From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934229AbdKBDRG (ORCPT ); Wed, 1 Nov 2017 23:17:06 -0400 Received: from mx1.redhat.com ([209.132.183.28]:36080 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751428AbdKBDRE (ORCPT ); Wed, 1 Nov 2017 23:17:04 -0400 DMARC-Filter: OpenDMARC Filter v1.3.2 mx1.redhat.com 62B138765C Authentication-Results: ext-mx02.extmail.prod.ext.phx2.redhat.com; dmarc=none (p=none dis=none) header.from=redhat.com Authentication-Results: ext-mx02.extmail.prod.ext.phx2.redhat.com; spf=fail smtp.mailfrom=jasowang@redhat.com Subject: Re: [PATCH net-next] vhost_net: conditionally enable tx polling To: "Michael S. Tsirkin" Cc: Matthew Rosato , kvm@vger.kernel.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, Wei Xu References: <1509445640-4085-1-git-send-email-jasowang@redhat.com> <20171031182637-mutt-send-email-mst@kernel.org> <20171101165953-mutt-send-email-mst@kernel.org> From: Jason Wang Message-ID: <386cdcbe-f69d-91a6-eac9-f0c3d834fc14@redhat.com> Date: Thu, 2 Nov 2017 11:16:56 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0 MIME-Version: 1.0 In-Reply-To: <20171101165953-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.26]); Thu, 02 Nov 2017 03:17:04 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2017年11月01日 23:03, Michael S. Tsirkin wrote: > On Wed, Nov 01, 2017 at 08:51:36PM +0800, Jason Wang wrote: >> >> On 2017年11月01日 00:36, Michael S. Tsirkin wrote: >>> On Tue, Oct 31, 2017 at 06:27:20PM +0800, Jason Wang wrote: >>>> We always poll tx for socket, this is sub optimal since: >>>> >>>> - we only want to be notified when sndbuf is available >>>> - this will slightly increase the waitqueue traversing time and more >>>> important, vhost could not benefit from commit >>>> commit 9e641bdcfa4e >>>> ("net-tun: restructure tun_do_read for better sleep/wakeup efficiency") >>>> even if we've stopped rx polling during handle_rx() since tx poll >>>> were still left in the waitqueue. >>>> >>>> Pktgen from a remote host to VM over mlx4 shows 5.5% improvements on >>>> rx PPS. (from 1.27Mpps to 1.34Mpps) >>>> >>>> Cc: Wei Xu >>>> Cc: Matthew Rosato >>>> Signed-off-by: Jason Wang >>>> --- >>> Now that vhost_poll_stop happens on data path >>> a lot, I'd say >>> if (poll->wqh) >>> there should be unlikely(). >> It has been there since 8241a1e466cd ("vhost_net: stop polling socket during >> rx processing"). So it will be used for rx path too which unlikely() does >> not work as well as the case in tx. > Worth testing, does not have to block this patch. Ok. >>> >>>> drivers/vhost/net.c | 11 ++++++++--- >>>> 1 file changed, 8 insertions(+), 3 deletions(-) >>>> >>>> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c >>>> index 68677d9..286c3e4 100644 >>>> --- a/drivers/vhost/net.c >>>> +++ b/drivers/vhost/net.c >>>> @@ -471,6 +471,7 @@ static void handle_tx(struct vhost_net *net) >>>> goto out; >>>> vhost_disable_notify(&net->dev, vq); >>>> + vhost_net_disable_vq(net, vq); >>>> hdr_size = nvq->vhost_hlen; >>>> zcopy = nvq->ubufs; >>>> @@ -556,6 +557,8 @@ static void handle_tx(struct vhost_net *net) >>>> % UIO_MAXIOV; >>>> } >>>> vhost_discard_vq_desc(vq, 1); >>>> + if (err == -EAGAIN) >>>> + vhost_net_enable_vq(net, vq); >>>> break; >>>> } >>>> if (err != len) >>> I would probably just enable it unconditionally here. Why not? >>> >> I thought we only care about the case of tun_sock_write_space() and for the >> errors other than -EAGAIN, they have nothing to do with polling. > We could thinkably get ENOMEM I guess. If we miss a code things > get stuck - It's just easier not to add extra code IMHO. ENOBUFS actually. I agree to remove the specific checking of -EAGAIN here. > >>>> @@ -1145,9 +1148,11 @@ static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd) >>>> r = vhost_vq_init_access(vq); >>>> if (r) >>>> goto err_used; >>>> - r = vhost_net_enable_vq(n, vq); >>>> - if (r) >>>> - goto err_used; >>>> + if (index == VHOST_NET_VQ_RX) { >>>> + r = vhost_net_enable_vq(n, vq); >>>> + if (r) >>>> + goto err_used; >>>> + } >>>> oldubufs = nvq->ubufs; >>>> nvq->ubufs = ubufs; >>> This last chunk seems questionable. If queue has stuff in it >>> when we connect the backend, we'll miss a wakeup. >>> I suspect this can happen during migration. >> Unless qemu pass a tap which s already had pending tx packets. >> >> I can remove this chuck, but if guest does not transmit any packet, rx can't >> benefit from this. >> >> Thanks > Not sure I understand the last sentence. vhost will stay > polling the socket - why is that a problem? No function problem at all. If guest does not transmit any packet, the patch does not have any effect. Will remove this chunk in next version. Thanks > >>> >>>> -- >>>> 2.7.4 >> _______________________________________________ >> Virtualization mailing list >> Virtualization@lists.linux-foundation.org >> https://lists.linuxfoundation.org/mailman/listinfo/virtualization