Re: [PATCH bpf-next v1 3/5] bpf: Introduce cgroup iter

[Tejun Heo]

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.

>    <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?

Thanks.

-- 
tejun