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Date:   Fri, 20 May 2022 15:19:19 -0700
From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
To:     Yosry Ahmed <yosryahmed@google.com>
Cc:     Hao Luo <haoluo@google.com>, Tejun Heo <tj@kernel.org>,
        Yonghong Song <yhs@fb.com>,
        Alexei Starovoitov <ast@kernel.org>,
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        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        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>,
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Subject: Re: [PATCH bpf-next v1 3/5] bpf: Introduce cgroup iter
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On Fri, May 20, 2022 at 02:18:42PM -0700, Yosry Ahmed wrote:
> >
> > The userspace needs to specify the identity of the cgroup, when
> > creating bpf_iter. This identity could be cgroup id or fd. This
> > identity needs to be converted to cgroup object somewhere before
> > passing into bpf program to use.
> 
> 
> Let's sum up the discussion here, I feel like we are losing track of
> the main problem. IIUC the main concern is that cgroup_iter is not
> effectively an iterator, it rather dumps information for one cgroup. I
> like the suggestion to make it iterate cgroups by default, and an
> optional cgroup_id parameter to make it only "iterate" this one
> cgroup.

We have bpf_map iterator that walks all bpf maps.
When map iterator is parametrized with map_fd the iterator walks
all elements of that map.
cgroup iterator should have similar semantics.
When non-parameterized it will walk all cgroups and their descendent
depth first way. I believe that's what Yonghong is proposing.
When parametrized it will start from that particular cgroup and
walk all descendant of that cgroup only.
The bpf prog can stop the iteration right away with ret 1.
Maybe we can add two parameters. One -> cgroup_fd to use and another ->
the order of iteration css_for_each_descendant_pre vs _post.
wdyt?