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

[Kyle Larose <eomereadig@gmail.com>]

On Tue, Jan 5, 2016 at 2:13 AM, Xie, Huawei <huawei.xie@intel.com> wrote:
> 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.
>

FWIW, I fixed this issue myself by just removing the if(num == 0)
checks entirely. I didn't see any benefit in short-circuiting a loop
which pretty much does nothing anyway when num == 0. Further, we only
hit this case when there's no packets to receive, which means there's
probably a few cycles to spare. This is even simpler.

> 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);
>>
>>
>