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SubjectRe: [PATCH bpf-next v1 3/5] bpf: Introduce cgroup iter
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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.
>

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