Date:   Mon, 1 Aug 2022 20:42:47 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Hao Luo <haoluo@...gle.com>, Tejun Heo <tj@...nel.org>,
        Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Shuah Khan <shuah@...nel.org>,
        Michal Hocko <mhocko@...nel.org>,
        KP Singh <kpsingh@...nel.org>,
        Benjamin Tissoires <benjamin.tissoires@...hat.com>,
        John Fastabend <john.fastabend@...il.com>,
        Michal Koutný <mkoutny@...e.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        David Rientjes <rientjes@...gle.com>,
        Stanislav Fomichev <sdf@...gle.com>,
        Greg Thelen <gthelen@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        bpf@...r.kernel.org, cgroups@...r.kernel.org
Subject: Re: [PATCH bpf-next v5 4/8] bpf: Introduce cgroup iter

On Fri, Jul 22, 2022 at 10:48 AM Yosry Ahmed <yosryahmed@...gle.com> wrote:
>
> From: Hao Luo <haoluo@...gle.com>
>
> Cgroup_iter is a type of bpf_iter. It walks over cgroups in three modes:
>
>  - walking a cgroup's descendants in pre-order.
>  - walking a cgroup's descendants in post-order.
>  - walking a cgroup's ancestors.
>
> When attaching cgroup_iter, one can set a cgroup to the iter_link
> created from attaching. This cgroup is passed as a file descriptor and
> serves as the starting point of the walk. If no cgroup is specified,
> the starting point will be the root cgroup.
>
> For walking descendants, one can specify the order: either pre-order or
> post-order. For walking ancestors, the walk starts at the specified
> cgroup and ends at the root.
>
> One can also terminate the walk early by returning 1 from the iter
> program.
>
> Note that because walking cgroup hierarchy holds cgroup_mutex, the iter
> program is called with cgroup_mutex held.
>
> Currently only one session is supported, which means, depending on the
> volume of data bpf program intends to send to user space, the number
> of cgroups that can be walked is limited. For example, given the current
> buffer size is 8 * PAGE_SIZE, if the program sends 64B data for each
> cgroup, the total number of cgroups that can be walked is 512. This is
> a limitation of cgroup_iter. If the output data is larger than the
> buffer size, the second read() will signal EOPNOTSUPP. In order to work
> around, the user may have to update their program to reduce the volume
> of data sent to output. For example, skip some uninteresting cgroups.
> In future, we may extend bpf_iter flags to allow customizing buffer
> size.
>
> Signed-off-by: Hao Luo <haoluo@...gle.com>
> Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>
> Acked-by: Yonghong Song <yhs@...com>
> ---
>  include/linux/bpf.h                           |   8 +
>  include/uapi/linux/bpf.h                      |  30 +++
>  kernel/bpf/Makefile                           |   3 +
>  kernel/bpf/cgroup_iter.c                      | 252 ++++++++++++++++++
>  tools/include/uapi/linux/bpf.h                |  30 +++
>  .../selftests/bpf/prog_tests/btf_dump.c       |   4 +-
>  6 files changed, 325 insertions(+), 2 deletions(-)
>  create mode 100644 kernel/bpf/cgroup_iter.c
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index a97751d845c9..9061618fe929 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -47,6 +47,7 @@ struct kobject;
>  struct mem_cgroup;
>  struct module;
>  struct bpf_func_state;
> +struct cgroup;
>
>  extern struct idr btf_idr;
>  extern spinlock_t btf_idr_lock;
> @@ -1717,7 +1718,14 @@ int bpf_obj_get_user(const char __user *pathname, int flags);
>         int __init bpf_iter_ ## target(args) { return 0; }
>
>  struct bpf_iter_aux_info {
> +       /* for map_elem iter */
>         struct bpf_map *map;
> +
> +       /* for cgroup iter */
> +       struct {
> +               struct cgroup *start; /* starting cgroup */
> +               int order;
> +       } cgroup;
>  };
>
>  typedef int (*bpf_iter_attach_target_t)(struct bpf_prog *prog,
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index ffcbf79a556b..fe50c2489350 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -87,10 +87,30 @@ struct bpf_cgroup_storage_key {
>         __u32   attach_type;            /* program attach type (enum bpf_attach_type) */
>  };
>
> +enum bpf_iter_cgroup_traversal_order {
> +       BPF_ITER_CGROUP_PRE = 0,        /* pre-order traversal */
> +       BPF_ITER_CGROUP_POST,           /* post-order traversal */
> +       BPF_ITER_CGROUP_PARENT_UP,      /* traversal of ancestors up to the root */

I've just put up my arguments why it's a good idea to also support a
"trivial" mode of only traversing specified cgroup and no descendants
or parents. Please see [0]. I think the same applies here, especially
considering that it seems like a good idea to support
task/task_vma/task_files iteration within a cgroup. So depending on
how successful I am in arguing for supporting task iterator with
target cgroup, I think we should reuse *exactly* this
bpf_iter_cgroup_traversal_order and how we specify cgroup (FD or ID,
see some more below) *as is* in task iterators as well. In the latter
case, having an ability to say "iterate task for only given cgroup" is
very useful, and for such mode all the PRE/POST/PARENT_UP is just an
unnecessary nuisance.

So please consider also adding and supporting BPF_ITER_CGROUP_SELF (or
whatever naming makes most sense).


Some more naming nits. I find BPF_ITER_CGROUP_PRE and
BPF_ITER_CGROUP_POST a bit confusing. Even internally in kernel we
have css_next_descendant_pre/css_next_descendant_post, so why not
reflect the fact that we are going to iterate descendants:
BPF_ITER_CGROUP_DESCENDANTS_{PRE,POST}. And now that we use
"descendants" terminology, PARENT_UP should be ANCESTORS. ANCESTORS_UP
probably is fine, but seems a bit redundant (unless we consider a
somewhat weird ANCESTORS_DOWN, where we find the furthest parent and
then descend through preceding parents until we reach specified
cgroup; seems a bit exotic).

  [0] https://lore.kernel.org/bpf/f92e20e9961963e20766e290ee6668edd4bacf06.camel@fb.com/T/#m5ce50632aa550dd87a99241efb168cbcde1ee98f

> +};
> +
>  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?

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

[...]

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