Re: [PATCH bpf-next v2 06/11] bpf: Introduce BPF_MAP_TYPE_STRUCT_OPS

[Martin Lau <kafai@fb.com>]

On Mon, Dec 23, 2019 at 01:44:29PM -0800, Andrii Nakryiko wrote:
> On Mon, Dec 23, 2019 at 11:58 AM Yonghong Song <yhs@fb.com> wrote:
> >
> >
> >
> > On 12/20/19 10:26 PM, Martin KaFai Lau wrote:
> > > The patch introduces BPF_MAP_TYPE_STRUCT_OPS.  The map value
> > > is a kernel struct with its func ptr implemented in bpf prog.
> > > This new map is the interface to register/unregister/introspect
> > > a bpf implemented kernel struct.
> > >
> > > The kernel struct is actually embedded inside another new struct
> > > (or called the "value" struct in the code).  For example,
> > > "struct tcp_congestion_ops" is embbeded in:
> > > struct bpf_struct_ops_tcp_congestion_ops {
> > >       refcount_t refcnt;
> > >       enum bpf_struct_ops_state state;
> > >       struct tcp_congestion_ops data;  /* <-- kernel subsystem struct here */
> > > }
> > > The map value is "struct bpf_struct_ops_tcp_congestion_ops".
> > > The "bpftool map dump" will then be able to show the
> > > state ("inuse"/"tobefree") and the number of subsystem's refcnt (e.g.
> > > number of tcp_sock in the tcp_congestion_ops case).  This "value" struct
> > > is created automatically by a macro.  Having a separate "value" struct
> > > will also make extending "struct bpf_struct_ops_XYZ" easier (e.g. adding
> > > "void (*init)(void)" to "struct bpf_struct_ops_XYZ" to do some
> > > initialization works before registering the struct_ops to the kernel
> > > subsystem).  The libbpf will take care of finding and populating the
> > > "struct bpf_struct_ops_XYZ" from "struct XYZ".
> > >
> > > Register a struct_ops to a kernel subsystem:
> > > 1. Load all needed BPF_PROG_TYPE_STRUCT_OPS prog(s)
> > > 2. Create a BPF_MAP_TYPE_STRUCT_OPS with attr->btf_vmlinux_value_type_id
> > >     set to the btf id "struct bpf_struct_ops_tcp_congestion_ops" of the
> > >     running kernel.
> > >     Instead of reusing the attr->btf_value_type_id,
> > >     btf_vmlinux_value_type_id s added such that attr->btf_fd can still be
> > >     used as the "user" btf which could store other useful sysadmin/debug
> > >     info that may be introduced in the furture,
> > >     e.g. creation-date/compiler-details/map-creator...etc.
> > > 3. Create a "struct bpf_struct_ops_tcp_congestion_ops" object as described
> > >     in the running kernel btf.  Populate the value of this object.
> > >     The function ptr should be populated with the prog fds.
> > > 4. Call BPF_MAP_UPDATE with the object created in (3) as
> > >     the map value.  The key is always "0".
> > >
> > > During BPF_MAP_UPDATE, the code that saves the kernel-func-ptr's
> > > args as an array of u64 is generated.  BPF_MAP_UPDATE also allows
> > > the specific struct_ops to do some final checks in "st_ops->init_member()"
> > > (e.g. ensure all mandatory func ptrs are implemented).
> > > If everything looks good, it will register this kernel struct
> > > to the kernel subsystem.  The map will not allow further update
> > > from this point.
> > >
> > > Unregister a struct_ops from the kernel subsystem:
> > > BPF_MAP_DELETE with key "0".
> > >
> > > Introspect a struct_ops:
> > > BPF_MAP_LOOKUP_ELEM with key "0".  The map value returned will
> > > have the prog _id_ populated as the func ptr.
> > >
> > > The map value state (enum bpf_struct_ops_state) will transit from:
> > > INIT (map created) =>
> > > INUSE (map updated, i.e. reg) =>
> > > TOBEFREE (map value deleted, i.e. unreg)
> > >
> > > The kernel subsystem needs to call bpf_struct_ops_get() and
> > > bpf_struct_ops_put() to manage the "refcnt" in the
> > > "struct bpf_struct_ops_XYZ".  This patch uses a separate refcnt
> > > for the purose of tracking the subsystem usage.  Another approach
> > > is to reuse the map->refcnt and then "show" (i.e. during map_lookup)
> > > the subsystem's usage by doing map->refcnt - map->usercnt to filter out
> > > the map-fd/pinned-map usage.  However, that will also tie down the
> > > future semantics of map->refcnt and map->usercnt.
> > >
> > > The very first subsystem's refcnt (during reg()) holds one
> > > count to map->refcnt.  When the very last subsystem's refcnt
> > > is gone, it will also release the map->refcnt.  All bpf_prog will be
> > > freed when the map->refcnt reaches 0 (i.e. during map_free()).
> > >
> > > Here is how the bpftool map command will look like:
> > > [root@arch-fb-vm1 bpf]# bpftool map show
> > > 6: struct_ops  name dctcp  flags 0x0
> > >       key 4B  value 256B  max_entries 1  memlock 4096B
> > >       btf_id 6
> > > [root@arch-fb-vm1 bpf]# bpftool map dump id 6
> > > [{
> > >          "value": {
> > >              "refcnt": {
> > >                  "refs": {
> > >                      "counter": 1
> > >                  }
> > >              },
> > >              "state": 1,
> >
> > The bpftool dump with "state" 1 is a little bit cryptic.
> > Since this is common for all struct_ops maps, can we
> > make it explicit, e.g., as enum values, like INIT/INUSE/TOBEFREE?
> 
> This can (and probably should) be done generically in bpftool for any
> field of enum type. Not blocking this patch set, though.
> 
> >
> > >              "data": {
> > >                  "list": {
> > >                      "next": 0,
> > >                      "prev": 0
> > >                  },
> > >                  "key": 0,
> > >                  "flags": 2,
> > >                  "init": 24,
> > >                  "release": 0,
> > >                  "ssthresh": 25,
> > >                  "cong_avoid": 30,
> > >                  "set_state": 27,
> > >                  "cwnd_event": 28,
> > >                  "in_ack_event": 26,
> > >                  "undo_cwnd": 29,
> > >                  "pkts_acked": 0,
> > >                  "min_tso_segs": 0,
> > >                  "sndbuf_expand": 0,
> > >                  "cong_control": 0,
> > >                  "get_info": 0,
> > >                  "name": [98,112,102,95,100,99,116,99,112,0,0,0,0,0,0,0
> > >                  ],
> 
> Same here, bpftool should be smart enough to figure out that this is a
> string, not just an array of bytes.

Agree on both above that bpftool can print better strings.
Those are generic improvements to bpftool and not specific
to a particular map type.

> 
> > >                  "owner": 0
> > >              }
> > >          }
> > >      }
> > > ]
> > >
> 
> [...]