Subject: Re: [PATCH bpf-next 6/6] ixgbe, xsk: finish napi loop if AF_XDP Rx queue is full

From: Jesper Dangaard Brouer <[email-masked]>
Date: Fri, 4 Sep 2020 17:35:40 +0200

On Fri,  4 Sep 2020 15:53:31 +0200
Björn Töpel <[email-masked]> wrote:

> From: Björn Töpel <[email-masked]>
> 
> Make the AF_XDP zero-copy path aware that the reason for redirect
> failure was due to full Rx queue. If so, exit the napi loop as soon as
> possible (exit the softirq processing), so that the userspace AF_XDP
> process can hopefully empty the Rx queue. This mainly helps the "one
> core scenario", where the userland process and Rx softirq processing
> is on the same core.
> 
> Note that the early exit can only be performed if the "need wakeup"
> feature is enabled, because otherwise there is no notification
> mechanism available from the kernel side.
> 
> This requires that the driver starts using the newly introduced
> xdp_do_redirect_ext() and xsk_do_redirect_rx_full() functions.
> 
> Signed-off-by: Björn Töpel <[email-masked]>
> ---
>  drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c | 23 ++++++++++++++------
>  1 file changed, 16 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c
> index 3771857cf887..a4aebfd986b3 100644
> --- a/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c
> +++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c
> @@ -93,9 +93,11 @@ int ixgbe_xsk_pool_setup(struct ixgbe_adapter *adapter,
>  
>  static int ixgbe_run_xdp_zc(struct ixgbe_adapter *adapter,
>  			    struct ixgbe_ring *rx_ring,
> -			    struct xdp_buff *xdp)
> +			    struct xdp_buff *xdp,
> +			    bool *early_exit)
>  {
>  	int err, result = IXGBE_XDP_PASS;
> +	enum bpf_map_type map_type;
>  	struct bpf_prog *xdp_prog;
>  	struct xdp_frame *xdpf;
>  	u32 act;
> @@ -116,8 +118,13 @@ static int ixgbe_run_xdp_zc(struct ixgbe_adapter *adapter,
>  		result = ixgbe_xmit_xdp_ring(adapter, xdpf);
>  		break;
>  	case XDP_REDIRECT:
> -		err = xdp_do_redirect(rx_ring->netdev, xdp, xdp_prog);
> -		result = !err ? IXGBE_XDP_REDIR : IXGBE_XDP_CONSUMED;
> +		err = xdp_do_redirect_ext(rx_ring->netdev, xdp, xdp_prog, &map_type);
> +		if (err) {
> +			*early_exit = xsk_do_redirect_rx_full(err, map_type);

Have you tried calling xdp_do_flush (that calls __xsk_map_flush()) and
(I guess) xsk_set_rx_need_wakeup() here, instead of stopping the loop?
(Or doing this in xsk core).

Looking at the code, the AF_XDP frames are "published" in the queue
rather late for AF_XDP.  Maybe in an orthogonal optimization, have you
considered "publishing" the ring producer when e.g. the queue is
half-full?

> +			result = IXGBE_XDP_CONSUMED;
> +		} else {
> +			result = IXGBE_XDP_REDIR;
> +		}
>  		break;
>  	default:
>  		bpf_warn_invalid_xdp_action(act);
> @@ -235,8 +242,8 @@ int ixgbe_clean_rx_irq_zc(struct ixgbe_q_vector *q_vector,
>  	unsigned int total_rx_bytes = 0, total_rx_packets = 0;
>  	struct ixgbe_adapter *adapter = q_vector->adapter;
>  	u16 cleaned_count = ixgbe_desc_unused(rx_ring);
> +	bool early_exit = false, failure = false;
>  	unsigned int xdp_res, xdp_xmit = 0;
> -	bool failure = false;
>  	struct sk_buff *skb;
>  
>  	while (likely(total_rx_packets < budget)) {
> @@ -288,7 +295,7 @@ int ixgbe_clean_rx_irq_zc(struct ixgbe_q_vector *q_vector,
>  
>  		bi->xdp->data_end = bi->xdp->data + size;
>  		xsk_buff_dma_sync_for_cpu(bi->xdp, rx_ring->xsk_pool);
> -		xdp_res = ixgbe_run_xdp_zc(adapter, rx_ring, bi->xdp);
> +		xdp_res = ixgbe_run_xdp_zc(adapter, rx_ring, bi->xdp, &early_exit);
>  
>  		if (xdp_res) {
>  			if (xdp_res & (IXGBE_XDP_TX | IXGBE_XDP_REDIR))
> @@ -302,6 +309,8 @@ int ixgbe_clean_rx_irq_zc(struct ixgbe_q_vector *q_vector,
>  
>  			cleaned_count++;
>  			ixgbe_inc_ntc(rx_ring);
> +			if (early_exit)
> +				break;
>  			continue;
>  		}
>  
> @@ -346,12 +355,12 @@ int ixgbe_clean_rx_irq_zc(struct ixgbe_q_vector *q_vector,
>  	q_vector->rx.total_bytes += total_rx_bytes;
>  
>  	if (xsk_uses_need_wakeup(rx_ring->xsk_pool)) {
> -		if (failure || rx_ring->next_to_clean == rx_ring->next_to_use)
> +		if (early_exit || failure || rx_ring->next_to_clean == rx_ring->next_to_use)
>  			xsk_set_rx_need_wakeup(rx_ring->xsk_pool);
>  		else
>  			xsk_clear_rx_need_wakeup(rx_ring->xsk_pool);
>  
> -		return (int)total_rx_packets;
> +		return early_exit ? 0 : (int)total_rx_packets;
>  	}
>  	return failure ? budget : (int)total_rx_packets;
>  }

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer