netdev - Re: [PATCH bpf-next 05/10] ixgbe: xsk: terminate NAPI when XSK Rx queue gets full

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Date:   Tue, 5 Apr 2022 14:36:41 +0200
From:   Jesper Dangaard Brouer <jbrouer@...hat.com>
To:     Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        bpf@...r.kernel.org, ast@...nel.org, daniel@...earbox.net,
        magnus.karlsson@...el.com, bjorn@...nel.org
Cc:     brouer@...hat.com, netdev@...r.kernel.org, maximmi@...dia.com,
        alexandr.lobakin@...el.com
Subject: Re: [PATCH bpf-next 05/10] ixgbe: xsk: terminate NAPI when XSK Rx
 queue gets full


On 05/04/2022 13.06, Maciej Fijalkowski wrote:
> Correlate -ENOBUFS that was returned from xdp_do_redirect() with a XSK
> Rx queue being full. In such case, terminate the softirq processing and
> let the user space to consume descriptors from XSK Rx queue so that
> there is room that driver can use later on.
> 
> Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
> ---
>   .../ethernet/intel/ixgbe/ixgbe_txrx_common.h  |  1 +
>   drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c  | 23 ++++++++++++-------
>   2 files changed, 16 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_txrx_common.h b/drivers/net/ethernet/intel/ixgbe/ixgbe_txrx_common.h
> index bba3feaf3318..f1f69ce67420 100644
> --- a/drivers/net/ethernet/intel/ixgbe/ixgbe_txrx_common.h
> +++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_txrx_common.h
> @@ -8,6 +8,7 @@
>   #define IXGBE_XDP_CONSUMED	BIT(0)
>   #define IXGBE_XDP_TX		BIT(1)
>   #define IXGBE_XDP_REDIR		BIT(2)
> +#define IXGBE_XDP_EXIT		BIT(3)
>   
>   #define IXGBE_TXD_CMD (IXGBE_TXD_CMD_EOP | \
>   		       IXGBE_TXD_CMD_RS)
> diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c
> index dd7ff66d422f..475244a2c6e4 100644
> --- a/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c
> +++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c
> @@ -109,9 +109,10 @@ static int ixgbe_run_xdp_zc(struct ixgbe_adapter *adapter,
>   
>   	if (likely(act == XDP_REDIRECT)) {
>   		err = xdp_do_redirect(rx_ring->netdev, xdp, xdp_prog);
> -		if (err)
> -			goto out_failure;
> -		return IXGBE_XDP_REDIR;
> +		if (!err)
> +			return IXGBE_XDP_REDIR;
> +		result = (err == -ENOBUFS) ? IXGBE_XDP_EXIT : IXGBE_XDP_CONSUMED;
> +		goto out_failure;
>   	}
>   
>   	switch (act) {
> @@ -130,6 +131,9 @@ static int ixgbe_run_xdp_zc(struct ixgbe_adapter *adapter,
>   		if (result == IXGBE_XDP_CONSUMED)
>   			goto out_failure;
>   		break;
> +	case XDP_DROP:
> +		result = IXGBE_XDP_CONSUMED;
> +		break;
>   	default:
>   		bpf_warn_invalid_xdp_action(rx_ring->netdev, xdp_prog, act);
>   		fallthrough;
> @@ -137,9 +141,6 @@ static int ixgbe_run_xdp_zc(struct ixgbe_adapter *adapter,
>   out_failure:
>   		trace_xdp_exception(rx_ring->netdev, xdp_prog, act);
>   		fallthrough; /* handle aborts by dropping packet */
> -	case XDP_DROP:
> -		result = IXGBE_XDP_CONSUMED;
> -		break;
>   	}
>   	return result;
>   }
> @@ -304,10 +305,16 @@ int ixgbe_clean_rx_irq_zc(struct ixgbe_q_vector *q_vector,
>   		xdp_res = ixgbe_run_xdp_zc(adapter, rx_ring, bi->xdp);
>   
>   		if (xdp_res) {
> -			if (xdp_res & (IXGBE_XDP_TX | IXGBE_XDP_REDIR))
> +			if (xdp_res == IXGBE_XDP_EXIT) {

Micro optimization note: Having this as the first if()-statement
defaults the compiler to think this is the likely() case. (But compilers
can obviously be smarter and can easily choose that the IXGBE_XDP_REDIR
branch is so simple that it takes it as the likely case).
Just wanted to mention this, given this is fash-path code.

> +				failure = true;
> +				xsk_buff_free(bi->xdp);
> +				ixgbe_inc_ntc(rx_ring);
> +				break;

I was wondering if we have a situation where we should set xdp_xmit bit
to trigger the call to xdp_do_flush_map later in function, but I assume
you have this covered.

> +			} else if (xdp_res & (IXGBE_XDP_TX | IXGBE_XDP_REDIR)) {
>   				xdp_xmit |= xdp_res;
> -			else
> +			} else {
>   				xsk_buff_free(bi->xdp);
> +			}
>   
>   			bi->xdp = NULL;
>   			total_rx_packets++;