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

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Date:   Thu, 28 Jan 2021 18:41:26 +0100
From:   Lorenzo Bianconi <lorenzo@...nel.org>
To:     Toshiaki Makita <toshiaki.makita1@...il.com>
Cc:     bpf@...r.kernel.org, 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.

ack, I will fix it in v2

> 
> > -		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'?)

right, I will fix it in v2

> 
> 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);

I agree, the code is more readable. I will fix it in v2.
I guess we can drop patch 2/3 and squash patch 1/3 and 3/3.

Regards,
Lorenzo

> 
> Toshiaki Makita

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