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

[Yonghong Song]

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.