[PATCH net-next v2 0/2] tcp: rehash onto different 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.

Three selftest scenarios verify the behavior: SYN retransmission, SYN/ACK
retransmission (server-side), and midstream RTO on an established
connection.

Changes since v1:
- 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
- Add comment explaining the ir_iif update rationale in tcp_check_req()
- 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; add tcp_synack_retries=10 on the server

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

 net/ipv4/tcp_input.c                       |   4 +-
 net/ipv4/tcp_minisocks.c                   |  13 +
 net/ipv4/tcp_plb.c                         |   1 +
 net/ipv4/tcp_timer.c                       |   1 +
 net/ipv6/inet6_connection_sock.c           |  11 +
 tools/testing/selftests/net/Makefile       |   1 +
 tools/testing/selftests/net/ecmp_rehash.sh | 361 +++++++++++++++++++++
 7 files changed, 391 insertions(+), 1 deletion(-)
 create mode 100755 tools/testing/selftests/net/ecmp_rehash.sh

-- 
2.52.0