Messages in this thread | | | Subject | Re: [PATCH net-next v2 3/5] virtio_ring: add packed ring support | From | Jason Wang <> | Date | Fri, 9 Nov 2018 18:04:42 +0800 |
| |
On 2018/11/9 上午11:58, Michael S. Tsirkin wrote: > 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?
There's no traffic control in XDP, so it was the only thing we can do.
> >>>> 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.
Let me test and submit a patch.
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; >>>>> } >>>>> >>>>>
| |