The core benches a subflow via the "stale" bit after
net.mptcp.stale_loss_cnt retransmission intervals without progress.
That bit is core-owned and RTO-coupled -- cleared as soon as a packet
is acked on the subflow. A scheduler has no way to bench a subflow for
its own reasons (high latency, instability) independently of ack
traffic.

"scheduled" doesn't help either: the core clears it after every
send/retrans pass, so it can't hold a standing "leave this alone"
decision, and nothing outside the scheduler that set it can see it.

Changes since v1:

- avoid now lives in the existing __unused:8 bitfield padding instead
  of its own bool. No struct growth (Paolo).
- the selftest scheduler sets avoid once, on the first get_send()
  call, and only reads it back afterward -- proving persistence
  scheduled can't do, instead of just claiming it. A get_send_calls
  counter, asserted >1, makes sure multiple rounds actually ran
  (Paolo).
- mptcp_subflow_get_send() and mptcp_subflow_get_retrans() -- the
  default subflow picker, active with or without a custom scheduler
  -- now skip an avoided subflow next to the existing active-subflow
  check. avoid is also exposed via MPTCP diag (Paolo).
- set_avoid() marked __bpf_kfunc: it has no C callers, unlike
  set_scheduled(), and could be dropped under LTO before
  resolve_btfids finds it in BTF (review bot).
- added the missing extern __ksym for mptcp_subflow_active() in the
  selftest, matching mptcp_bpf_burst.c (review bot).
- moved the set_avoid extern out of mptcp_bpf.h into
  mptcp_bpf_avoid.c: checkpatch flags new externs in shared headers,
  and it has only the one caller.

Probing an avoided subflow to decide when to clear it (issue #348) is
left out on purpose -- that's scheduler policy, not core mechanism.

Patch 1 adds the flag, the kfunc, default-scheduler enforcement, and
diag support. Patch 2 reworks the selftest to prove persistence and
picks up the missing extern.

Ran the full suite through mptcp-upstream-virtme-docker (auto-normal
and auto-btf) -- selftests, packetdrill, and BPF tests pass, avoid
subtest included.

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/349

Kalpan Jani (2):
  mptcp: sched: add subflow avoid flag and enforce it in core
  selftests: mptcp: bpf: exercise the subflow avoid flag across rounds

 include/uapi/linux/mptcp.h                    |  1 +
 net/mptcp/bpf.c                               |  1 +
 net/mptcp/diag.c                              |  2 +
 net/mptcp/protocol.c                          |  4 +-
 net/mptcp/protocol.h                          |  5 +-
 net/mptcp/sched.c                             |  6 ++
 .../testing/selftests/bpf/prog_tests/mptcp.c  | 16 +++++
 .../selftests/bpf/progs/mptcp_bpf_avoid.c     | 70 +++++++++++++++++++
 8 files changed, 102 insertions(+), 3 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/progs/mptcp_bpf_avoid.c

base-commit: 490b4823cb98b233af2b7562bddc90ecb5dfbf4e

-- 
2.43.0