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

["Wang, YuanX" <yuanx.wang@intel.com>]

Hi Maxime,

> -----Original Message-----
> From: Maxime Coquelin <maxime.coquelin@redhat.com>
> Sent: Wednesday, June 22, 2022 3:24 PM
> To: Wang, YuanX <yuanx.wang@intel.com>; Xia, Chenbo
> <chenbo.xia@intel.com>; dev@dpdk.org
> Cc: Hu, Jiayu <jiayu.hu@intel.com>; He, Xingguang
> <xingguang.he@intel.com>; stable@dpdk.org; Ling, WeiX
> <weix.ling@intel.com>
> Subject: Re: [PATCH v3] examples/vhost: fix retry logic on eth rx path
> 
> 
> 
> 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.

Yes, it looks like the wrong tag, Thanks for pointing out.
Will fix at next version.

Thanks,
Yuan

> 
> > Cc: stable@dpdk.org
> >
> > Signed-off-by: Yuan Wang <yuanx.wang@intel.com>
> > Tested-by: Wei Ling <weix.ling@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);