Re: [dpdk-dev] [PATCH] virtio: fix rx ring descriptor starvation

["Xie, Huawei" <huawei.xie@intel.com>]

On 12/17/2015 7:18 PM, Tom Kiely wrote:
>
>
> On 11/25/2015 05:32 PM, Xie, Huawei wrote:
>> On 11/13/2015 5:33 PM, Tom Kiely wrote:
>>> If all rx descriptors are processed while transient
>>> mbuf exhaustion is present, the rx ring ends up with
>>> no available descriptors. Thus no packets are received
>>> on that ring. Since descriptor refill is performed post
>>> rx descriptor processing, in this case no refill is
>>> ever subsequently performed resulting in permanent rx
>>> traffic drop.
>>>
>>> Signed-off-by: Tom Kiely <tkiely@brocade.com>
>>> ---
>>>   drivers/net/virtio/virtio_rxtx.c |    6 ++++--
>>>   1 file changed, 4 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/net/virtio/virtio_rxtx.c
>>> b/drivers/net/virtio/virtio_rxtx.c
>>> index 5770fa2..a95e234 100644
>>> --- a/drivers/net/virtio/virtio_rxtx.c
>>> +++ b/drivers/net/virtio/virtio_rxtx.c
>>> @@ -586,7 +586,8 @@ virtio_recv_pkts(void *rx_queue, struct rte_mbuf
>>> **rx_pkts, uint16_t nb_pkts)
>>>       if (likely(num > DESC_PER_CACHELINE))
>>>           num = num - ((rxvq->vq_used_cons_idx + num) %
>>> DESC_PER_CACHELINE);
>>>   -    if (num == 0)
>>> +    /* Refill free descriptors even if no pkts recvd */
>>> +    if (num == 0 && virtqueue_full(rxvq))
>> Should the return condition be that no used buffers and we have avail
>> descs in avail ring, i.e,
>>      num == 0 && rxvq->vq_free_cnt != rxvq->vq_nentries
>>
>> rather than
>>      num == 0 && rxvq->vq_free_cnt == 0
> Yes we could do that but I don't see a good reason to wait until the
> vq_free_cnt == vq_nentries
> before attempting the refill. The existing code will attempt refill
> even if only 1 packet was received
> and the free count is small. To me it seems safer to extend that to
> try refill even if no packet was received
> but the free count is non-zero.
The existing code attempt to refill only if 1 packet was received.

If we want to refill even no packet was received, then the strict
condition should be
    num == 0 && rxvq->vq_free_cnt != rxvq->vq_nentries

The safer condition, what you want to use,  should be
    num == 0 && !virtqueue_full(...)
rather than
    num == 0 && virtqueue_full(...)

We could simplify things a bit, just remove this check, if the following
receiving code already takes care of the "num == 0" condition.

I find virtqueue_full is confusing, maybe we could change it to some
other meaningful name.

>
>    Tom
>
>>>           return 0;
>>>         num = virtqueue_dequeue_burst_rx(rxvq, rcv_pkts, len, num);
>>> @@ -683,7 +684,8 @@ virtio_recv_mergeable_pkts(void *rx_queue,
>>>         virtio_rmb();
>>>   -    if (nb_used == 0)
>>> +    /* Refill free descriptors even if no pkts recvd */
>>> +    if (nb_used == 0 && virtqueue_full(rxvq))
>>>           return 0;
>>>         PMD_RX_LOG(DEBUG, "used:%d\n", nb_used);
>
>