linux-kernel - Re: [RFC PATCH bpf-next 8/9] bpf: Introduce cgroup iter

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Message-ID: <CA+khW7girnNwap1ABN1a4XuvkEEnmkztTV+fsuC3MsxNeB08Yg@mail.gmail.com>
Date:   Tue, 10 May 2022 14:12:16 -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

Hello Tejun,

On Tue, May 10, 2022 at 11:54 AM Tejun Heo <tj@...nel.org> wrote:
>
> Hello,
>
> On Tue, May 10, 2022 at 12:18:06AM +0000, Yosry Ahmed wrote:
> > From: Hao Luo <haoluo@...gle.com>
> >
> > Introduce a new type of iter prog: cgroup. Unlike other bpf_iter, this
> > iter doesn't iterate a set of kernel objects. Instead, it is supposed to
> > be parameterized by a cgroup id and prints only that cgroup. So one
> > needs to specify a target cgroup id when attaching this iter. The target
> > cgroup's state can be read out via a link of this iter.
>
> 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.

> Thanks.

>
> --
> tejun