netdev - Re: [PATCH bpf-next v3 00/13] bpf: Introduce selectable memcg for bpf map

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Message-ID: <CAADnVQJHtY0m5TnFxnn8w3xBUfAGNzbf1HmS3ChqcLJadEYJFg@mail.gmail.com>
Date:   Wed, 7 Sep 2022 19:43:33 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Yafang Shao <laoar.shao@...il.com>
Cc:     Roman Gushchin <roman.gushchin@...ux.dev>,
        Tejun Heo <tj@...nel.org>, 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>, Jiri Olsa <jolsa@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Zefan Li <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 v3 00/13] bpf: Introduce selectable memcg for bpf map

On Wed, Sep 7, 2022 at 7:37 PM Yafang Shao <laoar.shao@...il.com> wrote:
>
> On Thu, Sep 8, 2022 at 6:29 AM Roman Gushchin <roman.gushchin@...ux.dev> wrote:
> >
> > On Wed, Sep 07, 2022 at 05:43:31AM -1000, Tejun Heo wrote:
> > > Hello,
> > >
> > > On Fri, Sep 02, 2022 at 02:29:50AM +0000, Yafang Shao wrote:
> > > ...
> > > > This patchset tries to resolve the above two issues by introducing a
> > > > selectable memcg to limit the bpf memory. Currently we only allow to
> > > > select its ancestor to avoid breaking the memcg hierarchy further.
> > > > Possible use cases of the selectable memcg as follows,
> > >
> > > As discussed in the following thread, there are clear downsides to an
> > > interface which requires the users to specify the cgroups directly.
> > >
> > >  https://lkml.kernel.org/r/YwNold0GMOappUxc@slm.duckdns.org
> > >
> > > So, I don't really think this is an interface we wanna go for. I was hoping
> > > to hear more from memcg folks in the above thread. Maybe ping them in that
> > > thread and continue there?
> >
>
> Hi Roman,
>
> > As I said previously, I don't like it, because it's an attempt to solve a non
> > bpf-specific problem in a bpf-specific way.
> >
>
> Why do you still insist that bpf_map->memcg is not a bpf-specific
> issue after so many discussions?
> Do you charge the bpf-map's memory the same way as you charge the page
> caches or slabs ?
> No, you don't. You charge it in a bpf-specific way.
>
> > Yes, memory cgroups are not great for accounting of shared resources, it's well
> > known. This patchset looks like an attempt to "fix" it specifically for bpf maps
> > in a particular cgroup setup. Honestly, I don't think it's worth the added
> > complexity. Especially because a similar behaviour can be achieved simple
> > by placing the task which creates the map into the desired cgroup.
>
> Are you serious ?
> Have you ever read the cgroup doc? Which clearly describe the "No
> Internal Process Constraint".[1]
> Obviously you can't place the task in the desired cgroup, i.e. the parent memcg.
>
> [1] https://www.kernel.org/doc/Documentation/cgroup-v2.txt
>
> > Beatiful? Not. Neither is the proposed solution.
> >
>
> Is it really hard to admit a fault?

Yafang,

This attitude won't get you anywhere.

Selecting memcg by fd is no go.
You need to work with the community to figure out a solution
acceptable to maintainers of relevant subsystems.