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

["Jiang, YuX" <yux.jiang@intel.com>]

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.

Best regards,
Yu Jiang

> -----Original Message-----
> From: Hengqi Chen <hengqi.chen@gmail.com>
> Sent: Monday, June 9, 2025 3:24 PM
> To: dev@dpdk.org
> Cc: Hengqi Chen <hengqi.chen@gmail.com>; Baoyuan Li
> <updoing@sina.com>; Maxime Coquelin <maxime.coquelin@redhat.com>;
> Chenbo Xia <chenbox@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@sina.com>
> Signed-off-by: Hengqi Chen <hengqi.chen@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