Re: [PATCH net-next RFC 2/2] vhost_net: basic polling support

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Oct 22, 2015 at 01:27:29AM -0400, Jason Wang wrote:
> This patch tries to poll for new added tx buffer for a while at the
> end of tx processing. The maximum time spent on polling were limited
> through a module parameter. To avoid block rx, the loop will end it
> there's new other works queued on vhost so in fact socket receive
> queue is also be polled.
> 
> busyloop_timeout = 50 gives us following improvement on TCP_RR test:
> 
> size/session/+thu%/+normalize%
>     1/     1/   +5%/  -20%
>     1/    50/  +17%/   +3%

Is there a measureable increase in cpu utilization
with busyloop_timeout = 0?

> Signed-off-by: Jason Wang <jasowang@redhat.com>

We might be able to shave off the minor regression
by careful use of likely/unlikely, or maybe
deferring 

> ---
>  drivers/vhost/net.c | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 9eda69e..bbb522a 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -31,7 +31,9 @@
>  #include "vhost.h"
>  
>  static int experimental_zcopytx = 1;
> +static int busyloop_timeout = 50;
>  module_param(experimental_zcopytx, int, 0444);
> +module_param(busyloop_timeout, int, 0444);

Pls add a description, including the units and the special
value 0.

>  MODULE_PARM_DESC(experimental_zcopytx, "Enable Zero Copy TX;"
>  		                       " 1 -Enable; 0 - Disable");
>  
> @@ -287,12 +289,23 @@ static void vhost_zerocopy_callback(struct ubuf_info *ubuf, bool success)
>  	rcu_read_unlock_bh();
>  }
>  
> +static bool tx_can_busy_poll(struct vhost_dev *dev,
> +			     unsigned long endtime)
> +{
> +	unsigned long now = local_clock() >> 10;

local_clock might go backwards if we jump between CPUs.
One way to fix would be to record the CPU id and break
out of loop if that changes.

Also - defer this until we actually know we need it?

> +
> +	return busyloop_timeout && !need_resched() &&
> +	       !time_after(now, endtime) && !vhost_has_work(dev) &&
> +	       single_task_running();

signal pending as well?

> +}
> +
>  /* 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)
>  {
>  	struct vhost_net_virtqueue *nvq = &net->vqs[VHOST_NET_VQ_TX];
>  	struct vhost_virtqueue *vq = &nvq->vq;
> +	unsigned long endtime;
>  	unsigned out, in;
>  	int head;
>  	struct msghdr msg = {
> @@ -331,6 +344,8 @@ static void handle_tx(struct vhost_net *net)
>  			      % UIO_MAXIOV == nvq->done_idx))
>  			break;
>  
> +		endtime  = (local_clock() >> 10) + busyloop_timeout;
> +again:
>  		head = vhost_get_vq_desc(vq, vq->iov,
>  					 ARRAY_SIZE(vq->iov),
>  					 &out, &in,
> @@ -340,6 +355,10 @@ static void handle_tx(struct vhost_net *net)
>  			break;
>  		/* Nothing new?  Wait for eventfd to tell us they refilled. */
>  		if (head == vq->num) {
> +			if (tx_can_busy_poll(vq->dev, endtime)) {
> +				cpu_relax();
> +				goto again;
> +			}
>  			if (unlikely(vhost_enable_notify(&net->dev, vq))) {
>  				vhost_disable_notify(&net->dev, vq);
>  				continue;
> -- 
> 1.8.3.1