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Date:   Fri, 14 Feb 2020 16:13:18 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     Tejun Heo <tj@kernel.org>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Roman Gushchin <guro@fb.com>, linux-mm@kvack.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@fb.com
Subject: Re: [PATCH v2 3/3] mm: memcontrol: recursive memory.low protection
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On Fri 14-02-20 08:57:28, Tejun Heo wrote:
[...]

Sorry to skip over a large part of your response. The discussion in this
thread got quite fragmented already and I would really like to conclude
to something.

> > I believe I have already expressed the configurability concern elsewhere
> > in the email thread. It boils down to necessity to propagate
> > protection all the way up the hierarchy properly if you really need to
> > protect leaf cgroups that are organized without a resource control in
> > mind. Which is what systemd does.
> 
> But that doesn't work for other controllers at all. I'm having a
> difficult time imagining how making this one control mechanism work
> that way makes sense. Memory protection has to be configured together
> with IO protection to be actually effective.

Please be more specific. If the protected workload is mostly in-memory,
I do not really see how IO controller is relevant. See the example of
the DB setup I've mentioned elsewhere.

> As for cgroup hierarchy being unrelated to how controllers behave, it
> frankly reminds me of cgroup1 memcg flat hierarchy thing I'm not sure
> how that would actually work in terms of resource isolation. Also, I'm
> not sure how systemd forces such configurations and I'd think systemd
> folks would be happy to fix them if there are such problems. Is the
> point you're trying to make "because of systemd, we have to contort
> how memory controller behaves"?

No, I am just saying and as explained in reply to Johannes, there are
practical cases where the cgroup hierarchy reflects organizational
structure as well.
-- 
Michal Hocko
SUSE Labs