The SO_REUSEPORT option allows sockets to listen on the same port and to accept connections evenly. However, there is a defect in the current implementation[1]. When a SYN packet is received, the connection is tied to a listening socket. Accordingly, when the listener is closed, in-flight requests during the three-way handshake and child sockets in the accept queue are dropped even if other listeners on the same port could accept such connections. This situation can happen when various server management tools restart server (such as nginx) processes. For instance, when we change nginx configurations and restart it, it spins up new workers that respect the new configuration and closes all listeners on the old workers, resulting in the in-flight ACK of 3WHS is responded by RST. The SO_REUSEPORT option is excellent to improve scalability. On the other hand, as a trade-off, users have to know deeply how the kernel handles SYN packets and implement connection draining by eBPF[2]: 1. Stop routing SYN packets to the listener by eBPF. 2. Wait for all timers to expire to complete requests 3. Accept connections until EAGAIN, then close the listener. or 1. Start counting SYN packets and accept syscalls using eBPF map. 2. Stop routing SYN packets. 3. Accept connections up to the count, then close the listener. In either way, we cannot close a listener immediately. However, ideally, the application need not drain the not yet accepted sockets because 3WHS and tying a connection to a listener are just the kernel behaviour. The root cause is within the kernel, so the issue should be addressed in kernel space and should not be visible to user space. This patchset fixes it so that users need not take care of kernel implementation and connection draining. With this patchset, the kernel redistributes requests and connections from a listener to others in the same reuseport group at/after close() or shutdown() syscalls. Although some software does connection draining, there are still merits in migration. For some security reasons such as replacing TLS certificates, we may want to apply new settings as soon as possible and/or we may not be able to wait for connection draining. The sockets in the accept queue have not started application sessions yet. So, if we do not drain such sockets, they can be handled by the newer listeners and could have a longer lifetime. It is difficult to drain all connections in every case, but we can decrease such aborted connections by migration. In that sense, migration is always better than draining. Moreover, auto-migration simplifies userspace logic and also works well in a case where we cannot modify and build a server program to implement the workaround. Note that the source and destination listeners MUST have the same settings at the socket API level; otherwise, applications may face inconsistency and cause errors. In such a case, we have to use eBPF program to select a specific listener or to cancel migration. Link: [1] The SO_REUSEPORT socket option https://lwn.net/Articles/542629/ [2] Re: [PATCH 1/1] net: Add SO_REUSEPORT_LISTEN_OFF socket option as drain mode https://lore.kernel.org/netdev/1458828813.10868.65.camel@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/ Changelog: v2: * Do not save closed sockets in socks[] * Revert 607904c357c61adf20b8fd18af765e501d61a385 * Extract inet_csk_reqsk_queue_migrate() into a single patch * Change the spin_lock order to avoid lockdep warning * Add static to __reuseport_select_sock * Use refcount_inc_not_zero() in reuseport_select_migrated_sock() * Set the default attach type in bpf_prog_load_check_attach() * Define new proto of BPF_FUNC_get_socket_cookie * Fix test to be compiled successfully * Update commit messages v1: https://lore.kernel.org/netdev/20201201144418.35045-1-kuniyu@xxxxxxxxxxxx/ * Remove the sysctl option * Enable migration if eBPF progam is not attached * Add expected_attach_type to check if eBPF program can migrate sockets * Add a field to tell migration type to eBPF program * Support BPF_FUNC_get_socket_cookie to get the cookie of sk * Allocate an empty skb if skb is NULL * Pass req_to_sk(req)->sk_hash because listener's hash is zero * Update commit messages and coverletter RFC: https://lore.kernel.org/netdev/20201117094023.3685-1-kuniyu@xxxxxxxxxxxx/ Kuniyuki Iwashima (13): tcp: Allow TCP_CLOSE sockets to hold the reuseport group. bpf: Define migration types for SO_REUSEPORT. Revert "locking/spinlocks: Remove the unused spin_lock_bh_nested() API" tcp: Introduce inet_csk_reqsk_queue_migrate(). tcp: Set the new listener to migrated TFO requests. tcp: Migrate TCP_ESTABLISHED/TCP_SYN_RECV sockets in accept queues. tcp: Migrate TCP_NEW_SYN_RECV requests. bpf: Introduce two attach types for BPF_PROG_TYPE_SK_REUSEPORT. libbpf: Set expected_attach_type for BPF_PROG_TYPE_SK_REUSEPORT. bpf: Add migration to sk_reuseport_(kern|md). bpf: Support BPF_FUNC_get_socket_cookie() for BPF_PROG_TYPE_SK_REUSEPORT. bpf: Call bpf_run_sk_reuseport() for socket migration. bpf: Test BPF_SK_REUSEPORT_SELECT_OR_MIGRATE. include/linux/bpf.h | 1 + include/linux/filter.h | 4 +- include/linux/spinlock.h | 8 + include/linux/spinlock_api_smp.h | 2 + include/linux/spinlock_api_up.h | 1 + include/net/inet_connection_sock.h | 12 ++ include/net/request_sock.h | 13 ++ include/net/sock_reuseport.h | 15 +- include/uapi/linux/bpf.h | 25 +++ kernel/bpf/syscall.c | 13 ++ kernel/locking/spinlock.c | 8 + net/core/filter.c | 56 +++++- net/core/sock_reuseport.c | 96 +++++++--- net/ipv4/inet_connection_sock.c | 99 +++++++++- net/ipv4/inet_hashtables.c | 9 +- net/ipv4/tcp_ipv4.c | 9 +- net/ipv6/tcp_ipv6.c | 9 +- tools/include/uapi/linux/bpf.h | 25 +++ tools/lib/bpf/libbpf.c | 5 +- .../bpf/prog_tests/select_reuseport_migrate.c | 173 ++++++++++++++++++ .../bpf/progs/test_select_reuseport_migrate.c | 53 ++++++ 21 files changed, 590 insertions(+), 46 deletions(-) create mode 100644 tools/testing/selftests/bpf/prog_tests/select_reuseport_migrate.c create mode 100644 tools/testing/selftests/bpf/progs/test_select_reuseport_migrate.c -- 2.17.2 (Apple Git-113)
