[PATCH mptcp-next 0/2] mptcp: sched: let schedulers mark a subflow to avoid

The core benches a subflow via the "stale" bit after
net.mptcp.stale_loss_cnt retransmission intervals without progress. That
bit is owned by the core and tightly coupled to the RTO: it is cleared as
soon as a packet is acked on the subflow, in mptcp_subflow_active(). A
packet scheduler therefore cannot repurpose it to bench a subflow for its
own reasons (too high latency, too unstable) and keep it benched: the core
would clear it on the next ack, unrelated to the scheduler's intent.

Add a separate, scheduler-owned "avoid" flag on the subflow context, set
and cleared only through mptcp_subflow_set_avoid(), exposed to BPF
struct_ops schedulers as a kfunc (same registration and filter as
mptcp_subflow_set_scheduled(), so only schedulers can call it). The flag is
scheduler-local state: the scheduler sets it to remember a subflow should
be skipped and consults it when picking a subflow to send on. It lives in
the struct_group(reset) block, so it is cleared on subflow reset and a
re-established path is re-evaluated. The core send and retransmit paths are
left unchanged.

This is the "manage a new bit" option from issue #349, kept independent of
the RTO-driven stale bit as that issue requests. Regular probing of an
avoided subflow (#348) and any core-side consumption of the flag are left
for follow-up.

Patch 1 adds the flag, the setter and the kfunc. Patch 2 adds a BPF
struct_ops scheduler selftest that marks all but the first subflow avoided,
reads the flag back, and schedules only a non-avoided subflow; the existing
harness confirms the avoided path carries no data, exercising the kfunc
round-trip from eBPF.

Tested with the mptcp-upstream-virtme-docker CI (auto-normal and auto-btf):
all selftests, packetdrill and BPF tests pass, including the new avoid
subtest.

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

Kalpan Jani (2):
  mptcp: sched: add subflow avoid flag and kfunc
  selftests: mptcp: bpf: exercise the subflow avoid flag

 net/mptcp/bpf.c                               |  1 +
 net/mptcp/protocol.h                          |  3 ++
 net/mptcp/sched.c                             |  6 +++
 .../testing/selftests/bpf/prog_tests/mptcp.c  | 15 ++++++
 tools/testing/selftests/bpf/progs/mptcp_bpf.h |  2 +
 .../selftests/bpf/progs/mptcp_bpf_avoid.c     | 49 +++++++++++++++++++
 6 files changed, 76 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/progs/mptcp_bpf_avoid.c


base-commit: 490b4823cb98b233af2b7562bddc90ecb5dfbf4e
-- 
2.43.0