Re: [PATCH net-next v2 3/5] virtio_ring: add packed ring support

[Jason Wang <jasowang@redhat.com>]

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;
>>>    	}
>>>
>>>