[PATCH bpf-next 0/7] bpf: Add bpf_tcp_ops hooks for TCP AutoLOWAT.

[Kuniyuki Iwashima <kuniyu@google.com>]

This series introduces two callbacks for bpf_tcp_ops:

  .enqueue_rcvq(): invoked when TCP stack enqueues skb to
                   sk->sk_receive_queue

  .dequeue_rcvq(): invoked in tcp_cleanup_rbuf() after data
                   is dequeued from sk->sk_receive_queue

Those callbacks can be enabled on a per-socket basis by
bpf_setsockopt():

  int flags = BPF_SOCK_OPS_RCVQ_CB_FLAG;

  bpf_setsockopt(sk, SOL_TCP, TCP_BPF_SOCK_OPS_CB_FLAGS,
                 &flags, sizeof(flags));

or via the bpf_tcp_ops-specific helper added in the next patch:

  bpf_sock_ops_cb_flags_set(sk, BPF_SOCK_OPS_RCVQ_CB_FLAG);

This allows the BPF prog to dynamically adjust sk->sk_rcvlowat,
suppressing unnecessary EPOLLIN wakeups until sufficient data
is available in the receive queue.

This functionality, which we call "TCP AutoLOWAT", was originally
developed in 2020 by Tenzin Ukyab with the help of Soheil Hassas
Yeganeh, Arjun Roy, and Eric Dumazet.  It has served Google RPC
workloads for more than 5 years.

Combined with TCP RX zerocopy, this typically allows us to read an
entire RPC frame with just a single wakeup and a single system call.

While the original implementation was specialised for our
internal RPC format, this series introduces a more flexible
version by leveraging BPF.

The bpf prog in the last selftest patch closely mirrors the core
logic of the original implementation to provide a real-world
example.

Note that the new callbacks are not supported on legacy SOCK_OPS.

Legacy SOCK_OPS version:
  v3: https://lore.kernel.org/bpf/20260523083001.2911931-1-kuniyu@google.com/
  v2: https://lore.kernel.org/bpf/20260522074601.1658705-1-kuniyu@google.com/
  v1: https://lore.kernel.org/bpf/20260508073355.3916746-1-kuniyu@google.com/


Kuniyuki Iwashima (7):
  selftest: bpf: Use BPF_SOCK_OPS_ALL_CB_FLAGS + 1 for bad_cb_test_rv.
  bpf: tcp: Introduce bpf_tcp_ops.{enqueue,dequeue}_rcvq().
  bpf: tcp: Support bpf_sock_ops_cb_flags_set() for bpf_tcp_ops.
  tcp: Split out __tcp_set_rcvlowat().
  bpf: mptcp: Don't support BPF_SOCK_OPS_RCVQ_CB_FLAG.
  bpf: tcp: Add kfunc to adjust sk->sk_rcvlowat.
  selftest: bpf: Add test for bpf_tcp_ops.{enqueue,dequeue}_rcvq().

 include/net/tcp.h                             |  34 ++
 include/uapi/linux/bpf.h                      |  13 +-
 net/core/filter.c                             |  10 +-
 net/ipv4/bpf_tcp_ops.c                        |  96 ++++-
 net/ipv4/tcp.c                                |  14 +-
 net/ipv4/tcp_fastopen.c                       |   2 +
 net/ipv4/tcp_input.c                          |   4 +
 tools/include/uapi/linux/bpf.h                |  13 +-
 .../selftests/bpf/prog_tests/tcp_autolowat.c  | 350 ++++++++++++++++++
 .../selftests/bpf/prog_tests/tcpbpf_user.c    |   3 +-
 .../selftests/bpf/progs/bpf_tracing_net.h     |   2 +
 .../selftests/bpf/progs/tcp_autolowat.c       | 312 ++++++++++++++++
 .../selftests/bpf/progs/test_tcpbpf_kern.c    |   3 +-
 13 files changed, 842 insertions(+), 14 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/tcp_autolowat.c
 create mode 100644 tools/testing/selftests/bpf/progs/tcp_autolowat.c

-- 
2.55.0.1082.g2b9226bbc0-goog