[PATCH net-next v3 0/2] tcp: rehash onto different local ECMP path on retransmit timeout

[Neil Spring <ntspring@meta.com>]

Make TCP retransmission timeouts select a different ECMP path for IPv6.

Currently sk_rethink_txhash() changes the socket's txhash on RTO, but the
cached route is reused and the new hash is not propagated into the ECMP
path selection logic.  This series adds sk_dst_reset() alongside
sk_rethink_txhash() to force a fresh route lookup, and sets fl6->mp_hash
from sk_txhash so fib6_select_path() picks a path based on the new hash.

Five selftest scenarios verify the behavior across connection setup and
established flows, forward and reverse path failures, and PLB:

  - SYN retransmission (forward path blocked during setup)
  - SYN/ACK retransmission (reverse path blocked during setup)
  - Midstream RTO (forward path blocked on established connection)
  - Midstream ACK rehash (reverse path blocked on established connection)
  - PLB rehash (ECN-driven congestion on established connection)

Changes since v2: https://lore.kernel.org/netdev/20260408070514.1840227-1-ntspring@meta.com/
- Retitle "ECMP" to "local ECMP" to distinguish from remote ECMP
  (Neal Cardwell)
- Add fl6->mp_hash propagation in inet6_sk_rebuild_header() (af_inet6.c),
  covering the dst rebuild path used on established sockets
- Remove incorrect ir_iif update from tcp_check_req() in tcp_minisocks.c;
  the SYN/ACK rehash is already handled by tcp_rtx_synack() re-rolling
  txhash which feeds into inet6_csk_route_req()'s mp_hash
  (Eric Dumazet)
- Add ACK rehash and PLB rehash selftests
- Improve selftest reliability

Changes since v1: https://lore.kernel.org/netdev/20260408002802.2448424-1-ntspring@meta.com/
- Use tcp_rsk(req)->txhash instead of jhash_1word(req->num_retrans, ...)
  for ECMP path selection in inet6_csk_route_req(), making the request
  socket path consistent with the established socket path (Eric Dumazet)
- Add comments explaining the >> 1 shift for 31-bit mp_hash range
- Use socat -u (unidirectional) in selftest to avoid SIGPIPE race
- Increase tcp_syn_retries and tcp_syn_linear_timeouts to 25 for
  better rehash coverage

Neil Spring (2):
  tcp: rehash onto different local ECMP path on retransmit timeout
  selftests: net: add local ECMP rehash test

 net/ipv4/tcp_input.c                       |   4 +-
 net/ipv4/tcp_plb.c                         |   1 +
 net/ipv4/tcp_timer.c                       |   1 +
 net/ipv6/af_inet6.c                        |   3 +
 net/ipv6/inet6_connection_sock.c           |   6 +
 tools/testing/selftests/net/Makefile       |   1 +
 tools/testing/selftests/net/ecmp_rehash.sh | 569 +++++++++++++++++++++
 7 files changed, 584 insertions(+), 1 deletion(-)
 create mode 100755 tools/testing/selftests/net/ecmp_rehash.sh

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2.52.0