   Tue, 5 Sep 2023 08:54:46 -0700
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Michal Koutný <mkoutny@...e.com>
Cc:     Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Ivan Babrou <ivan@...udflare.com>, Tejun Heo <tj@...nel.org>,
        Waiman Long <longman@...hat.com>, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 3/4] mm: memcg: let non-unified root stats flushes help
 unified flushes

On Tue, Sep 5, 2023 at 7:10 AM Michal Koutný <mkoutny@...e.com> wrote:
>
> On Mon, Sep 04, 2023 at 05:41:10PM +0200, Michal Hocko <mhocko@...e.com> 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.