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Date:   Tue, 12 Sep 2023 13:03:20 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Yosry Ahmed <yosryahmed@google.com>
Cc:     Michal =?iso-8859-1?Q?Koutn=FD?= <mkoutny@suse.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Shakeel Butt <shakeelb@google.com>,
        Muchun Song <muchun.song@linux.dev>,
        Ivan Babrou <ivan@cloudflare.com>, Tejun Heo <tj@kernel.org>,
        Waiman Long <longman@redhat.com>, linux-mm@kvack.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 3/4] mm: memcg: let non-unified root stats flushes
 help unified flushes
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On Tue 05-09-23 08:54:46, Yosry Ahmed wrote:
> On Tue, Sep 5, 2023 at 7:10 AM Michal Koutný <mkoutny@suse.com> wrote:
> >
> > On Mon, Sep 04, 2023 at 05:41:10PM +0200, Michal Hocko <mhocko@suse.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.

Flaky or not, it might cause behavior difference and a subtle one. I can
imagine nohz and similar workloads wanting to (ab)use this to reduce
kernel footprint. If we really need this then make it obvious in the
changelog at least.

-- 
Michal Hocko
SUSE Labs