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SubjectRe: [PATCH net-next v2 3/5] virtio_ring: add packed ring support
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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;
>>>>> }
>>>>>
>>>>>

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