Re: [PATCH v3 4/5] mm: memcg: charge memcg percpu memory to the parent cgroup

From: "Michal Koutný" <mkoutny@suse.com>
To: Roman Gushchin <guro@fb.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Dennis Zhou <dennis@kernel.org>, Tejun Heo <tj@kernel.org>,
	Christoph Lameter <cl@linux.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Michal Hocko <mhocko@kernel.org>,
	Shakeel Butt <shakeelb@google.com>,
	linux-mm@kvack.org, kernel-team@fb.com,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 4/5] mm: memcg: charge memcg percpu memory to the parent cgroup
Date: Tue, 11 Aug 2020 16:47:54 +0200	[thread overview]
Message-ID: <20200811144754.GA45201@blackbook> (raw)
In-Reply-To: <20200807043717.GA1231562@carbon.DHCP.thefacebook.com>

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On Thu, Aug 06, 2020 at 09:37:17PM -0700, Roman Gushchin <guro@fb.com> wrote:
> In general, yes. But in this case I think it wouldn't be a good idea:
> most often cgroups are created by a centralized daemon (systemd),
> which is usually located in the root cgroup. Even if it's located not in
> the root cgroup, limiting it's memory will likely affect the whole system,
> even if only one specific limit was reached.
The generic scheme would be (assuming the no internal process
constraint, in the root too)

` root or delegated root
  ` manager-cgroup (systemd, docker, ...)
  ` [aggregation group(s)]
    ` job-group-1
    ` ...
    ` job-group-n

> If there is a containerized workload, which creates sub-cgroups,
> charging it's parent cgroup is perfectly effective.
No dispute about this in either approaches.

> And the opposite, if we'll charge the cgroup of a process, who created
> a cgroup, we'll not cover the most common case: systemd creating
> cgroups for all services in the system.
What I mean is that systemd should be charged for the cgroup creation.
Or more generally, any container/cgroup manager should be charged.
Consider a leak when it wouldn't remove spent cgroups, IMO the effect
(throttling, reclaim) should be exercised on such a culprit.

I don't think the existing workload (job-group-i above) should
unnecessarily suffer when only manager is acting up. Is that different
from your idea?

> Right, but it's quite unusual for tasks from one cgroup to create sub-cgroups
> in completely different cgroup. In this particular case there are tons of other
> ways how a task from C00 can hurt C1.
I agree with that.

If I haven't overlooked anything, this should be first case when
cgroup-related structures are accounted (please correct me).
So this is setting a precendent, if others show useful to be accounted
in the future too. I'm thinking about cpu_cgroup_css_alloc() that can
also allocate a lot (with big CPU count). The current approach would lead
situations where matching cpu and memory csses needn't to exist and that
would need special handling.

> On Thu, Aug 06, 2020 at 09:16:03PM -0700, Andrew Morton wrote:
> > These week-old issues appear to be significant.  Roman?  Or someone
> > else?
Despite my concerns, I don't think this is fundamental and can't be
changed later so it doesn't prevent the inclusion in 5.9 RC1.

Regards,
Michal

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