Re: [PATCH bpf-next v5 4/8] bpf: Introduce cgroup iter

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On Wed, Aug 3, 2022 at 1:40 PM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Wed, Aug 3, 2022 at 1:30 PM Hao Luo <haoluo@xxxxxxxxxx> wrote:
> >
> > On Tue, Aug 2, 2022 at 3:50 PM Andrii Nakryiko
> > <andrii.nakryiko@xxxxxxxxx> wrote:
> > >
> > > On Tue, Aug 2, 2022 at 3:27 PM Hao Luo <haoluo@xxxxxxxxxx> wrote:
> > > >
> > > > On Mon, Aug 1, 2022 at 8:43 PM Andrii Nakryiko
> > > > <andrii.nakryiko@xxxxxxxxx> wrote:
> > > > >
> > > > > On Fri, Jul 22, 2022 at 10:48 AM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
[...]
> > > > > > +};
> > > > > > +
> > > > > >  union bpf_iter_link_info {
> > > > > >         struct {
> > > > > >                 __u32   map_fd;
> > > > > >         } map;
> > > > > > +
> > > > > > +       /* cgroup_iter walks either the live descendants of a cgroup subtree, or the
> > > > > > +        * ancestors of a given cgroup.
> > > > > > +        */
> > > > > > +       struct {
> > > > > > +               /* Cgroup file descriptor. This is root of the subtree if walking
> > > > > > +                * descendants; it's the starting cgroup if walking the ancestors.
> > > > > > +                * If it is left 0, the traversal starts from the default cgroup v2
> > > > > > +                * root. For walking v1 hierarchy, one should always explicitly
> > > > > > +                * specify the cgroup_fd.
> > > > > > +                */
> > > > > > +               __u32   cgroup_fd;
> > > > >
> > > > > Now, similar to what I argued in regard of pidfd vs pid, I think the
> > > > > same applied to cgroup_fd vs cgroup_id. Why can't we support both?
> > > > > cgroup_fd has some benefits, but cgroup_id is nice due to simplicity
> > > > > and not having to open/close/keep extra FDs (which can add up if we
> > > > > want to periodically query something about a large set of cgroups).
> > > > > Please see my arguments from [0] above.
> > > > >
> > > > > Thoughts?
> > > > >
> > > >
> > > > We can support both, it's a good idea IMO. But what exactly is the
> > > > interface going to look like? Can you be more specific about that?
> > > > Below is something I tried based on your description.
> > > >
> > > > @@ -91,6 +91,18 @@ union bpf_iter_link_info {
> > > >         struct {
> > > >                 __u32   map_fd;
> > > >         } map;
> > > > +       struct {
> > > > +               /* PRE/POST/UP/SELF */
> > > > +               __u32 order;
> > > > +               struct {
> > > > +                       __u32 cgroup_fd;
> > > > +                       __u64 cgroup_id;
> > > > +               } cgroup;
> > > > +               struct {
> > > > +                       __u32 pid_fd;
> > > > +                       __u64 pid;
> > > > +               } task;
> > > > +       };
> > > >  };
> > > >
> > >
> > > So I wouldn't combine task and cgroup definition together, let's keep
> > > them independent.
> > >
> > > then for cgroup we can do something like:
> > >
> > > struct {
> > >     __u32 order;
> > >     __u32 cgroup_fd; /* cgroup_fd ^ cgroup_id, exactly one can be non-zero */
> > >     __u32 cgroup_id;
> > > } cgroup
> > >
> > > Similar idea with task, but it's a bit more complicated because there
> > > we have target that can be pid, pidfd, or cgroup (cgroup_fd and
> > > cgroup_id). I haven't put much thought into the best representation,
> > > though.
> > >
> >
> > The cgroup part sounds good to me. For the full picture, how about
> > this? I'm just trying  a prototype, hoping that it can help people to
> > get a clear picture.
> >
> > union bpf_iter_link_info {
> >           struct {
> >                   __u32   map_fd;
> >           } map;
> >           struct {
> >                   __u32   order; /* PRE/POST/UP/SELF */
> >                   __u32   cgroup_fd;
> >                   __u64   cgroup_id;
> >           } cgroup;
>
> lgtm
>
> >           struct {
> >                   __u32   pid;
> >                   __u32   pid_fd;
> >                   __u64   cgroup_id;
> >                   __u32   cgroup_fd;
> >                   __u32   mode; /* SELF or others */
>
> I'd move mode to be first. I'm undecided on using 4 separate fields
> for pid/pid_fd/cgroup_{id,fd} vs a single union (or just generic "u64
> target"  and then mode can define how we should treat target --
> whether it's pid, pid_fd, cgroup ID or FD. I'm fine either way, I
> think. But for cgroup case not having to duplicate PRE/POST/UP/SELF
> for cgroup id and then for cgroup fd seems like a win. So separate
> fields might be better. It's also pretty extendable. And I'm
> personally not worried about using few more bytes in bpf_attr for
> disjoin fields like this.
>

Sounds good. Thanks for clarification. Using separate fields looks
good to me. Since we settled on the cgroup part, I will apply update
in cgroup_iter v7.


> >           } task;
> > };
> >
> > > > > > +               __u32   traversal_order;
> > > > > > +       } cgroup;
> > > > > >  };
> > > > > >
> > > > > >  /* BPF syscall commands, see bpf(2) man-page for more details. */
> > > > >
> > > > > [...]



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