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From:   Hao Luo <haoluo@google.com>
Date:   Fri, 20 May 2022 14:49:45 -0700
Message-ID: <CA+khW7iDDkO3h5WQixEA=nUL-tBmCTh7fMAf3iwNy98UfM-k9g@mail.gmail.com>
Subject: Re: [PATCH bpf-next v1 3/5] bpf: Introduce cgroup iter
To:     Tejun Heo <tj@kernel.org>
Cc:     Yonghong Song <yhs@fb.com>, Yosry Ahmed <yosryahmed@google.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>,
        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>,
        Stanislav Fomichev <sdf@google.com>,
        David Rientjes <rientjes@google.com>,
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        Shakeel Butt <shakeelb@google.com>,
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Hi Tejun and Yonghong,

On Fri, May 20, 2022 at 12:42 PM Hao Luo <haoluo@google.com> wrote:
>
> Hi Tejun and Yonghong,
>
> On Fri, May 20, 2022 at 9:45 AM Tejun Heo <tj@kernel.org> wrote:
> > On Fri, May 20, 2022 at 09:29:43AM -0700, Yonghong Song wrote:
> > >    <various stats interested by the user>
> > >
> > > This way, user space can easily construct the cgroup hierarchy stat like
> > >                            cpu   mem   cpu pressure   mem pressure ...
> > >    cgroup1                 ...
> > >       child1               ...
> > >         grandchild1        ...
> > >       child2               ...
> > >    cgroup 2                ...
> > >       child 3              ...
> > >         ...                ...
> > >
> > > the bpf iterator can have additional parameter like
> > > cgroup_id = ... to only call bpf program once with that
> > > cgroup_id if specified.
>
> Yep, this should work. We just need to make the cgroup_id parameter
> optional. If it is specified when creating bpf_iter_link, we print for
> that cgroup only. If it is not specified, we iterate over all cgroups.
> If I understand correctly, sounds doable.
>

Yonghong, I realized that seek() which Tejun has been calling out, can
be used to specify the target cgroup, rather than adding a new
parameter. Maybe, we can pass cgroup_id to seek() on cgroup bpf_iter,
which will instruct read() to return the corresponding cgroup's stats.
On the other hand, reading without calling seek() beforehand will
return all the cgroups.

WDYT?