RE: [PATCHv17 bpf-next 1/6] bpf: run devmap xdp_prog on flush instead of bulk enqueue

[John Fastabend <john.fastabend@gmail.com>]

Hangbin Liu wrote:
> From: Jesper Dangaard Brouer <brouer@redhat.com>
> 
> This changes the devmap XDP program support to run the program when the
> bulk queue is flushed instead of before the frame is enqueued. This has
> a couple of benefits:
> 
> - It "sorts" the packets by destination devmap entry, and then runs the
>   same BPF program on all the packets in sequence. This ensures that we
>   keep the XDP program and destination device properties hot in I-cache.
> 
> - It makes the multicast implementation simpler because it can just
>   enqueue packets using bq_enqueue() without having to deal with the
>   devmap program at all.
> 
> The drawback is that if the devmap program drops the packet, the enqueue
> step is redundant. However, arguably this is mostly visible in a
> micro-benchmark, and with more mixed traffic the I-cache benefit should
> win out. The performance impact of just this patch is as follows:
> 
> The bq_xmit_all's logic is also refactored and error label is removed.
> When bq_xmit_all() is called from bq_enqueue(), another packet will
> always be enqueued immediately after, so clearing dev_rx, xdp_prog and
> flush_node in bq_xmit_all() is redundant. Let's move the clear to
> __dev_flush(), and only check them once in bq_enqueue() since they are
> all modified together.
> 
> By using xdp_redirect_map in sample/bpf and send pkts via pktgen cmd:
> ./pktgen_sample03_burst_single_flow.sh -i eno1 -d $dst_ip -m $dst_mac -t 10 -s 64
> 
> There are about +/- 0.1M deviation for native testing, the performance
> improved for the base-case, but some drop back with xdp devmap prog attached.
> 
> Version          | Test                           | Generic | Native | Native + 2nd xdp_prog
> 5.10 rc6         | xdp_redirect_map   i40e->i40e  |    2.0M |   9.1M |  8.0M
> 5.10 rc6         | xdp_redirect_map   i40e->veth  |    1.7M |  11.0M |  9.7M
> 5.10 rc6 + patch | xdp_redirect_map   i40e->i40e  |    2.0M |   9.5M |  7.5M
> 5.10 rc6 + patch | xdp_redirect_map   i40e->veth  |    1.7M |  11.6M |  9.1M
> 

[...]

> +static int dev_map_bpf_prog_run(struct bpf_prog *xdp_prog,
> +				struct xdp_frame **frames, int n,
> +				struct net_device *dev)
> +{
> +	struct xdp_txq_info txq = { .dev = dev };
> +	struct xdp_buff xdp;
> +	int i, nframes = 0;
> +
> +	for (i = 0; i < n; i++) {
> +		struct xdp_frame *xdpf = frames[i];
> +		u32 act;
> +		int err;
> +
> +		xdp_convert_frame_to_buff(xdpf, &xdp);
> +		xdp.txq = &txq;
> +
> +		act = bpf_prog_run_xdp(xdp_prog, &xdp);
> +		switch (act) {
> +		case XDP_PASS:
> +			err = xdp_update_frame_from_buff(&xdp, xdpf);
> +			if (unlikely(err < 0))
> +				xdp_return_frame_rx_napi(xdpf);
> +			else
> +				frames[nframes++] = xdpf;
> +			break;
> +		default:
> +			bpf_warn_invalid_xdp_action(act);
> +			fallthrough;
> +		case XDP_ABORTED:
> +			trace_xdp_exception(dev, xdp_prog, act);
> +			fallthrough;
> +		case XDP_DROP:
> +			xdp_return_frame_rx_napi(xdpf);
> +			break;
> +		}
> +	}
> +	return nframes; /* sent frames count */
> +}
> +
>  static void bq_xmit_all(struct xdp_dev_bulk_queue *bq, u32 flags)
>  {
>  	struct net_device *dev = bq->dev;
> -	int sent = 0, drops = 0, err = 0;
> +	unsigned int cnt = bq->count;
> +	int drops = 0, err = 0;
> +	int to_send = cnt;
> +	int sent = cnt;
>  	int i;
>  
> -	if (unlikely(!bq->count))
> +	if (unlikely(!cnt))
>  		return;
>  
> -	for (i = 0; i < bq->count; i++) {
> +	for (i = 0; i < cnt; i++) {
>  		struct xdp_frame *xdpf = bq->q[i];
>  
>  		prefetch(xdpf);
>  	}
>  
> -	sent = dev->netdev_ops->ndo_xdp_xmit(dev, bq->count, bq->q, flags);
> +	if (bq->xdp_prog) {
> +		to_send = dev_map_bpf_prog_run(bq->xdp_prog, bq->q, cnt, dev);
> +		if (!to_send) {
> +			sent = 0;
> +			goto out;
> +		}
> +		drops = cnt - to_send;
> +	}

I might be missing something about how *bq works here. What happens when
dev_map_bpf_prog_run returns to_send < cnt?

So I read this as it will send [0, to_send] and [to_send, cnt] will be
dropped? How do we know the bpf prog would have dropped the set,
[to_send+1, cnt]?

> +
> +	sent = dev->netdev_ops->ndo_xdp_xmit(dev, to_send, bq->q, flags);
>  	if (sent < 0) {
>  		err = sent;
>  		sent = 0;
> -		goto error;
> +
> +		/* If ndo_xdp_xmit fails with an errno, no frames have been
> +		 * xmit'ed and it's our responsibility to them free all.
> +		 */
> +		for (i = 0; i < cnt - drops; i++) {
> +			struct xdp_frame *xdpf = bq->q[i];
> +
> +			xdp_return_frame_rx_napi(xdpf);
> +		}
>  	}
> -	drops = bq->count - sent;
>  out:
> +	drops = cnt - sent;
>  	bq->count = 0;
>  
>  	trace_xdp_devmap_xmit(bq->dev_rx, dev, sent, drops, err);

I gather the remaining [to_send+1, cnt] packets are in fact dropped
because we set bq->count=0 and trace_xdp_devmap_xmit seems to think
they are dropped.

Also update the comment in trace_xdp_devmap_xmit,

   /* Catch API error of drv ndo_xdp_xmit sent more than count */

so that it reads to account for devmap progs as well?

> -	bq->dev_rx = NULL;
> -	__list_del_clearprev(&bq->flush_node);
>  	return;
> -error:
> -	/* If ndo_xdp_xmit fails with an errno, no frames have been
> -	 * xmit'ed and it's our responsibility to them free all.
> -	 */
> -	for (i = 0; i < bq->count; i++) {
> -		struct xdp_frame *xdpf = bq->q[i];
> -
> -		xdp_return_frame_rx_napi(xdpf);
> -		drops++;
> -	}
> -	goto out;
>  }

[...]
  
> -static struct xdp_buff *dev_map_run_prog(struct net_device *dev,
> -					 struct xdp_buff *xdp,
> -					 struct bpf_prog *xdp_prog)
> -{
> -	struct xdp_txq_info txq = { .dev = dev };
> -	u32 act;
> -
> -	xdp_set_data_meta_invalid(xdp);
> -	xdp->txq = &txq;
> -
> -	act = bpf_prog_run_xdp(xdp_prog, xdp);
> -	switch (act) {
> -	case XDP_PASS:
> -		return xdp;
> -	case XDP_DROP:
> -		break;
> -	default:
> -		bpf_warn_invalid_xdp_action(act);
> -		fallthrough;
> -	case XDP_ABORTED:
> -		trace_xdp_exception(dev, xdp_prog, act);
> -		break;
> -	}
> -
> -	xdp_return_buff(xdp);
> -	return NULL;
> -}
> -
>  int dev_xdp_enqueue(struct net_device *dev, struct xdp_buff *xdp,
>  		    struct net_device *dev_rx)
>  {
> -	return __xdp_enqueue(dev, xdp, dev_rx);
> +	return __xdp_enqueue(dev, xdp, dev_rx, NULL);
>  }
>  
>  int dev_map_enqueue(struct bpf_dtab_netdev *dst, struct xdp_buff *xdp,
> @@ -489,12 +516,7 @@ int dev_map_enqueue(struct bpf_dtab_netdev *dst, struct xdp_buff *xdp,
>  {
>  	struct net_device *dev = dst->dev;
>  
> -	if (dst->xdp_prog) {
> -		xdp = dev_map_run_prog(dev, xdp, dst->xdp_prog);
> -		if (!xdp)
> -			return 0;

So here it looks like dev_map_run_prog will not drop extra
packets, but will see every single packet.

Are we changing the semantics subtle here? This looks like
a problem to me. We should not drop packets in the new case
unless bpf program tells us to.

> -	}
> -	return __xdp_enqueue(dev, xdp, dev_rx);
> +	return __xdp_enqueue(dev, xdp, dev_rx, dst->xdp_prog);
>  }

Did I miss something? If not maybe a comment in the commit
message would help or in the code itself.

Thanks,
John