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

From: Jason Wang <[email-masked]>
Date: Fri, 22 Jan 2016 13:59:34 +0800

On 01/20/2016 10:35 PM, 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?

We can.

> First, this will reduce the good path overhead as you
> won't have to play with timers and preemption.

For good path, yes.

>
> Second, this will reduce the chance of a pagefault on avail ring read.

Yes.

>
>> +
>> +	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.

Ok.

>
>> +{
>> +	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?

This is tx mutex, an optimization here: both rx socket and tx ring is
polled.  So there's no need to tx notification anymore. This can save
lots of vmexits.

>
>
>> +		vhost_disable_notify(&net->dev, vq);
> This appears to be called after disable notify
> in handle_rx - so why disable here again?

It disable the tx notification instead of rx.

>
>> +
>> +		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.
> So IMHO we only need to poll for sk_receive_queue really.

Same as above, tx virt queue is being polled here.

>
>> +
>> +		preempt_enable();
>> +
>> +		if (vhost_enable_notify(&net->dev, vq))
>> +			vhost_poll_queue(&vq->poll);
> But vhost_enable_notify returns true on queue not empty.
> So in fact this will requeue on good path -
> does not make sense to me.

And still the same, it enables tx notification and eliminate the window
if a new tx buffer is available.

>
>> +		mutex_unlock(&vq->mutex);
>> +	}
>> +
> Same comment as for get vq desc here: don't slow
> down the good path.
>
>> +	return peek_head_len(sk);

ok.

>> +}
>> +
>>  /* This is a multi-buffer version of vhost_get_desc, that works if
>>   *	vq has read descriptors only.
>>   * @vq		- the relevant virtqueue
>> @@ -553,7 +615,7 @@ static void handle_rx(struct vhost_net *net)
>>  		vq->log : NULL;
>>  	mergeable = vhost_has_feature(vq, VIRTIO_NET_F_MRG_RXBUF);
>>  
>> -	while ((sock_len = peek_head_len(sock->sk))) {
>> +	while ((sock_len = vhost_net_peek_head_len(net, sock->sk))) {
>>  		sock_len += sock_hlen;
>>  		vhost_len = sock_len + vhost_hlen;
>>  		headcount = get_rx_bufs(vq, vq->heads, vhost_len,
>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>> index 4f45a03..b8ca873 100644
>> --- a/drivers/vhost/vhost.c
>> +++ b/drivers/vhost/vhost.c
>> @@ -285,6 +285,7 @@ static void vhost_vq_reset(struct vhost_dev *dev,
>>  	vq->memory = NULL;
>>  	vq->is_le = virtio_legacy_is_little_endian();
>>  	vhost_vq_reset_user_be(vq);
>> +	vq->busyloop_timeout = 0;
>>  }
>>  
>>  static int vhost_worker(void *data)
>> @@ -747,6 +748,7 @@ long vhost_vring_ioctl(struct vhost_dev *d, int ioctl, void __user *argp)
>>  	struct vhost_vring_state s;
>>  	struct vhost_vring_file f;
>>  	struct vhost_vring_addr a;
>> +	struct vhost_vring_busyloop_timeout t;
>>  	u32 idx;
>>  	long r;
>>  
>> @@ -919,6 +921,19 @@ long vhost_vring_ioctl(struct vhost_dev *d, int ioctl, void __user *argp)
>>  	case VHOST_GET_VRING_ENDIAN:
>>  		r = vhost_get_vring_endian(vq, idx, argp);
>>  		break;
>> +	case VHOST_SET_VRING_BUSYLOOP_TIMEOUT:
>> +		if (copy_from_user(&t, argp, sizeof(t))) {
>> +			r = -EFAULT;
>> +			break;
>> +		}
>> +		vq->busyloop_timeout = t.timeout;
>> +		break;
>> +	case VHOST_GET_VRING_BUSYLOOP_TIMEOUT:
>> +		t.index = idx;
>> +		t.timeout = vq->busyloop_timeout;
>> +		if (copy_to_user(argp, &t, sizeof(t)))
>> +			r = -EFAULT;
>> +		break;
>>  	default:
>>  		r = -ENOIOCTLCMD;
>>  	}
>> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
>> index 2f3c57c..4b7d4fa 100644
>> --- a/drivers/vhost/vhost.h
>> +++ b/drivers/vhost/vhost.h
>> @@ -115,6 +115,7 @@ struct vhost_virtqueue {
>>  	/* Ring endianness requested by userspace for cross-endian support. */
>>  	bool user_be;
>>  #endif
>> +	u32 busyloop_timeout;
>>  };
>>  
>>  struct vhost_dev {
>> diff --git a/include/uapi/linux/vhost.h b/include/uapi/linux/vhost.h
>> index ab373191..eaf6c33 100644
>> --- a/include/uapi/linux/vhost.h
>> +++ b/include/uapi/linux/vhost.h
>> @@ -27,6 +27,11 @@ struct vhost_vring_file {
>>  
>>  };
>>  
>> +struct vhost_vring_busyloop_timeout {
>> +	unsigned int index;
>> +	unsigned int timeout;
>> +};
>> +
> So why not reuse vhost_vring_state then?

Good idea.

>
>
>
>>  struct vhost_vring_addr {
>>  	unsigned int index;
>>  	/* Option flags. */
>> @@ -126,6 +131,12 @@ struct vhost_memory {
>>  #define VHOST_SET_VRING_CALL _IOW(VHOST_VIRTIO, 0x21, struct vhost_vring_file)
>>  /* Set eventfd to signal an error */
>>  #define VHOST_SET_VRING_ERR _IOW(VHOST_VIRTIO, 0x22, struct vhost_vring_file)
>> +/* Set busy loop timeout */
> Units?

Will add this. (us).

>
>> +#define VHOST_SET_VRING_BUSYLOOP_TIMEOUT _IOW(VHOST_VIRTIO, 0x23,	\
>> +					 struct vhost_vring_busyloop_timeout)
>> +/* Get busy loop timeout */
>> +#define VHOST_GET_VRING_BUSYLOOP_TIMEOUT _IOW(VHOST_VIRTIO, 0x24,	\
>> +					 struct vhost_vring_busyloop_timeout)
>>  
>>  /* VHOST_NET specific defines */
>>  
>> -- 
>> 2.5.0
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