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

[Yonghong Song <yhs@fb.com>]

On 7/22/22 1:33 PM, Hao Luo wrote:
> On Fri, Jul 22, 2022 at 11:36 AM Kumar Kartikeya Dwivedi
> <memxor@gmail.com> wrote:
>>
>> On Fri, 22 Jul 2022 at 19:52, Yosry Ahmed <yosryahmed@google.com> wrote:
>>>
>>> From: Hao Luo <haoluo@google.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@google.com>
>>> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
>>> Acked-by: Yonghong Song <yhs@fb.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 */
>>> +};
>>> +
>>>   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;
>>> +               __u32   traversal_order;
>>> +       } cgroup;
>>>   };
>>>
>>>   /* BPF syscall commands, see bpf(2) man-page for more details. */
>>> @@ -6136,6 +6156,16 @@ struct bpf_link_info {
>>>                                          __u32 map_id;
>>>                                  } map;
>>>                          };
>>> +                       union {
>>> +                               struct {
>>> +                                       __u64 cgroup_id;
>>> +                                       __u32 traversal_order;
>>> +                               } cgroup;
>>> +                       };
>>> +                       /* For new iters, if the first field is larger than __u32,
>>> +                        * the struct should be added in the second union. Otherwise,
>>> +                        * it will create holes before map_id, breaking uapi.
>>> +                        */
>>>                  } iter;
>>>                  struct  {
>>>                          __u32 netns_ino;
>>> diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile
>>> index 057ba8e01e70..00e05b69a4df 100644
>>> --- a/kernel/bpf/Makefile
>>> +++ b/kernel/bpf/Makefile
>>> @@ -24,6 +24,9 @@ endif
>>>   ifeq ($(CONFIG_PERF_EVENTS),y)
>>>   obj-$(CONFIG_BPF_SYSCALL) += stackmap.o
>>>   endif
>>> +ifeq ($(CONFIG_CGROUPS),y)
>>> +obj-$(CONFIG_BPF_SYSCALL) += cgroup_iter.o
>>> +endif
>>>   obj-$(CONFIG_CGROUP_BPF) += cgroup.o
>>>   ifeq ($(CONFIG_INET),y)
>>>   obj-$(CONFIG_BPF_SYSCALL) += reuseport_array.o
>>> diff --git a/kernel/bpf/cgroup_iter.c b/kernel/bpf/cgroup_iter.c
>>> new file mode 100644
>>> index 000000000000..1027faed0b8b
>>> --- /dev/null
>>> +++ b/kernel/bpf/cgroup_iter.c
>>> @@ -0,0 +1,252 @@
>>> +// SPDX-License-Identifier: GPL-2.0-only
>>> +/* Copyright (c) 2022 Google */
>>> +#include <linux/bpf.h>
>>> +#include <linux/btf_ids.h>
>>> +#include <linux/cgroup.h>
>>> +#include <linux/kernel.h>
>>> +#include <linux/seq_file.h>
>>> +
>>> +#include "../cgroup/cgroup-internal.h"  /* cgroup_mutex and cgroup_is_dead */
>>> +
>>> +/* cgroup_iter provides three modes of traversal to the cgroup hierarchy.
>>> + *
>>> + *  1. Walk the descendants of a cgroup in pre-order.
>>> + *  2. Walk the descendants of a cgroup in post-order.
>>> + *  2. Walk the ancestors of a cgroup.
>>> + *
>>> + * 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.
>>> + *
>>> + * 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, 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.
>>> + */
>>> +
>>> +struct bpf_iter__cgroup {
>>> +       __bpf_md_ptr(struct bpf_iter_meta *, meta);
>>> +       __bpf_md_ptr(struct cgroup *, cgroup);
>>> +};
>>> +
>>> +struct cgroup_iter_priv {
>>> +       struct cgroup_subsys_state *start_css;
>>> +       bool terminate;
>>> +       int order;
>>> +};
>>> +
>>> +static void *cgroup_iter_seq_start(struct seq_file *seq, loff_t *pos)
>>> +{
>>> +       struct cgroup_iter_priv *p = seq->private;
>>> +
>>> +       mutex_lock(&cgroup_mutex);
>>> +
>>> +       /* cgroup_iter doesn't support read across multiple sessions. */
>>> +       if (*pos > 0)
>>> +               return ERR_PTR(-EOPNOTSUPP);
>>> +
>>> +       ++*pos;
>>> +       p->terminate = false;
>>> +       if (p->order == BPF_ITER_CGROUP_PRE)
>>> +               return css_next_descendant_pre(NULL, p->start_css);
>>> +       else if (p->order == BPF_ITER_CGROUP_POST)
>>> +               return css_next_descendant_post(NULL, p->start_css);
>>> +       else /* BPF_ITER_CGROUP_PARENT_UP */
>>> +               return p->start_css;
>>> +}
>>> +
>>> +static int __cgroup_iter_seq_show(struct seq_file *seq,
>>> +                                 struct cgroup_subsys_state *css, int in_stop);
>>> +
>>> +static void cgroup_iter_seq_stop(struct seq_file *seq, void *v)
>>> +{
>>> +       /* pass NULL to the prog for post-processing */
>>> +       if (!v)
>>> +               __cgroup_iter_seq_show(seq, NULL, true);
>>> +       mutex_unlock(&cgroup_mutex);
>>
>> I'm just curious, but would it be a good optimization (maybe in a
>> follow up) to move this mutex_unlock before the check on v? That
>> allows you to store/buffer some info you want to print as a compressed
>> struct in a map, then write the full text to the seq_file outside the
>> cgroup_mutex lock in the post-processing invocation.
>>
>> It probably also allows you to walk the whole hierarchy, if one
>> doesn't want to run into seq_file buffer limit (or it can decide what
>> to print within the limit in the post processing invocation), or it
>> can use some out of band way (ringbuf, hashmap, etc.) to send the data
>> to userspace. But all of this can happen without holding cgroup_mutex
>> lock.
> 
> Thanks Kumar.
> 
> It sounds like an idea, but the key thing is not about moving
> cgroup_mutex unlock before the check IMHO. The user can achieve
> compression using the current infra. Compression could actually be
> done in the bpf program. user can define and output binary content and
> implement a userspace library to parse/decompress when reading out the
> data.

Right mutex_unlock() can be moved to the beginning of the
function since the cgroup is not used in
   __cgroup_iter_seq_show(seq, NULL, true)