RE: [PATCH v2] examples/vhost: fix retry logic on eth rx path

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

Hi Chenbo,

> -----Original Message-----
> From: Xia, Chenbo <chenbo.xia@intel.com>
> Sent: Tuesday, June 21, 2022 9:35 PM
> To: Wang, YuanX <yuanx.wang@intel.com>; maxime.coquelin@redhat.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 v2] examples/vhost: fix retry logic on eth rx path
> 
> Hi Yuan,
> 
> > -----Original Message-----
> > From: Wang, YuanX <yuanx.wang@intel.com>
> > Sent: Friday, June 17, 2022 3:02 PM
> > To: maxime.coquelin@redhat.com; Xia, Chenbo <chenbo.xia@intel.com>;
> > dev@dpdk.org
> > Cc: Hu, Jiayu <jiayu.hu@intel.com>; He, Xingguang
> > <xingguang.he@intel.com>; Wang, YuanX <yuanx.wang@intel.com>;
> > stable@dpdk.org; Ling, WeiX <weix.ling@intel.com>
> > Subject: [PATCH v2] examples/vhost: fix retry logic on eth rx path
> >
> > 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")
> > Cc: stable@dpdk.org
> >
> > Signed-off-by: Yuan Wang <yuanx.wang@intel.com>
> > Tested-by: Wei Ling <weix.ling@intel.com>
> > ---
> > V2: Rebase to 22.07 rc1.
> > ---
> >  examples/vhost/main.c | 27 ++++++++++-----------------
> >  1 file changed, 10 insertions(+), 17 deletions(-)
> >
> > diff --git a/examples/vhost/main.c b/examples/vhost/main.c index
> > e7fee5aa1b..e7a84333ed 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 +1383,20 @@ 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,
> > rx_count);
> 
> Looking at the code again, it seems current logic will enqueue more than
> rx_count packets and same packet may be sent multiple times as you are
> enqueue multiple times but using the same mbuf pointer and rx_count.
> 

Thanks for pointing that out, I did miss the packet index.
Will fix in the next version.

Thanks,
Yuan

> Thanks,
> Chenbo
> 
> >  		}
> >  	}
> >
> > -	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);
> > --
> > 2.25.1