[PATCH] net/virtio: Fix check of threshold for Tx freeing

[Maxime Coquelin]

Hi Yu, Hengqi,

First, thanks Yu for testing and reporting the performance degradation,
this is much appreciated!

On 7/4/25 1:29 PM, Jiang, YuX wrote:
> Hi Hengqi,
> 
> Can you help to check the ticket: https://bugs.dpdk.org/show_bug.cgi?id=1747? The perf drop is caused by your patch. Thanks so much.

The options I see are:
1. Revert the patch for v25.07, fix it and resubmit for v25.11.
2. Fix it on time for v25.07-rc3
3. Keep the patch in v25.07 and try to fix it in v25.11.

As I will be OoO starting this evening, I would be in favor of option 1.
What do you think?

Thanks,
Maxime

> Best regards,
> Yu Jiang
> 
>> -----Original Message-----
>> From: Hengqi Chen <hengqi.chen at gmail.com>
>> Sent: Monday, June 9, 2025 3:24 PM
>> To: dev at dpdk.org
>> Cc: Hengqi Chen <hengqi.chen at gmail.com>; Baoyuan Li
>> <updoing at sina.com>; Maxime Coquelin <maxime.coquelin at redhat.com>;
>> Chenbo Xia <chenbox at nvidia.com>
>> Subject: [PATCH] net/virtio: Fix check of threshold for Tx freeing
>>
>> Like most dirvers, make the fast path of virtio_xmit_cleanup() behave as
>> described by the comments of rte_eth_txconf::tx_free_thresh ([0]):
>>      Start freeing Tx buffers if there are
>>      less free descriptors than this value.
>>
>> The rationale behind this change is that:
>>    * vq->vq_nentries is set during device probe with the queue size specified
>>      by vhost backend, this value does not reflect the real nb_tx_desc
>>    * the real available tx desc is set to vq->vq_free_cnt via the nb_tx_desc
>>      param of rte_eth_tx_queue_setup() API
>>    * so `nb_used > vq->vq_nentries - vq->vq_free_thresh` could never be true
>>      if say nb_tx_desc=2048, vq->vq_nentries=4096, vq->vq_free_thresh=32,
>>      see bug report 1716 ([1]) for details.
>>
>> Bugzilla ID: 1716
>>
>>    [0]: https://github.com/DPDK/dpdk/commit/72514b5d5543
>>    [1]: https://bugs.dpdk.org/show_bug.cgi?id=1716
>>
>> Signed-off-by: Baoyuan Li <updoing at sina.com>
>> Signed-off-by: Hengqi Chen <hengqi.chen at gmail.com>
>> ---
>>   drivers/net/virtio/virtio_rxtx.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/net/virtio/virtio_rxtx.c b/drivers/net/virtio/virtio_rxtx.c
>> index edecd2011f..ab97f03d7d 100644
>> --- a/drivers/net/virtio/virtio_rxtx.c
>> +++ b/drivers/net/virtio/virtio_rxtx.c
>> @@ -1873,7 +1873,7 @@ virtio_xmit_pkts(void *tx_queue, struct rte_mbuf
>> **tx_pkts, uint16_t nb_pkts)
>>
>>   	nb_used = virtqueue_nused(vq);
>>
>> -	if (likely(nb_used > vq->vq_nentries - vq->vq_free_thresh))
>> +	if (likely(vq->vq_free_cnt < vq->vq_free_thresh))
>>   		virtio_xmit_cleanup(vq, nb_used);
>>
>>   	for (nb_tx = 0; nb_tx < nb_pkts; nb_tx++) {
>> --
>> 2.43.5
>