Re: [RFC PATCH bpf-next 8/9] bpf: Introduce cgroup iter

[Hao Luo]

Hello Tejun,

On Tue, May 10, 2022 at 11:54 AM Tejun Heo <tj@xxxxxxxxxx> wrote:
>
> Hello,
>
> On Tue, May 10, 2022 at 12:18:06AM +0000, Yosry Ahmed wrote:
> > From: Hao Luo <haoluo@xxxxxxxxxx>
> >
> > Introduce a new type of iter prog: cgroup. Unlike other bpf_iter, this
> > iter doesn't iterate a set of kernel objects. Instead, it is supposed to
> > be parameterized by a cgroup id and prints only that cgroup. So one
> > needs to specify a target cgroup id when attaching this iter. The target
> > cgroup's state can be read out via a link of this iter.
>
> Is there a reason why this can't be a proper iterator which supports
> lseek64() to locate a specific cgroup?
>

There are two reasons:

- Bpf_iter assumes no_llseek. I haven't looked closely on why this is
so and whether we can add its support.

- Second, the name 'iter' in this patch is misleading. What this patch
really does is reusing the functionality of dumping in bpf_iter.
'Dumper' is a better name. We want to create one file in bpffs for
each cgroup. We are essentially just iterating a set of one single
element.

> Thanks.

>
> --
> tejun