netdev - Re: [PATCH bpf-next v2 00/12] bpf: Introduce selectable memcg for bpf map

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Message-ID: <CALOAHbDcrj1ifFsNMHBEih5-SXY2rWViig4rQHi9N07JY6CjXA@mail.gmail.com>
Date:   Fri, 19 Aug 2022 09:09:25 +0800
From:   Yafang Shao <laoar.shao@...il.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, Martin Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        john fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, jolsa@...nel.org,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        lizefan.x@...edance.com, Cgroups <cgroups@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>
Subject: Re: [PATCH bpf-next v2 00/12] bpf: Introduce selectable memcg for bpf map

On Fri, Aug 19, 2022 at 6:33 AM Tejun Heo <tj@...nel.org> wrote:
>
> On Thu, Aug 18, 2022 at 12:20:33PM -1000, Tejun Heo wrote:
> > We have the exact same problem for any resources which span multiple
> > instances of a service including page cache, tmpfs instances and any other
> > thing which can persist longer than procss life time. My current opinion is
>
> To expand a bit more on this point, once we start including page cache and
> tmpfs, we now get entangled with memory reclaim which then brings in IO and
> not-yet-but-eventually CPU usage.

Introduce-a-new-layer vs introduce-a-new-cgroup, which one is more overhead?

> Once you start splitting the tree like
> you're suggesting here, all those will break down and now we have to worry
> about how to split resource accounting and control for the same entities
> across two split branches of the tree, which doesn't really make any sense.
>

The k8s has already been broken thanks to the memcg accounting on  bpf memory.
If you ignored it, I paste it below.
[0]"1. The memory usage is not consistent between the first generation and
new generations."

This issue will persist even if you introduce a new layer.

> So, we *really* don't wanna paint ourselves into that kind of a corner. This
> is a dead-end. Please ditch it.
>

It makes non-sensen to ditch it.
Because, the hierarchy I described in the commit log is *one* use case
of the selectable memcg, but not *the only one* use case of it. If you
dislike that hierarchy, I will remove it to avoid misleading you.

Even if you introduce a new layer, you still need the selectable memcg.
For example, to avoid the issue I described in [0],  you still need to
charge to the parent cgroup instead of the current cgroup.

That's why I described in the commit log that the selectable memcg is flexible.

-- 
Regards
Yafang