Re: [PATCH bpf-next v7 4/8] bpf: Introduce cgroup iter

[Hao Luo]

On Mon, Aug 8, 2022 at 5:19 PM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Fri, Aug 5, 2022 at 2:49 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                      |  38 +++
> >  kernel/bpf/Makefile                           |   3 +
> >  kernel/bpf/cgroup_iter.c                      | 286 ++++++++++++++++++
> >  tools/include/uapi/linux/bpf.h                |  38 +++
> >  .../selftests/bpf/prog_tests/btf_dump.c       |   4 +-
> >  6 files changed, 375 insertions(+), 2 deletions(-)
> >  create mode 100644 kernel/bpf/cgroup_iter.c
> >
> > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > index 20c26aed7896..09b5c2167424 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 */
> > +               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 59a217ca2dfd..4d758b2e70d6 100644
> > --- a/include/uapi/linux/bpf.h
> > +++ b/include/uapi/linux/bpf.h
> > @@ -87,10 +87,37 @@ struct bpf_cgroup_storage_key {
> >         __u32   attach_type;            /* program attach type (enum bpf_attach_type) */
> >  };
> >
> > +enum bpf_iter_order {
> > +       BPF_ITER_ORDER_DEFAULT = 0,     /* default order. */
>
> why is this default order necessary? It just adds confusion (I had to
> look up source code to know what is default order). I might have
> missed some discussion, so if there is some very good reason, then
> please document this in commit message. But I'd rather not do some
> magical default order instead. We can set 0 to mean invalid and error
> out, or just do SELF as the very first value (and if user forgot to
> specify more fancy mode, they hopefully will quickly discover this in
> their testing).
>

PRE/POST/UP are tree-specific orders. SELF applies on all iters and
yields only a single object. How does task_iter express a non-self
order? By non-self, I mean something like "I don't care about the
order, just scan _all_ the objects". And this "don't care" order, IMO,
may be the common case. I don't think everyone cares about walking
order for tasks. The DEFAULT is intentionally put at the first value,
so that if users don't care about order, they don't have to specify
this field.

If that sounds valid, maybe using "UNSPEC" instead of "DEFAULT" is better?

> > +       BPF_ITER_SELF,                  /* 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. */
> > +};
> > +
>
> This is a somewhat pedantic nit, so feel free to ignore completely,
> but don't DESCENDANTS_{PRE,POST} and ANCESTORS_UP also include "self"?
> As it is right now, BPF_ITER_SELF name might be read as implying that
> DESCENDANTS and ANCESTORS order don't include self. So I don't know,
> maybe BPF_ITER_SELF_ONLY would be a bit clearer?
>

No problem with that. I can update it in the next version.

>
> >  union bpf_iter_link_info {
> >         struct {
> >                 __u32   map_fd;
> >         } map;
> > +       struct {
> > +               /* Valid values include:
> > +                *  - BPF_ITER_ORDER_DEFAULT
> > +                *  - BPF_ITER_SELF
> > +                *  - BPF_ITER_DESCENDANTS_PRE
> > +                *  - BPF_ITER_DESCENDANTS_POST
> > +                *  - BPF_ITER_ANCESTORS_UP
> > +                * for cgroup_iter, DEFAULT is equivalent to DESCENDANTS_PRE.
> > +                */
> > +               __u32   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;
> >  };
> >
> >  /* BPF syscall commands, see bpf(2) man-page for more details. */
> > @@ -6134,11 +6161,22 @@ struct bpf_link_info {
> >                 struct {
> >                         __aligned_u64 target_name; /* in/out: target_name buffer ptr */
> >                         __u32 target_name_len;     /* in/out: target_name buffer len */
> > +
> > +                       /* If the iter specific field is 32 bits, it can be put
> > +                        * in the first or second union. Otherwise it should be
> > +                        * put in the second union.
> > +                        */
> >                         union {
> >                                 struct {
> >                                         __u32 map_id;
> >                                 } map;
> >                         };
> > +                       union {
> > +                               struct {
> > +                                       __u64 cgroup_id;
> > +                                       __u32 order;
> > +                               } cgroup;
> > +                       };
> >                 } iter;
>
> But other than above, I like how UAPI looks like, thanks!
>
> [...]
>
> > + *
> > + * For walking descendants, cgroup_iter can walk in either pre-order or
> > + * post-order. For walking ancestors, the iter walks up from a cgroup to
> > + * the root.
> > + *
> > + * The iter program can terminate the walk early by returning 1. Walk
> > + * continues if prog returns 0.
> > + *
> > + * The prog can check (seq->num == 0) to determine whether this is
> > + * the first element. The prog may also be passed a NULL cgroup,
> > + * which means the walk has completed and the prog has a chance to
> > + * do post-processing, such as outputing an epilogue.
>
> typo: outputting
>

Thanks for catching. Will fix.

> > + *
> > + * Note: the iter_prog 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.
> > + */
> > +
>
> [...]
>
> > +
> > +static void bpf_iter_cgroup_show_fdinfo(const struct bpf_iter_aux_info *aux,
> > +                                       struct seq_file *seq)
> > +{
> > +       char *buf;
> > +
> > +       buf = kzalloc(PATH_MAX, GFP_KERNEL);
> > +       if (!buf) {
> > +               seq_puts(seq, "cgroup_path:\t<unknown>\n");
> > +               goto show_order;
> > +       }
> > +
> > +       /* If cgroup_path_ns() fails, buf will be an empty string, cgroup_path
> > +        * will print nothing.
> > +        *
> > +        * Path is in the calling process's cgroup namespace.
> > +        */
> > +       cgroup_path_ns(aux->cgroup.start, buf, PATH_MAX,
> > +                      current->nsproxy->cgroup_ns);
> > +       seq_printf(seq, "cgroup_path:\t%s\n", buf);
> > +       kfree(buf);
> > +
> > +show_order:
> > +       if (aux->cgroup.order == BPF_ITER_DESCENDANTS_PRE)
> > +               seq_puts(seq, "order: pre\n");
> > +       else if (aux->cgroup.order == BPF_ITER_DESCENDANTS_POST)
> > +               seq_puts(seq, "order: post\n");
> > +       else if (aux->cgroup.order == BPF_ITER_ANCESTORS_UP)
> > +               seq_puts(seq, "order: up\n");
> > +       else /* BPF_ITER_SELF */
> > +               seq_puts(seq, "order: self\n");
>
> should we output "descendants_pre", "descendants_post", "ancestors_up"
> and "self" to match enum names more uniformly? We had similar
> discussion when Daniel Mueller was doing some clean up in bpftool and
> public's opinion was that uniform and consistent mapping between
> kernel enum and it's string representation is more valuable than
> shortness of the string.
>

I feel this is very nit, but can update in the next version. On a
second thought, I think, specifying "descendants", "ancestors" and
"self" are probably slightly better. Because doing so, people know
it's a tree when reading iter link info.

> > +}
> > +
>
> [...]