Messages in this thread Patch in this message | | | Subject | Re: [PATCH net-next v2 3/5] virtio_ring: add packed ring support | From | Jason Wang <> | Date | Fri, 9 Nov 2018 10:25:28 +0800 |
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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 */
> >> 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
> >>> 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; >>> } >>> >>>
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