Subject: Re: [PATCH V2 1/2] vhost_net: stop polling socket during rx processing

From: Jason Wang <[email-masked]>
Date: Tue, 31 May 2016 11:14:10 +0800

On 2016年05月30日 23:47, Michael S. Tsirkin wrote:
> On Mon, May 30, 2016 at 02:47:53AM -0400, Jason Wang wrote:
>> We don't stop rx polling socket during rx processing, this will lead
>> unnecessary wakeups from under layer net devices (E.g
>> sock_def_readable() form tun). Rx will be slowed down in this
>> way. This patch avoids this by stop polling socket during rx
>> processing. A small drawback is that this introduces some overheads in
>> light load case because of the extra start/stop polling, but single
>> netperf TCP_RR does not notice any change. In a super heavy load case,
>> e.g using pktgen to inject packet to guest, we get about ~8.8%
>> improvement on pps:
>>
>> before: ~1240000 pkt/s
>> after:  ~1350000 pkt/s
>>
>> Signed-off-by: Jason Wang <[email-masked]>
>> ---
>>   drivers/vhost/net.c | 56 +++++++++++++++++++++++++++--------------------------
>>   1 file changed, 29 insertions(+), 27 deletions(-)
>>
>> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
>> index 10ff494..e91603b 100644
>> --- a/drivers/vhost/net.c
>> +++ b/drivers/vhost/net.c
>> @@ -301,6 +301,32 @@ static bool vhost_can_busy_poll(struct vhost_dev *dev,
>>   	       !vhost_has_work(dev);
>>   }
>>   
>> +static void vhost_net_disable_vq(struct vhost_net *n,
>> +				 struct vhost_virtqueue *vq)
>> +{
>> +	struct vhost_net_virtqueue *nvq =
>> +		container_of(vq, struct vhost_net_virtqueue, vq);
>> +	struct vhost_poll *poll = n->poll + (nvq - n->vqs);
>> +	if (!vq->private_data)
>> +		return;
>> +	vhost_poll_stop(poll);
>> +}
>> +
>> +static int vhost_net_enable_vq(struct vhost_net *n,
>> +				struct vhost_virtqueue *vq)
>> +{
>> +	struct vhost_net_virtqueue *nvq =
>> +		container_of(vq, struct vhost_net_virtqueue, vq);
>> +	struct vhost_poll *poll = n->poll + (nvq - n->vqs);
>> +	struct socket *sock;
>> +
>> +	sock = vq->private_data;
>> +	if (!sock)
>> +		return 0;
>> +
>> +	return vhost_poll_start(poll, sock->file);
>> +}
>> +
>>   static int vhost_net_tx_get_vq_desc(struct vhost_net *net,
>>   				    struct vhost_virtqueue *vq,
>>   				    struct iovec iov[], unsigned int iov_size,
> BTW we might want to rename these functions, name no longer
> reflects function ...

Do you mean adding something reflect busy polling in the name? Then the 
name may be too long or have suggestion on the name?

>
>
>> @@ -627,6 +653,7 @@ static void handle_rx(struct vhost_net *net)
>>   	if (!sock)
>>   		goto out;
>>   	vhost_disable_notify(&net->dev, vq);
>> +	vhost_net_disable_vq(net, vq);
>>   
>>   	vhost_hlen = nvq->vhost_hlen;
>>   	sock_hlen = nvq->sock_hlen;
>> @@ -715,9 +742,10 @@ static void handle_rx(struct vhost_net *net)
>>   		total_len += vhost_len;
>>   		if (unlikely(total_len >= VHOST_NET_WEIGHT)) {
>>   			vhost_poll_queue(&vq->poll);
>> -			break;
>> +			goto out;
>>   		}
>>   	}
>> +	vhost_net_enable_vq(net, vq);
> OK so if sock is readable but RX VQ is empty, this will
> immediately schedule another round of handle_rx and so ad
> infinitum,
>
> Looks like a bug.

Yes it is, will change the above headcount check to:

                 /* OK, now we need to know about added descriptors. */
                 if (!headcount) {
                         if (unlikely(vhost_enable_notify(&net->dev, vq))) {
                                 /* They have slipped one in as we were
                                  * doing that: check again. */
vhost_disable_notify(&net->dev, vq);
                 continue;
                         }
                         /* Nothing new?  Wait for eventfd to tell us
                          * they refilled. */
                         goto out;
                 }

>
>
>>   out:
>>   	mutex_unlock(&vq->mutex);
>>   }
>> @@ -796,32 +824,6 @@ static int vhost_net_open(struct inode *inode, struct file *f)
>>   	return 0;
>>   }
>>   
>> -static void vhost_net_disable_vq(struct vhost_net *n,
>> -				 struct vhost_virtqueue *vq)
>> -{
>> -	struct vhost_net_virtqueue *nvq =
>> -		container_of(vq, struct vhost_net_virtqueue, vq);
>> -	struct vhost_poll *poll = n->poll + (nvq - n->vqs);
>> -	if (!vq->private_data)
>> -		return;
>> -	vhost_poll_stop(poll);
>> -}
>> -
>> -static int vhost_net_enable_vq(struct vhost_net *n,
>> -				struct vhost_virtqueue *vq)
>> -{
>> -	struct vhost_net_virtqueue *nvq =
>> -		container_of(vq, struct vhost_net_virtqueue, vq);
>> -	struct vhost_poll *poll = n->poll + (nvq - n->vqs);
>> -	struct socket *sock;
>> -
>> -	sock = vq->private_data;
>> -	if (!sock)
>> -		return 0;
>> -
>> -	return vhost_poll_start(poll, sock->file);
>> -}
>> -
>>   static struct socket *vhost_net_stop_vq(struct vhost_net *n,
>>   					struct vhost_virtqueue *vq)
>>   {
>> -- 
>> 1.8.3.1
> --
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