Subject: Re: [PATCH net 3/3] net: enetc: reset xdp_tx_in_flight when updating bpf program

From: Maciej Fijalkowski <[email-masked]>
Date: Fri, 20 Sep 2024 15:04:48 +0200

On Thu, Sep 19, 2024 at 04:41:04PM +0800, Wei Fang wrote:
> When running "xdp-bench tx eno0" to test the XDP_TX feature of ENETC
> on LS1028A, it was found that if the command was re-run multiple times,
> Rx could not receive the frames, and the result of xdo-bench showed
> that the rx rate was 0.
> 
> root@ls1028ardb:~# ./xdp-bench tx eno0
> Hairpinning (XDP_TX) packets on eno0 (ifindex 3; driver fsl_enetc)
> Summary                      2046 rx/s                  0 err,drop/s
> Summary                         0 rx/s                  0 err,drop/s
> Summary                         0 rx/s                  0 err,drop/s
> Summary                         0 rx/s                  0 err,drop/s
> 
> By observing the Rx PIR and CIR registers, we found that CIR is always
> equal to 0x7FF and PIR is always 0x7FE, which means that the Rx ring
> is full and can no longer accommodate other Rx frames. Therefore, it
> is obvious that the RX BD ring has not been cleaned up.
> 
> Further analysis of the code revealed that the Rx BD ring will only
> be cleaned if the "cleaned_cnt > xdp_tx_in_flight" condition is met.
> Therefore, some debug logs were added to the driver and the current
> values of cleaned_cnt and xdp_tx_in_flight were printed when the Rx
> BD ring was full. The logs are as follows.
> 
> [  178.762419] [XDP TX] >> cleaned_cnt:1728, xdp_tx_in_flight:2140
> [  178.771387] [XDP TX] >> cleaned_cnt:1941, xdp_tx_in_flight:2110
> [  178.776058] [XDP TX] >> cleaned_cnt:1792, xdp_tx_in_flight:2110
> 
> From the results, we can see that the maximum value of xdp_tx_in_flight
> has reached 2140. However, the size of the Rx BD ring is only 2048. This
> is incredible, so checked the code again and found that the driver did
> not reset xdp_tx_in_flight when installing or uninstalling bpf program,
> resulting in xdp_tx_in_flight still retaining the value after the last
> command was run.
> 
> Fixes: c33bfaf91c4c ("net: enetc: set up XDP program under enetc_reconfigure()")
> Cc: [email-masked]
> Signed-off-by: Wei Fang <[email-masked]>
> ---
>  drivers/net/ethernet/freescale/enetc/enetc.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/net/ethernet/freescale/enetc/enetc.c b/drivers/net/ethernet/freescale/enetc/enetc.c
> index 5830c046cb7d..3cff76923ab9 100644
> --- a/drivers/net/ethernet/freescale/enetc/enetc.c
> +++ b/drivers/net/ethernet/freescale/enetc/enetc.c
> @@ -2769,6 +2769,7 @@ static int enetc_reconfigure_xdp_cb(struct enetc_ndev_priv *priv, void *ctx)
>  	for (i = 0; i < priv->num_rx_rings; i++) {
>  		struct enetc_bdr *rx_ring = priv->rx_ring[i];
>  
> +		rx_ring->xdp.xdp_tx_in_flight = 0;

zero init is good but shouldn't you be draining these buffers when
removing XDP resources at least? what happens with DMA mappings that are
related to these cached buffers?

>  		rx_ring->xdp.prog = prog;
>  
>  		if (prog)
> -- 
> 2.34.1
> 
>