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

From: Jason Wang <[email-masked]>
Date: Fri, 23 Oct 2015 15:13:07 +0800

On 10/22/2015 05:33 PM, Michael S. Tsirkin wrote:
> 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?

Just run TCP_RR, no increasing. Will run a complete test on next version.

>
>> Signed-off-by: Jason Wang <[email-masked]>
> We might be able to shave off the minor regression
> by careful use of likely/unlikely, or maybe
> deferring 

Yes, but what did "deferring" mean here?
 
>
>> ---
>>  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.

Ok.

>
>>  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.

Right, or maybe disable preemption in this case?

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

Right.

>
>> +
>> +	return busyloop_timeout && !need_resched() &&
>> +	       !time_after(now, endtime) && !vhost_has_work(dev) &&
>> +	       single_task_running();
> signal pending as well?

Yes.

>> +}
>> +
>>  /* 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
> --
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