From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [PATCH net 2/4] vhost_net: rework on the lock ordering for busy polling Date: Mon, 10 Dec 2018 23:04:08 -0500 Message-ID: <20181210230106-mutt-send-email-mst@kernel.org> References: <20181210094454.21144-1-jasowang@redhat.com> <20181210094454.21144-3-jasowang@redhat.com> <20181210203119-mutt-send-email-mst@kernel.org> Mime-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: base64 Return-path: Content-Disposition: inline In-Reply-To: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: Jason Wang Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, virtualization@lists.linux-foundation.org List-Id: virtualization@lists.linuxfoundation.org T24gVHVlLCBEZWMgMTEsIDIwMTggYXQgMTE6MDY6NDNBTSArMDgwMCwgSmFzb24gV2FuZyB3cm90 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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 9EBCFC07E85 for ; Tue, 11 Dec 2018 04:04:11 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5412320811 for ; Tue, 11 Dec 2018 04:04:11 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 5412320811 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 S1730114AbeLKEEK (ORCPT ); Mon, 10 Dec 2018 23:04:10 -0500 Received: from mx1.redhat.com ([209.132.183.28]:47236 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727721AbeLKEEJ (ORCPT ); Mon, 10 Dec 2018 23:04:09 -0500 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 5F2BC369A0; Tue, 11 Dec 2018 04:04:09 +0000 (UTC) Received: from redhat.com (ovpn-120-187.rdu2.redhat.com [10.10.120.187]) by smtp.corp.redhat.com (Postfix) with ESMTP id 85BF660BF1; Tue, 11 Dec 2018 04:04:08 +0000 (UTC) Date: Mon, 10 Dec 2018 23:04:08 -0500 From: "Michael S. Tsirkin" To: Jason Wang Cc: kvm@vger.kernel.org, virtualization@lists.linux-foundation.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Tonghao Zhang Subject: Re: [PATCH net 2/4] vhost_net: rework on the lock ordering for busy polling Message-ID: <20181210230106-mutt-send-email-mst@kernel.org> References: <20181210094454.21144-1-jasowang@redhat.com> <20181210094454.21144-3-jasowang@redhat.com> <20181210203119-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.30]); Tue, 11 Dec 2018 04:04:09 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Dec 11, 2018 at 11:06:43AM +0800, Jason Wang wrote: > > 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. Right but it's all slow path - it happens when queue is otherwise empty. So this is what I am saying: let's drop the locks we hold around this. > > > > > 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 I'd rather we kept the fine grained locking. E.g. people are looking at splitting the tx and rx threads. But if not possible let's fix it cleanly with a coarse-grained one. A mess here will just create more trouble later. -- MST