Re: [dpdk-dev] [PATCH v2] net/af_packet: fix ignoring full ring on tx

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 9/13/2021 2:45 PM, Tudor Cornea wrote:
> The poll call can return POLLERR which is ignored, or it can return
> POLLOUT, even if there are no free frames in the mmap-ed area.
> 
> We can account for both of these cases by re-checking if the next
> frame is empty before writing into it.
> 
> Signed-off-by: Mihai Pogonaru <pogonarumihai@gmail.com>
> Signed-off-by: Tudor Cornea <tudor.cornea@gmail.com>
> ---
>  drivers/net/af_packet/rte_eth_af_packet.c | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
> 
> diff --git a/drivers/net/af_packet/rte_eth_af_packet.c b/drivers/net/af_packet/rte_eth_af_packet.c
> index b73b211..087c196 100644
> --- a/drivers/net/af_packet/rte_eth_af_packet.c
> +++ b/drivers/net/af_packet/rte_eth_af_packet.c
> @@ -216,6 +216,25 @@ eth_af_packet_tx(void *queue, struct rte_mbuf **bufs, uint16_t nb_pkts)
>  		    (poll(&pfd, 1, -1) < 0))
>  			break;
>  
> +		/*
> +		 * Poll can return POLLERR if the interface is down
> +		 *
> +		 * It will almost always return POLLOUT, even if there
> +		 * are no extra buffers available
> +		 *
> +		 * This happens, because packet_poll() calls datagram_poll()
> +		 * which checks the space left in the socket buffer and,
> +		 * in the case of packet_mmap, the default socket buffer length
> +		 * doesn't match the requested size for the tx_ring.
> +		 * As such, there is almost always space left in socket buffer,
> +		 * which doesn't seem to be correlated to the requested size
> +		 * for the tx_ring in packet_mmap.
> +		 *
> +		 * This results in poll() returning POLLOUT.
> +		 */
> +		if (ppd->tp_status != TP_STATUS_AVAILABLE)
> +			break;
> +

If 'POLLOUT' doesn't indicate that there is space in the buffer, what is the
point of the 'poll()' at all?

What can we test/reproduce the mentioned behavior? Or is there a way to fix the
behavior of poll() or use an alternative of it?


OK to break on the 'POLLERR', I guess it can be detected in the 'pfd.revent'.


>  		/* copy the tx frame data */
>  		pbuf = (uint8_t *) ppd + TPACKET2_HDRLEN -
>  			sizeof(struct sockaddr_ll);
>