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Date:   Fri, 20 May 2022 17:52:24 -0700
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Subject: Re: [PATCH bpf-next v1 3/5] bpf: Introduce cgroup iter
Content-Language: en-US
To:     Tejun Heo <tj@kernel.org>
Cc:     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>, 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>,
        Greg Thelen <gthelen@google.com>,
        Shakeel Butt <shakeelb@google.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
        Cgroups <cgroups@vger.kernel.org>
References: <20220520012133.1217211-1-yosryahmed@google.com>
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From:   Yonghong Song <yhs@fb.com>
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On 5/20/22 9:45 AM, Tejun Heo wrote:
> Hello, Yonghong.
> 
> On Fri, May 20, 2022 at 09:29:43AM -0700, Yonghong Song wrote:
>> Maybe you can have a bpf program signature like below:
>>
>> int BPF_PROG(dump_vmscan, struct bpf_iter_meta *meta, struct cgroup *cgrp,
>> struct cgroup *parent_cgrp)
>>
>> parent_cgrp is NULL when cgrp is the root cgroup.
>>
>> I would like the bpf program should send the following information to
>> user space:
>>     <parent cgroup dir name> <current cgroup dir name>
> 
> I don't think parent cgroup dir name would be sufficient to reconstruct the
> path given that multiple cgroups in different subtrees can have the same
> name. For live cgroups, userspace can find the path from id (or ino) without
> traversing anything by constructing the fhandle, open it open_by_handle_at()
> and then reading /proc/self/fd/$FD symlink -
> https://lkml.org/lkml/2020/12/2/1126. This isn't available for dead cgroups
> but I'm not sure how much that'd matter given that they aren't visible from
> userspace anyway.

passing id/ino to user space and then get directory name in userspace
should work just fine.

> 
>>     <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.
>>
>> The kernel part of cgroup_iter can call cgroup_rstat_flush()
>> before calling cgroup_iter bpf program.
>>
>> WDYT?
> 
> Would it work to just pass in @cgrp and provide a group of helpers so that
> the program can do whatever it wanna do including looking up the full path
> and passing that to userspace?

I am not super familiar with cgroup internals, I guess with cgroup + 
helpers to retrieve stats, or directly expose stats data structure
to bpf program. Either one is okay to me as long as we can get
desired results.

> 
> Thanks.
>