From: Tiwei Bie <tiwei.bie@intel.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>,
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
Subject: Re: [PATCH net-next v2 3/5] virtio_ring: add packed ring support
Date: Fri, 9 Nov 2018 09:50:11 +0800 [thread overview]
Message-ID: <20181109015011.GA24544@debian> (raw)
In-Reply-To: <20181108103155-mutt-send-email-mst@kernel.org>
On Thu, Nov 08, 2018 at 10:56:02AM -0500, Michael S. Tsirkin wrote:
> On Thu, Nov 08, 2018 at 07:51:48PM +0800, Tiwei Bie 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.
> >
> > Do you know why removing this notify in Tx will break "the
> > heuristics which KVM uses"? Or what does "the heuristics
> > which KVM uses" refer to?
>
> Yes. QEMU has a mode where it disables notifications and processes TX
> ring periodically from a timer. It's off by default but used to be on
> by default a long time ago. If ring becomes full this causes traffic
> stalls. As a work-around Rusty put in this hack to kick on ring full
> even with notifications disabled. It's easy enough to make sure QEMU
> does not combine devices with packed ring support with the timer hack.
> And I am guessing it's safe enough to also block that option completely
> e.g. when virtio 1.0 is enabled.
I see. Thanks!
>
> >
> > > 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. Consider the removal of lguest support recently,
> > > maybe we can drop this for split ring as well?
> > >
> > > Thanks
> > >
> > >
> > > >
> > > > commit 44653eae1407f79dff6f52fcf594ae84cb165ec4
> > > > Author: Rusty Russell<rusty@rustcorp.com.au>
> > > > 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<rusty@rustcorp.com.au>
> > > >
> > > > 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;
> > > > }
> > > >
> > > >
next prev parent reply other threads:[~2018-11-09 1:51 UTC|newest]
Thread overview: 53+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-07-11 2:27 [PATCH net-next v2 0/5] virtio: support packed ring Tiwei Bie
2018-07-11 2:27 ` [PATCH net-next v2 1/5] virtio: add packed ring definitions Tiwei Bie
2018-09-07 13:51 ` Michael S. Tsirkin
2018-09-10 2:13 ` Tiwei Bie
2018-09-12 12:53 ` Michael S. Tsirkin
2018-07-11 2:27 ` [PATCH net-next v2 2/5] virtio_ring: support creating packed ring Tiwei Bie
2018-09-07 14:03 ` Michael S. Tsirkin
2018-09-10 2:28 ` Tiwei Bie
2018-09-12 7:51 ` Tiwei Bie
2018-09-12 16:12 ` Michael S. Tsirkin
2018-09-12 12:45 ` Michael S. Tsirkin
2018-07-11 2:27 ` [PATCH net-next v2 3/5] virtio_ring: add packed ring support Tiwei Bie
2018-09-07 13:49 ` Michael S. Tsirkin
2018-09-10 2:03 ` Tiwei Bie
2018-09-12 16:22 ` Michael S. Tsirkin
2018-11-07 17:48 ` Michael S. Tsirkin
2018-11-08 1:38 ` Tiwei Bie
2018-11-08 8:18 ` Jason Wang
2018-11-08 11:51 ` Tiwei Bie
2018-11-08 15:56 ` Michael S. Tsirkin
2018-11-09 1:50 ` Tiwei Bie [this message]
2018-11-09 2:30 ` Jason Wang
2018-11-09 4:00 ` Michael S. Tsirkin
2018-11-09 10:05 ` Jason Wang
2018-11-08 14:14 ` Michael S. Tsirkin
2018-11-09 2:25 ` Jason Wang
2018-11-09 3:58 ` Michael S. Tsirkin
2018-11-09 10:04 ` Jason Wang
2018-07-11 2:27 ` [PATCH net-next v2 4/5] virtio_ring: add event idx support in packed ring Tiwei Bie
2018-09-07 14:10 ` Michael S. Tsirkin
2018-09-10 2:35 ` [virtio-dev] " Tiwei Bie
2018-07-11 2:27 ` [PATCH net-next v2 5/5] virtio_ring: enable " Tiwei Bie
2018-07-11 2:52 ` [PATCH net-next v2 0/5] virtio: support " Jason Wang
2018-07-12 21:44 ` David Miller
2018-07-13 0:52 ` Jason Wang
2018-07-13 3:26 ` Michael S. Tsirkin
2018-08-27 14:00 ` Michael S. Tsirkin
2018-08-28 5:51 ` [virtio-dev] " Jens Freimann
2018-09-07 1:22 ` Tiwei Bie
2018-09-07 13:00 ` Michael S. Tsirkin
2018-09-10 3:00 ` Tiwei Bie
2018-09-10 3:33 ` Jason Wang
2018-09-11 5:37 ` Tiwei Bie
2018-09-12 16:16 ` Michael S. Tsirkin
2018-09-13 8:59 ` Tiwei Bie
2018-09-13 9:47 ` Jason Wang
2018-10-10 14:36 ` Michael S. Tsirkin
2018-10-11 12:12 ` Tiwei Bie
2018-10-11 13:48 ` Michael S. Tsirkin
2018-10-11 14:13 ` Tiwei Bie
2018-10-11 14:17 ` Michael S. Tsirkin
2018-10-11 14:34 ` Tiwei Bie
2018-09-12 13:06 ` Michael S. Tsirkin
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