[PATCH v3] examples/vhost: fix retry logic on eth rx path

[Maxime Coquelin]

On 6/22/22 08:27, Yuan Wang wrote:
> drain_eth_rx() uses rte_vhost_avail_entries() to calculate
> the available entries to determine if a retry is required.
> However, this function only works with split rings, and
> calculating packed rings will return the wrong value and cause
> unnecessary retries resulting in a significant performance penalty.
> 
> This patch fix that by using the difference between tx/rx burst
> as the retry condition.
> 
> Fixes: 4ecf22e356de ("vhost: export device id as the interface to applications")

Are you sure about the Fixes tag?
It seems to be unrelated to the issue this patch is addressing.

> Cc: stable at dpdk.org
> 
> Signed-off-by: Yuan Wang <yuanx.wang at intel.com>
> Tested-by: Wei Ling <weix.ling at intel.com>
> ---
> V3: Fix mbuf index.
> V2: Rebase to 22.07 rc1.
> ---
>   examples/vhost/main.c | 28 +++++++++++-----------------
>   1 file changed, 11 insertions(+), 17 deletions(-)
> 
> diff --git a/examples/vhost/main.c b/examples/vhost/main.c
> index e7fee5aa1b..0fa6c096c8 100644
> --- a/examples/vhost/main.c
> +++ b/examples/vhost/main.c
> @@ -634,7 +634,7 @@ us_vhost_usage(const char *prgname)
>   {
>   	RTE_LOG(INFO, VHOST_CONFIG, "%s [EAL options] -- -p PORTMASK\n"
>   	"		--vm2vm [0|1|2]\n"
> -	"		--rx_retry [0|1] --mergeable [0|1] --stats [0-N]\n"
> +	"		--rx-retry [0|1] --mergeable [0|1] --stats [0-N]\n"
>   	"		--socket-file <path>\n"
>   	"		--nb-devices ND\n"
>   	"		-p PORTMASK: Set mask for ports to be used by application\n"
> @@ -1383,27 +1383,21 @@ drain_eth_rx(struct vhost_dev *vdev)
>   	if (!rx_count)
>   		return;
>   
> -	/*
> -	 * When "enable_retry" is set, here we wait and retry when there
> -	 * is no enough free slots in the queue to hold @rx_count packets,
> -	 * to diminish packet loss.
> -	 */
> -	if (enable_retry &&
> -	    unlikely(rx_count > rte_vhost_avail_entries(vdev->vid,
> -			VIRTIO_RXQ))) {
> -		uint32_t retry;
> +	enqueue_count = vdev_queue_ops[vdev->vid].enqueue_pkt_burst(vdev,
> +						VIRTIO_RXQ, pkts, rx_count);
>   
> -		for (retry = 0; retry < burst_rx_retry_num; retry++) {
> +	/* Retry if necessary */
> +	if (enable_retry && unlikely(enqueue_count < rx_count)) {
> +		uint32_t retry = 0;
> +
> +		while (enqueue_count < rx_count && retry++ < burst_rx_retry_num) {
>   			rte_delay_us(burst_rx_delay_time);
> -			if (rx_count <= rte_vhost_avail_entries(vdev->vid,
> -					VIRTIO_RXQ))
> -				break;
> +			enqueue_count += vdev_queue_ops[vdev->vid].enqueue_pkt_burst(vdev,
> +							VIRTIO_RXQ, &pkts[enqueue_count],
> +							rx_count - enqueue_count);
>   		}
>   	}
>   
> -	enqueue_count = vdev_queue_ops[vdev->vid].enqueue_pkt_burst(vdev,
> -					VIRTIO_RXQ, pkts, rx_count);
> -
>   	if (enable_stats) {
>   		__atomic_add_fetch(&vdev->stats.rx_total_atomic, rx_count,
>   				__ATOMIC_SEQ_CST);