Subject: Re: [PATCH V2 3/3] vhost_net: basic polling support

From: Yang Zhang <[email-masked]>
Date: Thu, 21 Jan 2016 10:11:35 +0800

On 2016/1/20 22:35, Michael S. Tsirkin wrote:
> On Tue, Dec 01, 2015 at 02:39:45PM +0800, Jason Wang wrote:
>> This patch tries to poll for new added tx buffer or socket receive
>> queue for a while at the end of tx/rx processing. The maximum time
>> spent on polling were specified through a new kind of vring ioctl.
>>
>> Signed-off-by: Jason Wang <[email-masked]>
>> ---
>>   drivers/vhost/net.c        | 72 ++++++++++++++++++++++++++++++++++++++++++----
>>   drivers/vhost/vhost.c      | 15 ++++++++++
>>   drivers/vhost/vhost.h      |  1 +
>>   include/uapi/linux/vhost.h | 11 +++++++
>>   4 files changed, 94 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
>> index 9eda69e..ce6da77 100644
>> --- a/drivers/vhost/net.c
>> +++ b/drivers/vhost/net.c
>> @@ -287,6 +287,41 @@ static void vhost_zerocopy_callback(struct ubuf_info *ubuf, bool success)
>>   	rcu_read_unlock_bh();
>>   }
>>
>> +static inline unsigned long busy_clock(void)
>> +{
>> +	return local_clock() >> 10;
>> +}
>> +
>> +static bool vhost_can_busy_poll(struct vhost_dev *dev,
>> +				unsigned long endtime)
>> +{
>> +	return likely(!need_resched()) &&
>> +	       likely(!time_after(busy_clock(), endtime)) &&
>> +	       likely(!signal_pending(current)) &&
>> +	       !vhost_has_work(dev) &&
>> +	       single_task_running();
>> +}
>> +
>> +static int vhost_net_tx_get_vq_desc(struct vhost_net *net,
>> +				    struct vhost_virtqueue *vq,
>> +				    struct iovec iov[], unsigned int iov_size,
>> +				    unsigned int *out_num, unsigned int *in_num)
>> +{
>> +	unsigned long uninitialized_var(endtime);
>> +
>> +	if (vq->busyloop_timeout) {
>> +		preempt_disable();
>> +		endtime = busy_clock() + vq->busyloop_timeout;
>> +		while (vhost_can_busy_poll(vq->dev, endtime) &&
>> +		       !vhost_vq_more_avail(vq->dev, vq))
>> +			cpu_relax();
>> +		preempt_enable();
>> +	}
>
> Isn't there a way to call all this after vhost_get_vq_desc?
> First, this will reduce the good path overhead as you
> won't have to play with timers and preemption.
>
> Second, this will reduce the chance of a pagefault on avail ring read.
>
>> +
>> +	return vhost_get_vq_desc(vq, vq->iov, ARRAY_SIZE(vq->iov),
>> +				 out_num, in_num, NULL, NULL);
>> +}
>> +
>>   /* Expects to be always run from workqueue - which acts as
>>    * read-size critical section for our kind of RCU. */
>>   static void handle_tx(struct vhost_net *net)
>> @@ -331,10 +366,9 @@ static void handle_tx(struct vhost_net *net)
>>   			      % UIO_MAXIOV == nvq->done_idx))
>>   			break;
>>
>> -		head = vhost_get_vq_desc(vq, vq->iov,
>> -					 ARRAY_SIZE(vq->iov),
>> -					 &out, &in,
>> -					 NULL, NULL);
>> +		head = vhost_net_tx_get_vq_desc(net, vq, vq->iov,
>> +						ARRAY_SIZE(vq->iov),
>> +						&out, &in);
>>   		/* On error, stop handling until the next kick. */
>>   		if (unlikely(head < 0))
>>   			break;
>> @@ -435,6 +469,34 @@ static int peek_head_len(struct sock *sk)
>>   	return len;
>>   }
>>
>> +static int vhost_net_peek_head_len(struct vhost_net *net, struct sock *sk)
>
> Need a hint that it's rx related in the name.
>
>> +{
>> +	struct vhost_net_virtqueue *nvq = &net->vqs[VHOST_NET_VQ_TX];
>> +	struct vhost_virtqueue *vq = &nvq->vq;
>> +	unsigned long uninitialized_var(endtime);
>> +
>> +	if (vq->busyloop_timeout) {
>> +		mutex_lock(&vq->mutex);
>
> This appears to be called under vq mutex in handle_rx.
> So how does this work then?
>
>
>> +		vhost_disable_notify(&net->dev, vq);
>
> This appears to be called after disable notify
> in handle_rx - so why disable here again?
>
>> +
>> +		preempt_disable();
>> +		endtime = busy_clock() + vq->busyloop_timeout;
>> +
>> +		while (vhost_can_busy_poll(&net->dev, endtime) &&
>> +		       skb_queue_empty(&sk->sk_receive_queue) &&
>> +		       !vhost_vq_more_avail(&net->dev, vq))
>> +			cpu_relax();
>
> This seems to mix in several items.
> RX queue is normally not empty. I don't think
> we need to poll for that.

I have seen the RX queue is easy to be empty under some extreme 
conditions like lots of small packet. So maybe the check is useful here.

-- 
best regards
yang