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

[Stephen Hemminger <stephen@networkplumber.org>]

On Mon, 19 Oct 2015 08:02:07 +0000
"Xie, Huawei" <huawei.xie@intel.com> wrote:

> 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.

The only case that matters is if virtio_xmit_cleanup can not
free up any slots.

> > This can help performance and simplifies loop.
> >
> > Signed-off-by: Stephen Hemminger <stephen@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;
>