Re: [PATCH bpf-next v9 1/5] bpf: Introduce cgroup iter

[Andrii Nakryiko]

On Tue, Aug 23, 2022 at 8:01 PM Hao Luo <haoluo@xxxxxxxxxx> wrote:
>
> Cgroup_iter is a type of bpf_iter. It walks over cgroups in four modes:
>
>  - walking a cgroup's descendants in pre-order.
>  - walking a cgroup's descendants in post-order.
>  - walking a cgroup's ancestors.
>  - process only the given cgroup.
>
> When attaching cgroup_iter, one can set a cgroup to the iter_link
> created from attaching. This cgroup is passed as a file descriptor
> or cgroup id and serves as the starting point of the walk. If no
> cgroup is specified, the starting point will be the root cgroup v2.
>
> 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, assuming PAGE_SIZE is 4kb, 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 kernel buffer size, after all data in the
> kernel buffer is consumed by user space, the subsequent read() syscall
> 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.
>
> Acked-by: Yonghong Song <yhs@xxxxxx>
> Acked-by: Tejun Heo <tj@xxxxxxxxxx>
> Signed-off-by: Hao Luo <haoluo@xxxxxxxxxx>
> ---
>  include/linux/bpf.h                           |   8 +
>  include/uapi/linux/bpf.h                      |  30 ++
>  kernel/bpf/Makefile                           |   3 +
>  kernel/bpf/cgroup_iter.c                      | 284 ++++++++++++++++++
>  tools/include/uapi/linux/bpf.h                |  30 ++
>  .../selftests/bpf/prog_tests/btf_dump.c       |   4 +-
>  6 files changed, 357 insertions(+), 2 deletions(-)
>  create mode 100644 kernel/bpf/cgroup_iter.c
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 39bd36359c1e..dc9bf5e95ebf 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -48,6 +48,7 @@ struct mem_cgroup;
>  struct module;
>  struct bpf_func_state;
>  struct ftrace_ops;
> +struct cgroup;
>
>  extern struct idr btf_idr;
>  extern spinlock_t btf_idr_lock;
> @@ -1730,7 +1731,14 @@ 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 */
> +               enum bpf_cgroup_iter_order 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 934a2a8beb87..1c4e1c583880 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -87,10 +87,29 @@ struct bpf_cgroup_storage_key {
>         __u32   attach_type;            /* program attach type (enum bpf_attach_type) */
>  };
>
> +enum bpf_cgroup_iter_order {
> +       BPF_ITER_ORDER_UNSPEC = 0,
> +       BPF_ITER_SELF_ONLY,             /* process only a single object. */
> +       BPF_ITER_DESCENDANTS_PRE,       /* walk descendants in pre-order. */
> +       BPF_ITER_DESCENDANTS_POST,      /* walk descendants in post-order. */
> +       BPF_ITER_ANCESTORS_UP,          /* walk ancestors upward. */
> +};

just skimming through this, I noticed that we have "enum
bpf_cgroup_iter_order" (good, I like) but BPF_ITER_xxx with no CGROUP
part in it (not good, don't like :). All the enumerator names have
global visibility, so it would probably be best for them to be
CGROUP-specific and roughly match the enum name itself:
BPF_CGROUP_ITER_SELF_ONLY, etc?

> +
>  union bpf_iter_link_info {
>         struct {
>                 __u32   map_fd;
>         } map;
> +       struct {
> +               enum bpf_cgroup_iter_order order;
> +
> +               /* At most one of cgroup_fd and cgroup_id can be non-zero. If
> +                * both are zero, the walk starts from the default cgroup v2
> +                * root. For walking v1 hierarchy, one should always explicitly
> +                * specify cgroup_fd.
> +                */
> +               __u32   cgroup_fd;
> +               __u64   cgroup_id;
> +       } cgroup;
>  };

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