Re: [dpdk-dev] [PATCH] ixgbe: avoid unnessary break when checking at the tail of rx hwring

[Jianbo Liu <jianbo.liu@linaro.org>]

Hi Wenzhuo,

On 16 March 2016 at 14:06, Lu, Wenzhuo <wenzhuo.lu@intel.com> wrote:
> HI Jianbo,
>
>
>> -----Original Message-----
>> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Jianbo Liu
>> Sent: Monday, March 14, 2016 10:26 PM
>> To: Zhang, Helin; Ananyev, Konstantin; dev@dpdk.org
>> Cc: Jianbo Liu
>> Subject: [dpdk-dev] [PATCH] ixgbe: avoid unnessary break when checking at the
>> tail of rx hwring
>>
>> When checking rx ring queue, it's possible that loop will break at the tail while
>> there are packets still in the queue header.
> Would you like to give more details about in what scenario this issue will be hit? Thanks.
>

vPMD will place extra RTE_IXGBE_DESCS_PER_LOOP - 1 number of empty
descriptiors at the end of hwring to avoid overflow when do checking
on rx side.

For the loop in _recv_raw_pkts_vec(), we check 4 descriptors each
time. If all 4 DD are set, and all 4 packets are received.That's OK in
the middle.
But if come to the end of hwring, and less than 4 descriptors left, we
still need to check 4 descriptors at the same time, so the extra empty
descriptors are checked with them.
This time, the number of received packets is apparently less than 4,
and we break out of the loop because of the condition "var !=
RTE_IXGBE_DESCS_PER_LOOP".
So the problem arises. It is possible that there could be more packets
at the hwring beginning that still waiting for being received.
I think this fix can avoid this situation, and at least reduce the
latency for the packets in the header.

Thanks!
Jianbo