Re: [dpdk-stable] [PATCH] net/af_xdp: fix Tx halt when no recv packets

["Zhang, Qi Z" <qi.z.zhang@intel.com>]

> -----Original Message-----
> From: Ye, Xiaolong
> Sent: Tuesday, September 10, 2019 9:54 PM
> To: Zhang, Qi Z <qi.z.zhang@intel.com>
> Cc: Yigit, Ferruh <ferruh.yigit@intel.com>; Loftus, Ciara
> <ciara.loftus@intel.com>; dev@dpdk.org; stable@dpdk.org; Karlsson,
> Magnus <magnus.karlsson@intel.com>
> Subject: Re: [PATCH] net/af_xdp: fix Tx halt when no recv packets
> 
> On 09/10, Zhang, Qi Z wrote:
> >
> >
> >> -----Original Message-----
> >> From: Ye, Xiaolong
> >> Sent: Tuesday, September 10, 2019 12:13 AM
> >> To: Yigit, Ferruh <ferruh.yigit@intel.com>; Loftus, Ciara
> >> <ciara.loftus@intel.com>; Ye, Xiaolong <xiaolong.ye@intel.com>;
> >> Zhang, Qi Z <qi.z.zhang@intel.com>
> >> Cc: dev@dpdk.org; stable@dpdk.org
> >> Subject: [PATCH] net/af_xdp: fix Tx halt when no recv packets
> >>
> >> The kernel only consumes Tx packets if we have some Rx traffic on
> >> specified queue or we have called send(). So we need to issue a
> >> send() even when the allocation fails so that kernel will start to consume
> packets again.
> >
> >So "allocation fails" means " xsk_ring_prod__reserve" fail right?
> 
> Yes.
> 
> >I don't understand when xsk_ring_prod__needs_wakeup is true why kernel
> >will stop Tx packet at this situation would you share more insight?
> 
> Actually, the fail case is xsk_ring_prod__needs_wakeup is false, then we
> can't issue a send() when xsk_ring_prod__reserve fails.

Sorry, I think my question should be for the case when xsk_ring_prod__needs_wakeup is false,
I don't understand why we need to handle different at below two situations
1. when xsk_ring_prod__reserve fails
2. normal tx scenario.
My understanding is when xsk_ring_prod__needs_wakeup(tx) is false, which means Tx is ongoing, we don't need to wake up kernel to continue.

> 
> Thanks,
> Xiaolong
> 
> >
> >Thanks
> >Qi
> >
> >>
> >> Commit 45bba02c95b0 ("net/af_xdp: support need wakeup feature")
> >> breaks above rule by adding some condition to send, this patch fixes
> >> it while still keeps the need_wakeup feature for Tx.
> >>
> >> Fixes: 45bba02c95b0 ("net/af_xdp: support need wakeup feature")
> >> Cc: stable@dpdk.org
> >>
> >> Signed-off-by: Xiaolong Ye <xiaolong.ye@intel.com>
> >> ---
> >>  drivers/net/af_xdp/rte_eth_af_xdp.c | 28
> >> ++++++++++++++--------------
> >>  1 file changed, 14 insertions(+), 14 deletions(-)
> >>
> >> diff --git a/drivers/net/af_xdp/rte_eth_af_xdp.c
> >> b/drivers/net/af_xdp/rte_eth_af_xdp.c
> >> index 41ed5b2af..e496e9aaa 100644
> >> --- a/drivers/net/af_xdp/rte_eth_af_xdp.c
> >> +++ b/drivers/net/af_xdp/rte_eth_af_xdp.c
> >> @@ -286,19 +286,16 @@ kick_tx(struct pkt_tx_queue *txq)  {
> >>  	struct xsk_umem_info *umem = txq->pair->umem;
> >>
> >> -#if defined(XDP_USE_NEED_WAKEUP)
> >> -	if (xsk_ring_prod__needs_wakeup(&txq->tx))
> >> -#endif
> >> -		while (send(xsk_socket__fd(txq->pair->xsk), NULL,
> >> -			    0, MSG_DONTWAIT) < 0) {
> >> -			/* some thing unexpected */
> >> -			if (errno != EBUSY && errno != EAGAIN && errno != EINTR)
> >> -				break;
> >> -
> >> -			/* pull from completion queue to leave more space */
> >> -			if (errno == EAGAIN)
> >> -				pull_umem_cq(umem, ETH_AF_XDP_TX_BATCH_SIZE);
> >> -		}
> >> +	while (send(xsk_socket__fd(txq->pair->xsk), NULL,
> >> +		    0, MSG_DONTWAIT) < 0) {
> >> +		/* some thing unexpected */
> >> +		if (errno != EBUSY && errno != EAGAIN && errno != EINTR)
> >> +			break;
> >> +
> >> +		/* pull from completion queue to leave more space */
> >> +		if (errno == EAGAIN)
> >> +			pull_umem_cq(umem, ETH_AF_XDP_TX_BATCH_SIZE);
> >> +	}
> >>  	pull_umem_cq(umem, ETH_AF_XDP_TX_BATCH_SIZE);  }
> >>
> >> @@ -367,7 +364,10 @@ eth_af_xdp_tx(void *queue, struct rte_mbuf
> >> **bufs, uint16_t nb_pkts)
> >>
> >>  	xsk_ring_prod__submit(&txq->tx, nb_pkts);
> >>
> >> -	kick_tx(txq);
> >> +#if defined(XDP_USE_NEED_WAKEUP)
> >> +	if (xsk_ring_prod__needs_wakeup(&txq->tx))
> >> +#endif
> >> +		kick_tx(txq);
> >>
> >>  	txq->stats.tx_pkts += nb_pkts;
> >>  	txq->stats.tx_bytes += tx_bytes;
> >> --
> >> 2.17.1
> >