[dpdk-dev] [PATCH 1/5] virtio: clean up space checks on xmit

Xie, Huawei huawei.xie at intel.com
Mon Oct 19 10:02:07 CEST 2015


On 10/19/2015 1:16 PM, Stephen Hemminger wrote:
> The space check for transmit ring only needs a single conditional.
> I.e only need to recheck for space if there was no space in first check.
I see you reorganized the code. It is more clear and readable with your
change, but no check is removed.
We could remove the check after virtio_xmit_cleanup. In current
implementation, we assume the virtio_xmit_cleanup and vq_ring_free_chain
always succeed.
> This can help performance and simplifies loop.
>
> Signed-off-by: Stephen Hemminger <stephen at networkplumber.org>
> ---
>  drivers/net/virtio/virtio_rxtx.c | 66 ++++++++++++++++------------------------
>  1 file changed, 27 insertions(+), 39 deletions(-)
>
> diff --git a/drivers/net/virtio/virtio_rxtx.c b/drivers/net/virtio/virtio_rxtx.c
> index c5b53bb..5b50ed0 100644
> --- a/drivers/net/virtio/virtio_rxtx.c
> +++ b/drivers/net/virtio/virtio_rxtx.c
> @@ -745,7 +745,6 @@ uint16_t
>  virtio_xmit_pkts(void *tx_queue, struct rte_mbuf **tx_pkts, uint16_t nb_pkts)
>  {
>  	struct virtqueue *txvq = tx_queue;
> -	struct rte_mbuf *txm;
>  	uint16_t nb_used, nb_tx;
>  	int error;
>  
> @@ -759,57 +758,46 @@ virtio_xmit_pkts(void *tx_queue, struct rte_mbuf **tx_pkts, uint16_t nb_pkts)
>  	if (likely(nb_used > txvq->vq_nentries - txvq->vq_free_thresh))
>  		virtio_xmit_cleanup(txvq, nb_used);
>  
> -	nb_tx = 0;
> +	for (nb_tx = 0; nb_tx < nb_pkts; nb_tx++) {
> +		struct rte_mbuf *txm = tx_pkts[nb_tx];
> +		int need = txm->nb_segs - txvq->vq_free_cnt + 1;
>  
> -	while (nb_tx < nb_pkts) {
> -		/* Need one more descriptor for virtio header. */
> -		int need = tx_pkts[nb_tx]->nb_segs - txvq->vq_free_cnt + 1;
> -
> -		/*Positive value indicates it need free vring descriptors */
> +		/* Positive value indicates it need free vring descriptors */
As you fix the comment, if it is correct, could you do s/need/needs/ as
well, :)?
>  		if (unlikely(need > 0)) {
>  			nb_used = VIRTQUEUE_NUSED(txvq);
>  			virtio_rmb();
>  			need = RTE_MIN(need, (int)nb_used);
>  
>  			virtio_xmit_cleanup(txvq, need);
> -			need = (int)tx_pkts[nb_tx]->nb_segs -
> -				txvq->vq_free_cnt + 1;
> -		}
> -
> -		/*
> -		 * Zero or negative value indicates it has enough free
> -		 * descriptors to use for transmitting.
> -		 */
> -		if (likely(need <= 0)) {
> -			txm = tx_pkts[nb_tx];
> -
> -			/* Do VLAN tag insertion */
> -			if (unlikely(txm->ol_flags & PKT_TX_VLAN_PKT)) {
> -				error = rte_vlan_insert(&txm);
> -				if (unlikely(error)) {
> -					rte_pktmbuf_free(txm);
> -					++nb_tx;
> -					continue;
> -				}
Still need is rechecked here. It could be removed if virtio_xmit_cleanup
ensures need would be <= 0 after the call.
> +			need = txm->nb_segs - txvq->vq_free_cnt + 1;
> +			if (unlikely(need > 0)) {
> +				PMD_TX_LOG(ERR,
> +					   "No free tx descriptors to transmit");
> +				break;
>  			}
> +		}
>  
> -			/* Enqueue Packet buffers */
> -			error = virtqueue_enqueue_xmit(txvq, txm);
> +		/* Do VLAN tag insertion */
> +		if (unlikely(txm->ol_flags & PKT_TX_VLAN_PKT)) {
> +			error = rte_vlan_insert(&txm);
>  			if (unlikely(error)) {
> -				if (error == ENOSPC)
> -					PMD_TX_LOG(ERR, "virtqueue_enqueue Free count = 0");
> -				else if (error == EMSGSIZE)
> -					PMD_TX_LOG(ERR, "virtqueue_enqueue Free count < 1");
> -				else
> -					PMD_TX_LOG(ERR, "virtqueue_enqueue error: %d", error);
> -				break;
> +				rte_pktmbuf_free(txm);
> +				continue;
>  			}
> -			nb_tx++;
> -			txvq->bytes += txm->pkt_len;
> -		} else {
> -			PMD_TX_LOG(ERR, "No free tx descriptors to transmit");
> +		}
> +
> +		/* Enqueue Packet buffers */
> +		error = virtqueue_enqueue_xmit(txvq, txm);
> +		if (unlikely(error)) {
> +			if (error == ENOSPC)
> +				PMD_TX_LOG(ERR, "virtqueue_enqueue Free count = 0");
> +			else if (error == EMSGSIZE)
> +				PMD_TX_LOG(ERR, "virtqueue_enqueue Free count < 1");
> +			else
> +				PMD_TX_LOG(ERR, "virtqueue_enqueue error: %d", error);
>  			break;
>  		}
> +		txvq->bytes += txm->pkt_len;
>  	}
>  
>  	txvq->packets += nb_tx;



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