From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [PATCH net-next v2 3/5] virtio_ring: add packed ring support Date: Thu, 8 Nov 2018 22:58:44 -0500 Message-ID: <20181108225555-mutt-send-email-mst@kernel.org> References: <20180711022711.7090-1-tiwei.bie@intel.com> <20180711022711.7090-4-tiwei.bie@intel.com> <20181107123933-mutt-send-email-mst@kernel.org> <20181108013759.GA20591@debian> <2d46a41e-bc00-276a-e19a-105c9dffc75a@redhat.com> <20181108091337-mutt-send-email-mst@kernel.org> <21d6dbd9-8f78-6939-0e80-27b470aeb00a@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Cc: Tiwei Bie , virtualization@lists.linux-foundation.org, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, virtio-dev@lists.oasis-open.org, wexu@redhat.com, jfreimann@redhat.com To: Jason Wang Return-path: Sender: List-Post: List-Help: List-Unsubscribe: List-Subscribe: Content-Disposition: inline In-Reply-To: <21d6dbd9-8f78-6939-0e80-27b470aeb00a@redhat.com> List-Id: netdev.vger.kernel.org On Fri, Nov 09, 2018 at 10:25:28AM +0800, Jason Wang wrote: > > On 2018/11/8 下午10:14, Michael S. Tsirkin wrote: > > On Thu, Nov 08, 2018 at 04:18:25PM +0800, Jason Wang wrote: > > > On 2018/11/8 上午9:38, Tiwei Bie wrote: > > > > > > + > > > > > > + if (vq->vq.num_free < descs_used) { > > > > > > + pr_debug("Can't add buf len %i - avail = %i\n", > > > > > > + descs_used, vq->vq.num_free); > > > > > > + /* FIXME: for historical reasons, we force a notify here if > > > > > > + * there are outgoing parts to the buffer. Presumably the > > > > > > + * host should service the ring ASAP. */ > > > > > I don't think we have a reason to do this for packed ring. > > > > > No historical baggage there, right? > > > > Based on the original commit log, it seems that the notify here > > > > is just an "optimization". But I don't quite understand what does > > > > the "the heuristics which KVM uses" refer to. If it's safe to drop > > > > this in packed ring, I'd like to do it. > > > > > > According to the commit log, it seems like a workaround of lguest networking > > > backend. I agree to drop it, we should not have such burden. > > > > > > But we should notice that, with this removed, the compare between packed vs > > > split is kind of unfair. > > I don't think this ever triggers to be frank. When would it? > > > I think it can happen e.g in the path of XDP transmission in > __virtnet_xdp_xmit_one(): > > >         err = virtqueue_add_outbuf(sq->vq, sq->sg, 1, xdpf, GFP_ATOMIC); >         if (unlikely(err)) >                 return -ENOSPC; /* Caller handle free/refcnt */ > I see. We used to do it for regular xmit but stopped doing it. Is it fine for xdp then? > > > > > Consider the removal of lguest support recently, > > > maybe we can drop this for split ring as well? > > > > > > Thanks > > If it's helpful, then for sure we can drop it for virtio 1. > > Can you see any perf differences at all? With which device? > > > I don't test but consider the case of XDP_TX in guest plus vhost_net in > host. Since vhost_net is half duplex, it's pretty easier to trigger this > condition. > > Thanks Sounds reasonable. Worth testing before we change things though. > > > > > > > commit 44653eae1407f79dff6f52fcf594ae84cb165ec4 > > > > Author: Rusty Russell > > > > Date: Fri Jul 25 12:06:04 2008 -0500 > > > > > > > > virtio: don't always force a notification when ring is full > > > > We force notification when the ring is full, even if the host has > > > > indicated it doesn't want to know. This seemed like a good idea at > > > > the time: if we fill the transmit ring, we should tell the host > > > > immediately. > > > > Unfortunately this logic also applies to the receiving ring, which is > > > > refilled constantly. We should introduce real notification thesholds > > > > to replace this logic. Meanwhile, removing the logic altogether breaks > > > > the heuristics which KVM uses, so we use a hack: only notify if there are > > > > outgoing parts of the new buffer. > > > > Here are the number of exits with lguest's crappy network implementation: > > > > Before: > > > > network xmit 7859051 recv 236420 > > > > After: > > > > network xmit 7858610 recv 118136 > > > > Signed-off-by: Rusty Russell > > > > > > > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c > > > > index 72bf8bc09014..21d9a62767af 100644 > > > > --- a/drivers/virtio/virtio_ring.c > > > > +++ b/drivers/virtio/virtio_ring.c > > > > @@ -87,8 +87,11 @@ static int vring_add_buf(struct virtqueue *_vq, > > > > if (vq->num_free < out + in) { > > > > pr_debug("Can't add buf len %i - avail = %i\n", > > > > out + in, vq->num_free); > > > > - /* We notify*even if* VRING_USED_F_NO_NOTIFY is set here. */ > > > > - vq->notify(&vq->vq); > > > > + /* FIXME: for historical reasons, we force a notify here if > > > > + * there are outgoing parts to the buffer. Presumably the > > > > + * host should service the ring ASAP. */ > > > > + if (out) > > > > + vq->notify(&vq->vq); > > > > END_USE(vq); > > > > return -ENOSPC; > > > > } > > > > > > > >