unnecessary rx callbacks when zero packets

[Ferruh Yigit]

On 1/8/2024 3:20 PM, Konstantin Ananyev wrote:
> 
> rx callbacks when zero packets
>>
>> I noticed while looking at packet capture that currently the receive callbacks
>> get called even if there are no packets. This seems unnecessary since if
>> nb_rx is zero, then there are no packets to look at.  My one concern is that
>> an application could be using callbacks as some form of scheduling mechanism
>> which would be broken.
> 
> As I remember, original idea was to allow callbacks to inject new packets if needed.
> 

Right, callback function updating 'nb_rx' enables this.

Also Honnappa has a good point that callback can be used to calculate
zero packet polls.

These points are not documented and not obvious from the code,
@Stephen does it make sense to document that callback function called
with zero packet intentionally, in the 'rte_eth_rx_burst()' function
comment?

>>
>> The change would be:
>>
>>
>> diff --git a/lib/ethdev/rte_ethdev.h b/lib/ethdev/rte_ethdev.h
>> index 21e3a21903ec..f64bf977c46e 100644
>> --- a/lib/ethdev/rte_ethdev.h
>> +++ b/lib/ethdev/rte_ethdev.h
>> @@ -6077,7 +6077,7 @@ rte_eth_rx_burst(uint16_t port_id, uint16_t queue_id,
>>         nb_rx = p->rx_pkt_burst(qd, rx_pkts, nb_pkts);
>>
>>  #ifdef RTE_ETHDEV_RXTX_CALLBACKS
>> -       {
>> +       if (nb_rx > 0) {
>>                 void *cb;
>>
>>                 /* rte_memory_order_release memory order was used when the