From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-dy1-f178.google.com (mail-dy1-f178.google.com [74.125.82.178]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id A3A103537D7 for ; Thu, 2 Apr 2026 20:49:53 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=74.125.82.178 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775162995; cv=none; b=a/TqVTGcLiFaB9VTBRIQlt0hGGOKWm45xJ5cz9NLE5gFEWeDsDTt7PR7AM2WbPyw/ijllTRc8zdExlskdw0xid9Kw8+z1R+ZGxmp/o4LbTXVp06haNIaA2Ssy/FDMKlX0yjh873te+TKpBcvQStItWKJTQo+GkjmXQu2NAKddV0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775162995; c=relaxed/simple; bh=/NOWjdCDjnSGtjfZ0scHtF9gHAc7SZGxwt0BAwxtaiE=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version; b=jhLDfWtvFJvpfDyYlrFw/+DLyclUjwguD3ObwvrpJOinnkfdrrYAaO6X16nMAd0xJfSLfMiYWzfNf4+qBYWm9KWd5e7iZp6Qrj5ZG5kudK0wHewjQuGbP4EBNMgyUFJb1w4Hzezp7TplUC5ZecPoYHaZMjdXpIpSnwLHKUnszgU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com; spf=pass smtp.mailfrom=gmail.com; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b=WmENch9G; arc=none smtp.client-ip=74.125.82.178 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=gmail.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="WmENch9G" Received: by mail-dy1-f178.google.com with SMTP id 5a478bee46e88-2ba895adfeaso1597496eec.0 for ; Thu, 02 Apr 2026 13:49:53 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20251104; t=1775162993; x=1775767793; darn=vger.kernel.org; h=content-transfer-encoding:mime-version:message-id:date:subject:cc :to:from:from:to:cc:subject:date:message-id:reply-to; bh=SNWS0mYa+ildstE8KT3uWmZVAb+dtMF6/uKnjB13/tk=; b=WmENch9GfqskIqXVKLnP4o4inEiqBdYVov9v6fCOMn492L7WjFulQcy6HLLJEgOg3r bHON+SXP/lhATCq9Pz7y4PtspJsEH9vViZ6Mhu4bSE0a1oCXmVnb3vk5faAE2jNJomVc Fid2YrjVVldIr+ZMmqUGIzceC34DDeLbZlK8Esl9Z0IXC/vhi4uLDyCv7N9AXXVCswDT E/mz1VVcMtQGEWykaApzCaUsEyX6a423lUcn8mWGha+xXdOOmMlryT2MhskIZJmslsH+ CQ0qtVyD64grOCM19Hj6yTuHmSs+zaDAtDRHPxRP0L2rXdJwR4M9tphF1jfbGAz7kQz4 ASkw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1775162993; x=1775767793; h=content-transfer-encoding:mime-version:message-id:date:subject:cc :to:from:x-gm-gg:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to; bh=SNWS0mYa+ildstE8KT3uWmZVAb+dtMF6/uKnjB13/tk=; b=opzSRX2bW4OB1g0Qe4vHoFLZmBYU1ODLmA4edJXNY+P+Utf1qv+YKnjDEAJ2U6i986 cR13+gWXkQLraJJCPXYbK4ojPA9nrPrraU4ukCJrsogQ0T/5cgUQrjLrmtvpQ4Eh8Kxs qI/qHPBYwWNAnXXpPXOF2pvb49bGkOm2rw+5d9vMRX4Ay2BeLDHe1qzoRvcS1dxyxEz1 F3eZdRKTIY0rQODM8mw5fe2VY8uL+tTl7CQJLcwtdQX/TIKTA36X2uBTzSnMPUISlCjA urvpHUs3jOZL74JcVEnOZyOuB+ZL6xfoRWkw5Son8JtutN8wIS7Dm3qUt5A/TLL7mTqw aGaA== X-Forwarded-Encrypted: i=1; AJvYcCXmq7M/edwEUvKf/EV502//FyALDUS1aKYTw4G8RK8SLZyPV31//kEfTf4cx4yAANYCVuQCsf8=@vger.kernel.org X-Gm-Message-State: AOJu0YwIvpRcqzSsIzsUnZf64weIUVXrCfIWlXjleEVEFBn8ZpXbPxlZ QckZAb469eFeRpgEnS0w8l6fvBnY/emtdCofovwN1PIy4GOCv1w6Xyu+ X-Gm-Gg: AeBDieu10YUMgvtvd3CgVhpbHYzaWkSzHuTRB1136sfe3leA1aBE0tNeXZcpMoPo28L Z9p+K14h/tvbNKTpPiVYo4D6wTlUIeG/xMaJzr2INkyJv34Qj6BSXZorq6UUAYmX2+V83InuGpn IRRqDevV2JE/GJeO9Jm68v+vm56Tt8PZW/prAIDLXNUmiCtxQMH7gcPJ6w665sfYCkH35iGDKTI 3qR7aj3F6cqbyTo+ibFiKeiqpp6f1Ml/25ueXNH85mlaf9PWhPfbxziUiyvk3y7TJZnhZXhKTfB N7GbmQ+E7f9UUQVmbxzKDJs5uO7mMVgzR0aV1ZokJd/pSOr+Jv3q9wppusbvAsvtlJPbMqDpYgB /yBuV2qIQ4igbfCqrnALWCeE/hnNeUesmfqY40FAPnN6oH247dtTxVqEm1VgzhzeqRgxjbMpeU2 vmV3mw3zHCv1soBVwzhImWAC7zFeZHxshDPwViHc96AtZwJ5bf+tBGnY+tOk7v X-Received: by 2002:a05:7300:bc86:b0:2cb:53f5:553e with SMTP id 5a478bee46e88-2cbf64ab2c5mr405861eec.0.1775162992583; Thu, 02 Apr 2026 13:49:52 -0700 (PDT) Received: from elmini.. (c-98-42-127-152.hsd1.ca.comcast.net. [98.42.127.152]) by smtp.gmail.com with ESMTPSA id 5a478bee46e88-2ca7c20c30asm3575737eec.15.2026.04.02.13.49.51 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 02 Apr 2026 13:49:51 -0700 (PDT) From: Laurence Rowe To: Stefano Garzarella Cc: "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , virtualization@lists.linux.dev, netdev@vger.kernel.org, Laurence Rowe , Bobby Eshleman Subject: [PATCH net-next v2] vsock: avoid timeout for non-blocking accept() with empty backlog Date: Thu, 2 Apr 2026 13:49:18 -0700 Message-ID: <20260402204918.130395-1-laurencerowe@gmail.com> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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 Reviewed-by: Bobby Eshleman --- Patch v2 with feedback from Stefano Garzarella and Bobby Eshleman: - Move prepare_to_wait before release_sock to match previous behaviour. - Simplify not connected case. 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 | 15 ++++++--------- 1 file changed, 6 insertions(+), 9 deletions(-) diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c index e4756604d5..b8794ea0f0 100644 --- a/net/vmw_vsock/af_vsock.c +++ b/net/vmw_vsock/af_vsock.c @@ -1864,10 +1864,10 @@ 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) { + prepare_to_wait(sk_sleep(listener), &wait, TASK_INTERRUPTIBLE); release_sock(listener); timeout = schedule_timeout(timeout); finish_wait(sk_sleep(listener), &wait); @@ -1876,17 +1876,14 @@ 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 (!connected) { + err = -EAGAIN; + } if (connected) { sk_acceptq_removed(listener); base-commit: f35340f2d653f1003602878403c901396ab03c17 -- 2.43.0