Subject: Re: [V2 PATCH 9/9] vhost: zerocopy: poll vq in zerocopy callback

From: Jason Wang <[email-masked]>
Date: Mon, 21 May 2012 14:05:24 +0800

On 05/18/2012 11:29 PM, Shirley Ma wrote:
> On Fri, 2012-05-18 at 17:58 +0800, Jason Wang wrote:
>> On 05/17/2012 11:34 PM, Shirley Ma wrote:
>>> On Thu, 2012-05-17 at 10:50 +0800, Jason Wang wrote:
>>>> The problem is we may stop the tx queue when there no enough
>> capacity
>>>> to
>>>> place packets, at this moment  we depends on the tx interrupt to
>>>> re-enable the tx queue. So if we didn't poll the vhost during
>>>> callback,
>>>> guest may lose the tx interrupt to re-enable the tx queue which
>> could
>>>> stall the whole tx queue.
>>> VHOST_MAX_PEND should handle the capacity.
>>>
>>> Hasn't the above situation been handled in handle_tx() code?:
>>> ...
>>>                           if (unlikely(num_pends>   VHOST_MAX_PEND)) {
>>>                                   tx_poll_start(net, sock);
>>>
>> set_bit(SOCK_ASYNC_NOSPACE,&sock->flags);
>>>                                   break;
>>>                           }
>>> ...
>>>
>>> Thanks
>>> Shirley
>> It may not help in because:
>>
>> - tx polling depends on skb_orphan() which is often called by device
>> driver when it place the packet into the queue of the devices instead
>> of  when the packets were sent. So it was too early for vhost to be
>> notified.
> Then do you think it's better to replace with vhost_poll_queue here
> instead?

Just like what does this patch do - calling vhost_poll_queue() in 
vhost_zerocopy_callback().
>> - it only works when the pending DMAs exceeds VHOST_MAX_PEND, it's
>> highly possible that guest needs to be notified when the pending
>> packets
>> isn't so much.
> In which situation the guest needs to be notified when there is no TX
> besides buffers run out?

Consider guest call virtqueue_enable_cb_delayed() which means it only 
need to be notified when 3/4 of pending buffers ( about 178 buffers 
(256-MAX_SKB_FRAGS-2)*3/4 ) were sent by host. So vhost_net would notify 
guest when about 60 buffers were pending. Since tx polling is only 
enabled when pending packets exceeds VHOST_MAX_PEND 128, so tx work 
would not be notified to run and guest would never get the interrupt it 
expected to re-enable the queue.

And just like what we've discussed, tx polling based adding and 
signaling is too early for vhost.
>> So this piece of code may not help and could be removed and we need
>> to
>> poll the virt-queue during zerocopy callback ( through it could be
>> further optimized but may not be easy).
> Thanks
> Shirley
>
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