Re: [PATCH 19.11] net/af_packet: fix ignoring full ring on Tx

[Christian Ehrhardt <christian.ehrhardt@canonical.com>]

On Wed, Dec 1, 2021 at 12:02 PM Tudor Cornea <tudor.cornea@gmail.com> wrote:
>
> [ upstream commit f86d553cc180f9a65e115edb5641a49bbf2cf2f0 ]


Thanks, applied

> 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.
>
> We have attempted to reproduce this issue with pktgen-dpdk, using
> the following configuration.
>
> pktgen -l 1-4 -n 4 --proc-type=primary --no-pci --no-telemetry \
>     --no-huge -m 512 \
>     --vdev=net_af_packet0,iface=eth1,blocksz=16384,framesz=8192, \
>     framecnt=2048,qpairs=1,qdisc_bypass=0 \
>     -- \
>     -P \
>     -T \
>     -m "3.0" \
>     -f themes/black-yellow.theme
>
> We configure a low tx rate (~ 335 packets / second) and a small
> packet size, of about 300 Bytes from the pktgen CLI.
>
> set 0 size 300
> set 0 rate 0.008
> set 0 burst 1
> start 0
>
> After bringing the interface down, and up again, we seem to arrive
> in a state in which the tx rate is inconsistent, and does not recover.
>
> ifconfig eth1 down; sleep 7; ifconfig eth1 up
>
> [1] http://code.dpdk.org/pktgen-dpdk/pktgen-20.11.2/source/INSTALL.md
>
> Fixes: 364e08f2bbc0 ("af_packet: add PMD for AF_PACKET-based virtual devices")
>
> Signed-off-by: Mihai Pogonaru <pogonarumihai@gmail.com>
> Signed-off-by: Tudor Cornea <tudor.cornea@gmail.com>
> Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>
> ---
>  drivers/net/af_packet/rte_eth_af_packet.c | 26 ++++++++++++++++++++++++--
>  1 file changed, 24 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/net/af_packet/rte_eth_af_packet.c b/drivers/net/af_packet/rte_eth_af_packet.c
> index 00387ed..dbbe8e5 100644
> --- a/drivers/net/af_packet/rte_eth_af_packet.c
> +++ b/drivers/net/af_packet/rte_eth_af_packet.c
> @@ -213,8 +213,30 @@ eth_af_packet_tx(void *queue, struct rte_mbuf **bufs, uint16_t nb_pkts)
>                 }
>
>                 /* point at the next incoming frame */
> -               if ((ppd->tp_status != TP_STATUS_AVAILABLE) &&
> -                   (poll(&pfd, 1, -1) < 0))
> +               if (ppd->tp_status != TP_STATUS_AVAILABLE) {
> +                       if (poll(&pfd, 1, -1) < 0)
> +                               break;
> +
> +                       /* poll() can return POLLERR if the interface is down */
> +                       if (pfd.revents & POLLERR)
> +                               break;
> +               }
> +
> +               /*
> +                * poll() 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;
>
>                 /* copy the tx frame data */
> --
> 2.7.4
>


-- 
Christian Ehrhardt
Staff Engineer, Ubuntu Server
Canonical Ltd