From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754749AbdKAPDl (ORCPT ); Wed, 1 Nov 2017 11:03:41 -0400 Received: from mx1.redhat.com ([209.132.183.28]:40332 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752629AbdKAPDi (ORCPT ); Wed, 1 Nov 2017 11:03:38 -0400 DMARC-Filter: OpenDMARC Filter v1.3.2 mx1.redhat.com 9D40CC0587DC Authentication-Results: ext-mx08.extmail.prod.ext.phx2.redhat.com; dmarc=none (p=none dis=none) header.from=redhat.com Authentication-Results: ext-mx08.extmail.prod.ext.phx2.redhat.com; spf=fail smtp.mailfrom=mst@redhat.com Date: Wed, 1 Nov 2017 17:03:37 +0200 From: "Michael S. Tsirkin" To: Jason Wang Cc: Matthew Rosato , kvm@vger.kernel.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, Wei Xu Subject: Re: [PATCH net-next] vhost_net: conditionally enable tx polling Message-ID: <20171101165953-mutt-send-email-mst@kernel.org> References: <1509445640-4085-1-git-send-email-jasowang@redhat.com> <20171031182637-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-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.32]); Wed, 01 Nov 2017 15:03:38 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. > > > > > > > > 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. > > > @@ -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? > > > > > > > -- > > > 2.7.4 > > _______________________________________________ > Virtualization mailing list > Virtualization@lists.linux-foundation.org > https://lists.linuxfoundation.org/mailman/listinfo/virtualization