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

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Message-ID: <YxkVq4S1Eoa4edjZ@P9FQF9L96D.corp.robot.car>
Date:   Wed, 7 Sep 2022 15:28:53 -0700
From:   Roman Gushchin <roman.gushchin@...ux.dev>
To:     Tejun Heo <tj@...nel.org>
Cc:     Yafang Shao <laoar.shao@...il.com>, ast@...nel.org,
        daniel@...earbox.net, andrii@...nel.org, kafai@...com,
        songliubraving@...com, yhs@...com, john.fastabend@...il.com,
        kpsingh@...nel.org, sdf@...gle.com, haoluo@...gle.com,
        jolsa@...nel.org, hannes@...xchg.org, mhocko@...nel.org,
        shakeelb@...gle.com, songmuchun@...edance.com,
        akpm@...ux-foundation.org, lizefan.x@...edance.com,
        cgroups@...r.kernel.org, netdev@...r.kernel.org,
        bpf@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH bpf-next v3 00/13] bpf: Introduce selectable memcg for
 bpf map

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?

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.

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.
Beatiful? Not. Neither is the proposed solution.

Thanks!