Subject: Re: [Xen-devel] [PATCHv1] xen-netfront: always keep the Rx ring full of requests

From: annie li <[email-masked]>
Date: Tue, 07 Oct 2014 09:12:30 -0400

On 2014/10/7 5:43, David Vrabel wrote:
> On 06/10/14 22:07, David Miller wrote:
>> From: annie li <[email-masked]>
>> Date: Mon, 06 Oct 2014 14:41:48 -0400
>>
>>> On 2014/10/6 12:00, David Vrabel wrote:
>>>>>>     +    queue->rx.req_prod_pvt = req_prod;
>>>>>> +
>>>>>> +    /* Not enough requests? Try again later. */
>>>>>> +    if (req_prod - queue->rx.rsp_cons < NET_RX_SLOTS_MIN) {
>>>>>> +        mod_timer(&queue->rx_refill_timer, jiffies + (HZ/10));
>>>>>> +        return;
>>>>> If the previous for loop breaks because of failure of
>>>>> xennet_alloc_one_rx_buffer, then notify_remote_via_irq is missed here
>>>>> if
>>>>> the code returns directly.
>>>> This is deliberate -- there's no point notifying the backend if there
>>>> aren't enough requests for the next packet.  Since we don't know what
>>>> the next packet might be we assume it's the largest possible.
>>> That makes sense.
>>> However, the largest packet case does not happen so
>>> frequently. Moreover, netback checks the slots every incoming skb
>>> requires in xenvif_rx_ring_slots_available, not only concerning the
>>> largest case.
> An upcoming change to netback will cause it to wait for enough slots for
> the largest possible packet.

Netback knows the exact slot number that incoming skb will consumes, is 
there any reason to let it wait for the largest possible packets?

Thanks
Annie