From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.9 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 35F36C04EB8 for ; Tue, 11 Dec 2018 03:06:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E6E5A2064D for ; Tue, 11 Dec 2018 03:06:55 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org E6E5A2064D Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729820AbeLKDGy (ORCPT ); Mon, 10 Dec 2018 22:06:54 -0500 Received: from mx1.redhat.com ([209.132.183.28]:41824 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728393AbeLKDGy (ORCPT ); Mon, 10 Dec 2018 22:06:54 -0500 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 896C4C058CB0; Tue, 11 Dec 2018 03:06:53 +0000 (UTC) Received: from [10.72.12.184] (ovpn-12-184.pek2.redhat.com [10.72.12.184]) by smtp.corp.redhat.com (Postfix) with ESMTPS id C3D7A5D962; Tue, 11 Dec 2018 03:06:47 +0000 (UTC) Subject: Re: [PATCH net 2/4] vhost_net: rework on the lock ordering for busy polling To: "Michael S. Tsirkin" Cc: kvm@vger.kernel.org, virtualization@lists.linux-foundation.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Tonghao Zhang References: <20181210094454.21144-1-jasowang@redhat.com> <20181210094454.21144-3-jasowang@redhat.com> <20181210203119-mutt-send-email-mst@kernel.org> From: Jason Wang Message-ID: Date: Tue, 11 Dec 2018 11:06:43 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1 MIME-Version: 1.0 In-Reply-To: <20181210203119-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.32]); Tue, 11 Dec 2018 03:06:53 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2018/12/11 上午9:34, Michael S. Tsirkin wrote: > On Mon, Dec 10, 2018 at 05:44:52PM +0800, Jason Wang wrote: >> When we try to do rx busy polling in tx path in commit 441abde4cd84 >> ("net: vhost: add rx busy polling in tx path"), we lock rx vq mutex >> after tx vq mutex is held. This may lead deadlock so we try to lock vq >> one by one in commit 78139c94dc8c ("net: vhost: lock the vqs one by >> one"). With this commit, we avoid the deadlock with the assumption >> that handle_rx() and handle_tx() run in a same process. But this >> commit remove the protection for IOTLB updating which requires the >> mutex of each vq to be held. >> >> To solve this issue, the first step is to have a exact same lock >> ordering for vhost_net. This is done through: >> >> - For handle_rx(), if busy polling is enabled, lock tx vq immediately. >> - For handle_tx(), always lock rx vq before tx vq, and unlock it if >> busy polling is not enabled. >> - Remove the tricky locking codes in busy polling. >> >> With this, we can have a exact same lock ordering for vhost_net, this >> allows us to safely revert commit 78139c94dc8c ("net: vhost: lock the >> vqs one by one") in next patch. >> >> The patch will add two more atomic operations on the tx path during >> each round of handle_tx(). 1 byte TCP_RR does not notice such >> overhead. >> >> Fixes: commit 78139c94dc8c ("net: vhost: lock the vqs one by one") >> Cc: Tonghao Zhang >> Signed-off-by: Jason Wang >> --- >> drivers/vhost/net.c | 18 +++++++++++++++--- >> 1 file changed, 15 insertions(+), 3 deletions(-) >> >> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c >> index ab11b2bee273..5f272ab4d5b4 100644 >> --- a/drivers/vhost/net.c >> +++ b/drivers/vhost/net.c >> @@ -513,7 +513,6 @@ static void vhost_net_busy_poll(struct vhost_net *net, >> struct socket *sock; >> struct vhost_virtqueue *vq = poll_rx ? tvq : rvq; >> >> - mutex_lock_nested(&vq->mutex, poll_rx ? VHOST_NET_VQ_TX: VHOST_NET_VQ_RX); >> vhost_disable_notify(&net->dev, vq); >> sock = rvq->private_data; >> >> @@ -543,8 +542,6 @@ static void vhost_net_busy_poll(struct vhost_net *net, >> vhost_net_busy_poll_try_queue(net, vq); >> else if (!poll_rx) /* On tx here, sock has no rx data. */ >> vhost_enable_notify(&net->dev, rvq); >> - >> - mutex_unlock(&vq->mutex); >> } >> >> static int vhost_net_tx_get_vq_desc(struct vhost_net *net, >> @@ -913,10 +910,16 @@ static void handle_tx_zerocopy(struct vhost_net *net, struct socket *sock) >> static void handle_tx(struct vhost_net *net) >> { >> struct vhost_net_virtqueue *nvq = &net->vqs[VHOST_NET_VQ_TX]; >> + struct vhost_net_virtqueue *nvq_rx = &net->vqs[VHOST_NET_VQ_RX]; >> struct vhost_virtqueue *vq = &nvq->vq; >> + struct vhost_virtqueue *vq_rx = &nvq_rx->vq; >> struct socket *sock; >> >> + mutex_lock_nested(&vq_rx->mutex, VHOST_NET_VQ_RX); >> mutex_lock_nested(&vq->mutex, VHOST_NET_VQ_TX); >> + if (!vq->busyloop_timeout) >> + mutex_unlock(&vq_rx->mutex); >> + >> sock = vq->private_data; >> if (!sock) >> goto out; >> @@ -933,6 +936,8 @@ static void handle_tx(struct vhost_net *net) >> handle_tx_copy(net, sock); >> >> out: >> + if (vq->busyloop_timeout) >> + mutex_unlock(&vq_rx->mutex); >> mutex_unlock(&vq->mutex); >> } >> > So rx mutex taken on tx path now. And tx mutex is on rc path ... This > is just messed up. Why can't tx polling drop rx lock before > getting the tx lock and vice versa? Because we want to poll both tx and rx virtqueue at the same time (vhost_net_busy_poll()).     while (vhost_can_busy_poll(endtime)) {         if (vhost_has_work(&net->dev)) {             *busyloop_intr = true;             break;         }         if ((sock_has_rx_data(sock) &&              !vhost_vq_avail_empty(&net->dev, rvq)) ||             !vhost_vq_avail_empty(&net->dev, tvq))             break;         cpu_relax();     } And we disable kicks and notification for better performance. > > Or if we really wanted to force everything to be locked at > all times, let's just use a single mutex. > > > We could, but it might requires more changes which could be done for -next I believe. Thanks