Re: AF_XDP busy poll receives packets in batches of 8 on i40e

On 3/17/2022 8:34 AM, Federico Parola wrote:
>>     We observed similar "batching" behavior on i40e devices late
>> last year in ordinary use (not XDP, but using SR-IOV VFs).  We
>> instrumented the drivers at the send and receive sides, and determined
>> that it appeared to be a behavior of the receiving device itself, i.e.,
>> packets 1 - 7 would be held indefinitely (as I recall, no interrupt or
>> update of the RX ring pointers) until packet 8 arrives, at which point
>> all 8 were delivered simultaneously.
>>
>>     The issue was evidently in the firmware, and was resolved after
>> a firmware upgrade.
> 
> Hi Jay,
> I just updated the firmware to the latest version (v8.50 from v8.30) but 
> unfortunately the problem is still there.
> However I'm experiencing the problem only when using AF_XDP in busy poll 
> mode, all other modes (standard AF_XDP and normal packet reception) work 
> just fine.
> Maybe the two problems are correlated in some way.

This sounds related to the WB_ON_ITR feature in our hardware. If 
interrupts are disabled the driver needs to set that bit (and an ITR 
value) so that packets get written back in a timely manner and don't 
just wait for a cache line edge (I bet your Cache Line Size value in 
PCIe space (lspci) is set to 128?)

Maybe something to chase there? We may also be able to look into it if 
it wasn't fixed already upstream.