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Sender: Tejun Heo <htejun@gmail.com>
Date:   Thu, 19 May 2022 21:41:23 -1000
From:   Tejun Heo <tj@kernel.org>
To:     Yosry Ahmed <yosryahmed@google.com>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
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        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>, Hao Luo <haoluo@google.com>,
        Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Shuah Khan <shuah@kernel.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Michal Hocko <mhocko@kernel.org>,
        Stanislav Fomichev <sdf@google.com>,
        David Rientjes <rientjes@google.com>,
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        Shakeel Butt <shakeelb@google.com>,
        linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
        bpf@vger.kernel.org, cgroups@vger.kernel.org
Subject: Re: [PATCH bpf-next v1 3/5] bpf: Introduce cgroup iter
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 <20220520012133.1217211-4-yosryahmed@google.com>
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On Fri, May 20, 2022 at 01:21:31AM +0000, Yosry Ahmed wrote:
> From: Hao Luo <haoluo@google.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.
> 
> Signed-off-by: Hao Luo <haoluo@google.com>
> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>

This could be me not understanding why it's structured this way but it keeps
bothering me that this is adding a cgroup iterator which doesn't iterate
cgroups. If all that's needed is extracting information from a specific
cgroup, why does this need to be an iterator? e.g. why can't I use
BPF_PROG_TEST_RUN which looks up the cgroup with the provided ID, flushes
rstat, retrieves whatever information necessary and returns that as the
result?

Thanks.

-- 
tejun