Linux Virtualization - Sep 2017 - [PATCH net] vhost_net: correctly check tx avail during rx busy polling

We check tx avail through vhost_enable_notify() in the past which is
wrong since it only checks whether or not guest has filled more
available buffer since last avail idx synchronization which was just
done by vhost_vq_avail_empty() before. What we really want is checking
pending buffers in the avail ring. Fix this by calling
vhost_vq_avail_empty() instead.

This issue could be noticed by doing netperf TCP_RR benchmark as
client from guest (but not host). With this fix, TCP_RR from guest to
localhost restores from 1375.91 trans per sec to 55235.28 trans per
sec on my laptop (Intel(R) Core(TM) i7-5600U CPU @ 2.60GHz).

Fixes: 030881372460 ("vhost_net: basic polling support")
Signed-off-by: Jason Wang <jasowang at redhat.com>
---
- The patch is needed for -stable
---
 drivers/vhost/net.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index 06d0448..1b68253 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -634,7 +634,7 @@ static int vhost_net_rx_peek_head_len(struct vhost_net *net,
struct sock *sk)
 
 		preempt_enable();
 
-		if (vhost_enable_notify(&net->dev, vq))
+		if (!vhost_vq_avail_empty(&net->dev, vq))
 			vhost_poll_queue(&vq->poll);
 		mutex_unlock(&vq->mutex);
 
-- 
2.7.4

On Fri, Sep 01, 2017 at 05:02:50PM +0800, Jason Wang
wrote:
> We check tx avail through vhost_enable_notify() in the past which is
> wrong since it only checks whether or not guest has filled more
> available buffer since last avail idx synchronization which was just
> done by vhost_vq_avail_empty() before. What we really want is checking
> pending buffers in the avail ring.
These are rx buffers, right? I'm not even sure why do we need to poll
for them. Running out of rx buffers is a slow path.
> Fix this by calling
> vhost_vq_avail_empty() instead.
> 
> This issue could be noticed by doing netperf TCP_RR benchmark as
> client from guest (but not host). With this fix, TCP_RR from guest to
> localhost restores from 1375.91 trans per sec to 55235.28 trans per
> sec on my laptop (Intel(R) Core(TM) i7-5600U CPU @ 2.60GHz).
> 
> Fixes: 030881372460 ("vhost_net: basic polling support")
> Signed-off-by: Jason Wang <jasowang at redhat.com>
> ---
> - The patch is needed for -stable
> ---
>  drivers/vhost/net.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 06d0448..1b68253 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -634,7 +634,7 @@ static int vhost_net_rx_peek_head_len(struct vhost_net
*net, struct sock *sk)
In fact why does it poll the ring at all? I thought this function's
job is to poll the socket, isn't it?

>  
>  		preempt_enable();
>  
> -		if (vhost_enable_notify(&net->dev, vq))
> +		if (!vhost_vq_avail_empty(&net->dev, vq))
>  			vhost_poll_queue(&vq->poll);
>  		mutex_unlock(&vq->mutex);

Adding more contex:

                mutex_lock(&vq->mutex);
                vhost_disable_notify(&net->dev, vq);

                preempt_disable();
                endtime = busy_clock() + vq->busyloop_timeout;

                while (vhost_can_busy_poll(&net->dev, endtime) &&
                       !sk_has_rx_data(sk) &&
                       vhost_vq_avail_empty(&net->dev, vq))
                        cpu_relax();
                
                preempt_enable();
        
                if (vhost_enable_notify(&net->dev, vq))
                        vhost_poll_queue(&vq->poll);
                mutex_unlock(&vq->mutex);

                len = peek_head_len(rvq, sk);


If you drop this we'll exit the function with notifications
disabled. Seems wrong to me.

>  
> -- 
> 2.7.4

On 2017?09?01? 23:51, Michael S. Tsirkin wrote:
> On Fri, Sep 01, 2017 at 05:02:50PM +0800, Jason Wang wrote:
>> We check tx avail through vhost_enable_notify() in the past which is
>> wrong since it only checks whether or not guest has filled more
>> available buffer since last avail idx synchronization which was just
>> done by vhost_vq_avail_empty() before. What we really want is checking
>> pending buffers in the avail ring.
> These are rx buffers, right? I'm not even sure why do we need to poll
> for them. Running out of rx buffers is a slow path.
Actually it polls for tx buffer here. I admit the code (or probably the 
variable name) is confusing here.
>
>> Fix this by calling
>> vhost_vq_avail_empty() instead.
>>
>> This issue could be noticed by doing netperf TCP_RR benchmark as
>> client from guest (but not host). With this fix, TCP_RR from guest to
>> localhost restores from 1375.91 trans per sec to 55235.28 trans per
>> sec on my laptop (Intel(R) Core(TM) i7-5600U CPU @ 2.60GHz).
>>
>> Fixes: 030881372460 ("vhost_net: basic polling support")
>> Signed-off-by: Jason Wang <jasowang at redhat.com>
>> ---
>> - The patch is needed for -stable
>> ---
>>   drivers/vhost/net.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
>> index 06d0448..1b68253 100644
>> --- a/drivers/vhost/net.c
>> +++ b/drivers/vhost/net.c
>> @@ -634,7 +634,7 @@ static int vhost_net_rx_peek_head_len(struct
vhost_net *net, struct sock *sk)
> In fact why does it poll the ring at all? I thought this function's
> job is to poll the socket, isn't it?
Tx notification is disabled to try to avoid vmexits, so we poll tx avail 
buffers too.
>
>
>>   
>>   		preempt_enable();
>>   
>> -		if (vhost_enable_notify(&net->dev, vq))
>> +		if (!vhost_vq_avail_empty(&net->dev, vq))
>>   			vhost_poll_queue(&vq->poll);
>>   		mutex_unlock(&vq->mutex);
>
> Adding more contex:
>
>                  mutex_lock(&vq->mutex);
>                  vhost_disable_notify(&net->dev, vq);
>
>                  preempt_disable();
>                  endtime = busy_clock() + vq->busyloop_timeout;
>
>                  while (vhost_can_busy_poll(&net->dev, endtime)
&&
>                         !sk_has_rx_data(sk) &&
>                         vhost_vq_avail_empty(&net->dev, vq))
>                          cpu_relax();
>                  
>                  preempt_enable();
>          
>                  if (vhost_enable_notify(&net->dev, vq))
>                          vhost_poll_queue(&vq->poll);
>                  mutex_unlock(&vq->mutex);
>
>                  len = peek_head_len(rvq, sk);
>
>
> If you drop this we'll exit the function with notifications
> disabled. Seems wrong to me.
Yes, will fix this in V2.

Thanks
>
>>   
>> -- 
>> 2.7.4