bpf: Run devmap xdp_prog on flush instead of bulk enqueue

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:

Using 2 10Gb i40e NIC, redirecting one to another, or into a veth interface,
which do XDP_DROP on veth peer. With xdp_redirect_map in sample/bpf, 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
5.12 rc4         | xdp_redirect_map   i40e->i40e  |    1.9M |   9.6M |  8.4M
5.12 rc4         | xdp_redirect_map   i40e->veth  |    1.7M |  11.7M |  9.8M
5.12 rc4 + patch | xdp_redirect_map   i40e->i40e  |    1.9M |   9.8M |  8.0M
5.12 rc4 + patch | xdp_redirect_map   i40e->veth  |    1.7M |  12.0M |  9.4M

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. Move the clear to __dev_flush(),
and only check them once in bq_enqueue() since they are all modified
together.

This change also has the side effect of extending the lifetime of the
RCU-protected xdp_prog that lives inside the devmap entries: Instead of
just living for the duration of the XDP program invocation, the
reference now lives all the way until the bq is flushed. This is safe
because the bq flush happens at the end of the NAPI poll loop, so
everything happens between a local_bh_disable()/local_bh_enable() pair.
However, this is by no means obvious from looking at the call sites; in
particular, some drivers have an additional rcu_read_lock() around only
the XDP program invocation, which only confuses matters further.
Cleaning this up will be done in a separate patch series.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20210519090747.1655268-2-liuhangbin@gmail.com

kernel/bpf/devmap.c

@@ -57,6 +57,7 @@ struct xdp_dev_bulk_queue {
 	struct list_head flush_node;
 	struct net_device *dev;
 	struct net_device *dev_rx;
+	struct bpf_prog *xdp_prog;
 	unsigned int count;
 };
 
@@ -326,22 +327,71 @@ bool dev_map_can_have_prog(struct bpf_map *map)
 	return false;
 }
 
+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, err = 0;
+	int sent = 0, drops = 0, err = 0;
+	unsigned int cnt = bq->count;
+	int to_send = 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)
+			goto out;
+
+		drops = cnt - to_send;
+	}
+
+	sent = dev->netdev_ops->ndo_xdp_xmit(dev, to_send, bq->q, flags);
 	if (sent < 0) {
 		/* If ndo_xdp_xmit fails with an errno, no frames have
 		 * been xmit'ed.
@@ -353,13 +403,13 @@ static void bq_xmit_all(struct xdp_dev_bulk_queue *bq, u32 flags)
 	/* If not all frames have been transmitted, it is our
 	 * responsibility to free them
 	 */
-	for (i = sent; unlikely(i < bq->count); i++)
+	for (i = sent; unlikely(i < to_send); i++)
 		xdp_return_frame_rx_napi(bq->q[i]);
 
-	trace_xdp_devmap_xmit(bq->dev_rx, dev, sent, bq->count - sent, err);
+out:
-	bq->dev_rx = NULL;
+	drops = cnt - sent;
 	bq->count = 0;
-	__list_del_clearprev(&bq->flush_node);
+	trace_xdp_devmap_xmit(bq->dev_rx, dev, sent, drops, err);
 }
 
 /* __dev_flush is called from xdp_do_flush() which _must_ be signaled
@@ -377,8 +427,12 @@ void __dev_flush(void)
 	struct list_head *flush_list = this_cpu_ptr(&dev_flush_list);
 	struct xdp_dev_bulk_queue *bq, *tmp;
 
-	list_for_each_entry_safe(bq, tmp, flush_list, flush_node)
+	list_for_each_entry_safe(bq, tmp, flush_list, flush_node) {
 		bq_xmit_all(bq, XDP_XMIT_FLUSH);
+		bq->dev_rx = NULL;
+		bq->xdp_prog = NULL;
+		__list_del_clearprev(&bq->flush_node);
+	}
 }
 
 /* rcu_read_lock (from syscall and BPF contexts) ensures that if a delete and/or
@@ -401,7 +455,7 @@ static void *__dev_map_lookup_elem(struct bpf_map *map, u32 key)
  * Thus, safe percpu variable access.
  */
 static void bq_enqueue(struct net_device *dev, struct xdp_frame *xdpf,
-		       struct net_device *dev_rx)
+		       struct net_device *dev_rx, struct bpf_prog *xdp_prog)
 {
 	struct list_head *flush_list = this_cpu_ptr(&dev_flush_list);
 	struct xdp_dev_bulk_queue *bq = this_cpu_ptr(dev->xdp_bulkq);
@@ -412,18 +466,22 @@ static void bq_enqueue(struct net_device *dev, struct xdp_frame *xdpf,
 	/* Ingress dev_rx will be the same for all xdp_frame's in
 	 * bulk_queue, because bq stored per-CPU and must be flushed
 	 * from net_device drivers NAPI func end.
+	 *
+	 * Do the same with xdp_prog and flush_list since these fields
+	 * are only ever modified together.
 	 */
-	if (!bq->dev_rx)
+	if (!bq->dev_rx) {
 		bq->dev_rx = dev_rx;
+		bq->xdp_prog = xdp_prog;
+		list_add(&bq->flush_node, flush_list);
+	}
 
 	bq->q[bq->count++] = xdpf;
-
-	if (!bq->flush_node.prev)
-		list_add(&bq->flush_node, flush_list);
 }
 
 static inline int __xdp_enqueue(struct net_device *dev, struct xdp_buff *xdp,
-			       struct net_device *dev_rx)
+				struct net_device *dev_rx,
+				struct bpf_prog *xdp_prog)
 {
 	struct xdp_frame *xdpf;
 	int err;
@@ -439,42 +497,14 @@ static inline int __xdp_enqueue(struct net_device *dev, struct xdp_buff *xdp,
 	if (unlikely(!xdpf))
 		return -EOVERFLOW;
 
-	bq_enqueue(dev, xdpf, dev_rx);
+	bq_enqueue(dev, xdpf, dev_rx, xdp_prog);
 	return 0;
 }
 
-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,
@@ -482,12 +512,7 @@ int dev_map_enqueue(struct bpf_dtab_netdev *dst, struct xdp_buff *xdp,
 {
 	struct net_device *dev = dst->dev;
 
-	if (dst->xdp_prog) {
+	return __xdp_enqueue(dev, xdp, dev_rx, dst->xdp_prog);
-		xdp = dev_map_run_prog(dev, xdp, dst->xdp_prog);
-		if (!xdp)
-			return 0;
-	}
-	return __xdp_enqueue(dev, xdp, dev_rx);
 }
 
 int dev_map_generic_redirect(struct bpf_dtab_netdev *dst, struct sk_buff *skb,