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

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Wed, 27 Jan 2021 10:41:44 +0100

John Fastabend <[email-masked]> writes:

> Hangbin Liu wrote:
>> From: Jesper Dangaard Brouer <[email-masked]>
>> 
>> 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]?

Because dev_map_bpf_prog_run() compacts the array:

+		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;

[...]

>>  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.

It's not a change in semantics (see above), but I'll grant you that it's
subtle :)

-Toke