As it was shown in the report by Alexander Azimov, hash rethink at the client-side may lead to connection timeout toward stateful anycast services. Tom Herbert created a patchset to address this issue by applying hash rethink only after a negative routing event (3RTOs) [1]. This change also affects server-side behavior, which we found undesirable. This patchset changes defaults in a way to make them safe: hash rethink at the client-side is disabled and enabled at the server-side upon each RTO event or in case of duplicate acknowledgments. This patchset provides two options to change default behaviour. The hash rethink may be disabled at the server-side by the new sysctl option. Changes in the sysctl option don't affect default behavior at the client-side. Hash rethink can also be enabled/disabled with socket option or bpf syscalls which ovewrite both default and sysctl settings. This socket option is available on both client and server-side. This should provide mechanics to enable hash rethink inside administrative domain, such as DC, where hash rethink at the client-side can be desirable. [1] https://lore.kernel.org/netdev/20210809185314.38187-1-tom@xxxxxxxxxxxxxxx/ v2: - Changed sysctl default to ENABLED in all patches. Reduced sysctl and socket option size to u8. Fixed netns bug reported by kernel test robot. v3: - Fixed bug with bad u8 comparison. Moved sk_txrehash to use less bytes in struct. Added WRITE_ONCE() in setsockopt in and READ_ONCE() in tcp_rtx_synack. v4: - Rebase and add documentation for sysctl option. v5: - Move sk_txrehash out of busy poll ifdef. Akhmat Karakotov (5): txhash: Make rethinking txhash behavior configurable via sysctl txhash: Add socket option to control TX hash rethink behavior txhash: Add txrehash sysctl description bpf: Add SO_TXREHASH setsockopt tcp: Change SYN ACK retransmit behaviour to account for rehash Documentation/admin-guide/sysctl/net.rst | 9 ++++++++ arch/alpha/include/uapi/asm/socket.h | 2 ++ arch/mips/include/uapi/asm/socket.h | 2 ++ arch/parisc/include/uapi/asm/socket.h | 2 ++ arch/sparc/include/uapi/asm/socket.h | 2 ++ include/net/netns/core.h | 1 + include/net/sock.h | 28 +++++++++++++----------- include/uapi/asm-generic/socket.h | 2 ++ include/uapi/linux/socket.h | 4 ++++ net/core/filter.c | 10 +++++++++ net/core/net_namespace.c | 2 ++ net/core/sock.c | 14 ++++++++++++ net/core/sysctl_net_core.c | 14 ++++++++++-- net/ipv4/inet_connection_sock.c | 3 +++ net/ipv4/tcp_output.c | 4 +++- 15 files changed, 83 insertions(+), 16 deletions(-) -- 2.17.1
