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

[Hao Luo]

Hi Tejun and Yonghong,

On Fri, May 20, 2022 at 12:42 PM Hao Luo <haoluo@xxxxxxxxxx> wrote:
>
> Hi Tejun and Yonghong,
>
> On Fri, May 20, 2022 at 9:45 AM Tejun Heo <tj@xxxxxxxxxx> 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?