A common pattern in epoll network servers is to eagerly accept all pending connections from the non-blocking listening socket after epoll_wait indicates the socket is ready by calling accept in a loop until EAGAIN is returned indicating that the backlog is empty.
Scheduling a timeout for a non-blocking accept with an empty backlog meant AF_VSOCK sockets used by epoll network servers incurred hundreds of microseconds of additional latency per accept loop compared to AF_INET or AF_UNIX sockets. Signed-off-by: Laurence Rowe <[email protected]> --- This fixes the observed issue for me: 1. With loopback vsock on the host running Linux v6.19.10 built with config-6.17.0-19-generic from Ubuntu 24.04 and make olddefconfig. 2. With Firecracker guests with current torvalds/master, v6.19.10, and amazonlinux/microvm-kernel-6.1.166-24.303.amzn2023 used in Firecracker CI and examples. (Firecracker guest vsocks are unix sockets on the host side so this fix works there with just a fixed guest kernel.) I struggled to build a generic 6.1.166 kernel that worked as a Firecracker guest but the patch applies (conflict due to change of `flags` to `arg->flags` in surrounding context) so I believe it should work for generic v6.1.166 kernel. Alternatively a minimal version of this fix is to just wrap the `schedule_timeout` in an `if (timeout != 0)` but that leaves an unnecessary additional `lock_sock` call. There are ftrace's and reproduction tools at: https://github.com/lrowe/linux-vsock-accept-timeout-investigation --- net/vmw_vsock/af_vsock.c | 16 +++++++--------- 1 file changed, 7 insertions(+), 9 deletions(-) diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c index 2f7d94d682..483889b6d8 100644 --- a/net/vmw_vsock/af_vsock.c +++ b/net/vmw_vsock/af_vsock.c @@ -1850,11 +1850,11 @@ static int vsock_accept(struct socket *sock, struct socket *newsock, * created upon connection establishment. */ timeout = sock_rcvtimeo(listener, arg->flags & O_NONBLOCK); - prepare_to_wait(sk_sleep(listener), &wait, TASK_INTERRUPTIBLE); while ((connected = vsock_dequeue_accept(listener)) == NULL && - listener->sk_err == 0) { + listener->sk_err == 0 && timeout != 0) { release_sock(listener); + prepare_to_wait(sk_sleep(listener), &wait, TASK_INTERRUPTIBLE); timeout = schedule_timeout(timeout); finish_wait(sk_sleep(listener), &wait); lock_sock(listener); @@ -1862,17 +1862,15 @@ static int vsock_accept(struct socket *sock, struct socket *newsock, if (signal_pending(current)) { err = sock_intr_errno(timeout); goto out; - } else if (timeout == 0) { - err = -EAGAIN; - goto out; } - - prepare_to_wait(sk_sleep(listener), &wait, TASK_INTERRUPTIBLE); } - finish_wait(sk_sleep(listener), &wait); - if (listener->sk_err) + if (listener->sk_err) { err = -listener->sk_err; + } else if (timeout == 0 && connected == NULL) { + err = -EAGAIN; + goto out; + } if (connected) { sk_acceptq_removed(listener); -- 2.43.0
