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

[Hao Luo]

On Fri, May 20, 2022 at 5:58 PM Yonghong Song <yhs@xxxxxx> wrote:
> On 5/20/22 2:49 PM, Hao Luo wrote:
> > 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.
>
> Currently, seek is not supported for bpf_iter.
>
> const struct file_operations bpf_iter_fops = {
>          .open           = iter_open,
>          .llseek         = no_llseek,
>          .read           = bpf_seq_read,
>          .release        = iter_release,
> };
>
> But if seek() works, I don't mind to remove this restriction.
> But not sure what to seek. Do you mean to provide a cgroup_fd/cgroup_id
> as the seek() syscall parameter? This may work.

Yes, passing a cgroup_id as the seek() syscall parameter was what I meant.

Tejun previously requested us to support seek() for a proper iterator.
Since Alexei has a nice solution that all of us have ack'ed, I am not
sure whether we still want to add seek() for bpf_iter as Tejun asked.
I guess not.

>
> But considering we have parameterized example (map_fd) and
> in the future, we may have other parameterized bpf_iter
> (e.g., for one task). Maybe parameter-based approach is better.
>

Acknowledged.

> >
> > WDYT?