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

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Date:	Thu, 21 Jan 2016 14:39:51 +0800
From:	Yang Zhang <yang.zhang.wz@...il.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Jason Wang <jasowang@...hat.com>, kvm@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2 3/3] vhost_net: basic polling support

On 2016/1/21 13:13, Michael S. Tsirkin wrote:
> On Thu, Jan 21, 2016 at 10:11:35AM +0800, Yang Zhang wrote:
>> 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 <jasowang@...hat.com>
>>>> ---
>>>>   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.
>
> It's not useful *here*.
> If you have an rx packet but no space in the ring,
> this will exit immediately.

Indeed!

>
> It might be useful elsewhere but I doubt it -
> if rx ring is out of buffers, you are better off
> backing out and giving guest some breathing space.
>
>> --
>> best regards
>> yang


-- 
best regards
yang