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

Started by"Michael S. Tsirkin" <mst@redhat.com>
First post2016-01-20 15:40 +0100
Last post2016-01-22 07:10 +0100
Articles 5 — 3 participants

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  Re: [PATCH V2 3/3] vhost_net: basic polling support "Michael S. Tsirkin" <mst@redhat.com> - 2016-01-20 15:40 +0100
    Re: [PATCH V2 3/3] vhost_net: basic polling support Yang Zhang <yang.zhang.wz@gmail.com> - 2016-01-21 03:20 +0100
      Re: [PATCH V2 3/3] vhost_net: basic polling support "Michael S. Tsirkin" <mst@redhat.com> - 2016-01-21 06:20 +0100
        Re: [PATCH V2 3/3] vhost_net: basic polling support Yang Zhang <yang.zhang.wz@gmail.com> - 2016-01-21 07:50 +0100
    Re: [PATCH V2 3/3] vhost_net: basic polling support Jason Wang <jasowang@redhat.com> - 2016-01-22 07:10 +0100

#1313253 — Re: [PATCH V2 3/3] vhost_net: basic polling support

From"Michael S. Tsirkin" <mst@redhat.com>
Date2016-01-20 15:40 +0100
SubjectRe: [PATCH V2 3/3] vhost_net: basic polling support
Message-ID<qT25s-7Pe-19@gated-at.bofh.it>
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@redhat.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.
So IMHO we only need to poll for sk_receive_queue really.

> +
> +		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.

> +		mutex_unlock(&vq->mutex);
> +	}
> +

Same comment as for get vq desc here: don't slow
down the good path.

> +	return peek_head_len(sk);
> +}
> +
>  /* 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?




>  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?

> +#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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#1313816

FromYang Zhang <yang.zhang.wz@gmail.com>
Date2016-01-21 03:20 +0100
Message-ID<qTd0R-6UH-1@gated-at.bofh.it>
In reply to#1313253
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@redhat.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.

-- 
best regards
yang

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#1313887

From"Michael S. Tsirkin" <mst@redhat.com>
Date2016-01-21 06:20 +0100
Message-ID<qTfP5-x3-19@gated-at.bofh.it>
In reply to#1313816
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@redhat.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.

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

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#1313956

FromYang Zhang <yang.zhang.wz@gmail.com>
Date2016-01-21 07:50 +0100
Message-ID<qThe9-1qJ-1@gated-at.bofh.it>
In reply to#1313887
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@redhat.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

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#1314765

FromJason Wang <jasowang@redhat.com>
Date2016-01-22 07:10 +0100
Message-ID<qTD50-8pm-5@gated-at.bofh.it>
In reply to#1313253

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 <jasowang@redhat.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?

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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