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Date:   Tue, 10 May 2022 15:49:59 -0700
From:   Hao Luo <haoluo@...gle.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Yosry Ahmed <yosryahmed@...gle.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Shuah Khan <shuah@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Michal Hocko <mhocko@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Greg Thelen <gthelen@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        bpf@...r.kernel.org, cgroups@...r.kernel.org
Subject: Re: [RFC PATCH bpf-next 8/9] bpf: Introduce cgroup iter

On Tue, May 10, 2022 at 3:07 PM Tejun Heo <tj@...nel.org> wrote:
>
> Hello,
>
> On Tue, May 10, 2022 at 02:12:16PM -0700, Hao Luo wrote:
> > > Is there a reason why this can't be a proper iterator which supports
> > > lseek64() to locate a specific cgroup?
> > >
> >
> > There are two reasons:
> >
> > - Bpf_iter assumes no_llseek. I haven't looked closely on why this is
> > so and whether we can add its support.
> >
> > - Second, the name 'iter' in this patch is misleading. What this patch
> > really does is reusing the functionality of dumping in bpf_iter.
> > 'Dumper' is a better name. We want to create one file in bpffs for
> > each cgroup. We are essentially just iterating a set of one single
> > element.
>
> I see. I'm just shooting in the dark without context but at least in
> principle there's no reason why cgroups wouldn't be iterable, so it might be
> something worth at least thinking about before baking in the interface.
>

Yep. Conceptually there should be no problem to iterate cgroups in the
system. It may be better to have two independent bpf objects: bpf_iter
and bpf_dumper. In our use case, we want bpf_dumper, which just
exports data out through fs interface.

Hao

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