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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 20:32:25 +0200
Message-ID: <20200811183225.GA62582@blackbook> (raw)
In-Reply-To: <20200811165527.GA1507044@carbon.DHCP.thefacebook.com>


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On Tue, Aug 11, 2020 at 09:55:27AM -0700, Roman Gushchin <guro@fb.com> wrote:
> As I said, there are 2 problems with charging systemd (or a similar daemon):
> 1) It often belongs to the root cgroup.
This doesn't hold for systemd (if we agree that systemd is the most
common case).

> 2) OOMing or failing some random memory allocations is a bad way
>    to "communicate" a memory shortage to the daemon.
>    What we really want is to prevent creating a huge number of cgroups
There's cgroup.max.descendants for that...

>    (including dying cgroups) in some specific sub-tree(s).
...oh, so is this limiting the number of cgroups or limiting resources
used?

>    OOMing the daemon or returning -ENOMEM to some random syscalls
>    will not help us to reach the goal and likely will bring a bad
>    experience to a user.
If we reach the situation when memory for cgroup operations is tight,
it'll disappoint the user either way.
My premise is that a running workload is more valuable than the
accompanying manager.

> In a generic case I don't see how we can charge the cgroup which
> creates cgroups without solving these problems first.
In my understanding, "onbehalveness" is a concept useful for various
kernel threads doing deferred work. Here it's promoted to user processes
managing cgroups.

> And if there is a very special case where we have to limit it,
> we can just add an additional layer:
> 
> ` root or delegated root
>    ` manager-parent-cgroup-with-a-limit
>      ` manager-cgroup (systemd, docker, ...)
>    ` [aggregation group(s)]
>      ` job-group-1
>      ` ...
>      ` job-group-n
If the charge goes to the parent of created cgroup (job-cgroup-i here),
then the layer adds nothing. Am I missing something?

> I'd definitely charge the parent cgroup in all similar cases.
(This would mandate the controllers on the unified hierarchy, which is
fine IMO.) Then the order of enabling controllers on a subtree (e.g.
cpu,memory vs memory,cpu) by the manager would yield different charging.
This seems wrong^W confusing to me. 


Thanks,
Michal

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  reply index

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-06-23 18:45 [PATCH v3 0/5] mm: memcg accounting of percpu memory Roman Gushchin
2020-06-23 18:45 ` [PATCH v3 1/5] percpu: return number of released bytes from pcpu_free_area() Roman Gushchin
2020-06-24  0:58   ` Shakeel Butt
2020-06-23 18:45 ` [PATCH v3 2/5] mm: memcg/percpu: account percpu memory to memory cgroups Roman Gushchin
2020-06-24  1:25   ` Shakeel Butt
2020-06-23 18:45 ` [PATCH v3 3/5] mm: memcg/percpu: per-memcg percpu memory statistics Roman Gushchin
2020-06-24  1:35   ` Shakeel Butt
2020-08-11 15:05   ` Johannes Weiner
2020-06-23 18:45 ` [PATCH v3 4/5] mm: memcg: charge memcg percpu memory to the parent cgroup Roman Gushchin
2020-06-24  1:40   ` Shakeel Butt
2020-06-24  1:49     ` Roman Gushchin
2020-07-29 17:10   ` Michal Koutný
2020-08-07  4:16     ` Andrew Morton
2020-08-07  4:37       ` Roman Gushchin
2020-08-10 19:33         ` Roman Gushchin
2020-08-11 14:47         ` Michal Koutný
2020-08-11 16:55           ` Roman Gushchin
2020-08-11 18:32             ` Michal Koutný [this message]
2020-08-11 19:32               ` Roman Gushchin
2020-08-12 16:28                 ` Michal Koutný
2020-08-11 15:27   ` Johannes Weiner
2020-08-11 17:06     ` Roman Gushchin
2020-08-13  9:16       ` Naresh Kamboju
2020-08-13 23:27         ` Stephen Rothwell

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