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Thu, 02 Apr 2026 10:19:41 -0700 (PDT) Date: Thu, 2 Apr 2026 10:19:37 -0700 From: Bobby Eshleman To: Laurence Rowe Cc: Stefano Garzarella , virtualization@lists.linux.dev, netdev@vger.kernel.org Subject: Re: [PATCH] vsock: avoid timeout for non-blocking accept() with empty backlog Message-ID: References: <20260402044637.73531-1-laurencerowe@gmail.com> Precedence: bulk X-Mailing-List: virtualization@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260402044637.73531-1-laurencerowe@gmail.com> On Wed, Apr 01, 2026 at 09:46:37PM -0700, Laurence Rowe wrote: > 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 > --- > > 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; > + } I wonder if this goto can be omitted since the following 'if (connected)' guards the connected != NULL case? I don't have a strong opinion, just noticed it would keep if-else symmetrical. All-in-all, LGTM. Reviewed-by: Bobby Eshleman