netdev - Re: [PATCH bpf-next 1/3] net: veth: introduce bulking for XDP

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Date:   Fri, 29 Jan 2021 00:17:27 +0900
From:   Toshiaki Makita <toshiaki.makita1@...il.com>
To:     Lorenzo Bianconi <lorenzo@...nel.org>, bpf@...r.kernel.org
Cc:     netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
        ast@...nel.org, daniel@...earbox.net, lorenzo.bianconi@...hat.com,
        brouer@...hat.com, toke@...hat.com
Subject: Re: [PATCH bpf-next 1/3] net: veth: introduce bulking for XDP_PASS

On 2021/01/27 3:41, Lorenzo Bianconi wrote:
> Introduce bulking support for XDP_PASS verdict forwarding skbs to
> the networking stack
> 
> Signed-off-by: Lorenzo Bianconi <lorenzo@...nel.org>
> ---
>   drivers/net/veth.c | 43 ++++++++++++++++++++++++++-----------------
>   1 file changed, 26 insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/net/veth.c b/drivers/net/veth.c
> index 6e03b619c93c..23137d9966da 100644
> --- a/drivers/net/veth.c
> +++ b/drivers/net/veth.c
> @@ -35,6 +35,7 @@
>   #define VETH_XDP_HEADROOM	(XDP_PACKET_HEADROOM + NET_IP_ALIGN)
>   
>   #define VETH_XDP_TX_BULK_SIZE	16
> +#define VETH_XDP_BATCH		8
>   
>   struct veth_stats {
>   	u64	rx_drops;
> @@ -787,27 +788,35 @@ static int veth_xdp_rcv(struct veth_rq *rq, int budget,
>   	int i, done = 0;
>   
>   	for (i = 0; i < budget; i++) {
> -		void *ptr = __ptr_ring_consume(&rq->xdp_ring);
> -		struct sk_buff *skb;
> +		void *frames[VETH_XDP_BATCH];
> +		void *skbs[VETH_XDP_BATCH];
> +		int i, n_frame, n_skb = 0;

'i' is a shadowed variable. I think this may be confusing.

>   
> -		if (!ptr)
> +		n_frame = __ptr_ring_consume_batched(&rq->xdp_ring, frames,
> +						     VETH_XDP_BATCH);

This apparently exceeds the budget.
This will process budget*VETH_XDP_BATCH packets at most.
(You are probably aware of this because you return 'i' instead of 'done'?)

Also I'm not sure if we need to introduce __ptr_ring_consume_batched() here.
The function just does __ptr_ring_consume() n times.

IIUC Your final code looks like this:

for (budget) {
	n_frame = __ptr_ring_consume_batched(VETH_XDP_BATCH);
	for (n_frame) {
		if (frame is XDP)
			xdpf[n_xdpf++] = to_xdp(frame);
		else
			skbs[n_skb++] = frame;
	}

	if (n_xdpf)
		veth_xdp_rcv_batch(xdpf);

	for (n_skb) {
		skb = veth_xdp_rcv_skb(skbs[i]);
		napi_gro_receive(skb);
	}
}

Your code processes VETH_XDP_BATCH packets at a time no matter whether each of them 
is xdp_frame or skb, but I think you actually want to process VETH_XDP_BATCH 
xdp_frames at a time?
Then, why not doing like this?

for (budget) {
	ptr = __ptr_ring_consume();
	if (ptr is XDP) {
		if (n_xdpf >= VETH_XDP_BATCH) {
			veth_xdp_rcv_batch(xdpf);
			n_xdpf = 0;
		}
		xdpf[n_xdpf++] = to_xdp(ptr);
	} else {
		skb = veth_xdp_rcv_skb(ptr);
		napi_gro_receive(skb);
	}
}
if (n_xdpf)
	veth_xdp_rcv_batch(xdpf);

Toshiaki Makita