[PATCH bpf-next v2 0/6] Transit between BPF TCP congestion controls.

[Kui-Feng Lee <kuifeng@meta.com>]

Major changes:

 - Create bpf_links in the kernel for BPF struct_ops to register and
   unregister it.

 - Enables switching between implementations of bpf-tcp-cc under a
   name instantly by replacing the backing struct_ops map of a
   bpf_link.

Previously, BPF struct_ops didn't go off, as even when the user
program creating it was terminated, none of these ever were pinned.
For instance, the TCP congestion control subsystem indirectly
maintains a reference count on the struct_ops of any registered BPF
implemented algorithm. Thus, the algorithm won't be deactivated until
someone deliberately unregisters it.  For compatibility with other BPF
programs, bpf_links have been created to work in coordination with
struct_ops maps. This ensures that the registration and unregistration
of these respective maps is carried out at the start and end of the
bpf_link.

We also faced complications when attempting to replace an existing TCP
congestion control algorithm with a new implementation on the fly. A
struct_ops map was used to register a TCP congestion control algorithm
with a unique name.  We had to either register the alternative
implementation with a new name and move over or unregister the current
one before being able to reregistration with the same name.  To fix
this problem, we can an option to migrate the registration of the
algorithm from struct_ops maps to bpf_links. By modifying the backing
map of a bpf_link, it suddenly becomes possible to replace an existing
TCP congestion control algorithm with ease.

The major differences from v1:

 - Added bpf_struct_ops_link to replace the previous union-based
   approach.

 - Added UNREG and TOBEUNREG to the state of bpf_struct_ops_map.

   - bpf_struct_ops_transit_state() maintains state transitions.

 - Fixed synchronization issue.

 - Prepare kernel vdata of struct_ops during the loading phase of
   bpf_object.

 - Merged previous patch 3 to patch 1.

v1: https://lore.kernel.org/bpf/20230214221718.503964-1-kuifeng@meta.com/

Kui-Feng Lee (6):
  bpf: Create links for BPF struct_ops maps.
  net: Update an existing TCP congestion control algorithm.
  libbpf: Create a bpf_link in bpf_map__attach_struct_ops().
  bpf: Update the struct_ops of a bpf_link.
  libbpf: Update a bpf_link with another struct_ops.
  selftests/bpf: Test switching TCP Congestion Control algorithms.

 include/linux/bpf.h                           |  13 +
 include/net/tcp.h                             |   2 +
 include/uapi/linux/bpf.h                      |  20 +-
 kernel/bpf/bpf_struct_ops.c                   | 445 +++++++++++++++++-
 kernel/bpf/syscall.c                          |  58 ++-
 net/bpf/bpf_dummy_struct_ops.c                |   6 +
 net/ipv4/bpf_tcp_ca.c                         |   8 +-
 net/ipv4/tcp_cong.c                           |  58 ++-
 tools/include/uapi/linux/bpf.h                |  12 +-
 tools/lib/bpf/bpf.c                           |   2 +
 tools/lib/bpf/libbpf.c                        | 120 ++++-
 tools/lib/bpf/libbpf.h                        |   1 +
 tools/lib/bpf/libbpf.map                      |   2 +
 .../selftests/bpf/prog_tests/bpf_tcp_ca.c     |  48 ++
 .../selftests/bpf/progs/tcp_ca_update.c       |  62 +++
 15 files changed, 791 insertions(+), 66 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/progs/tcp_ca_update.c

-- 
2.30.2