[PATCH v2 2/2] gro: optimise redundant parsing of packets

[Richard Gobert]

Currently the IPv6 extension headers are parsed twice: first in
ipv6_gro_receive, and then again in ipv6_gro_complete.

By using the new ->transport_proto field, and also storing the size of the
network header, we can avoid parsing extension headers a second time in
ipv6_gro_complete (which saves multiple memory dereferences and conditional
checks inside ipv6_exthdrs_len for a varying amount of extension headers in IPv6
packets).

The implementation had to handle both inner and outer layers in case of
encapsulation (as they can't use the same field).

Performance tests for TCP stream over IPv6 with a varying amount of extension
headers demonstrate throughput improvement of ~0.7%.

In addition, I fixed a potential existing problem:
 - The call to skb_set_inner_network_header at the beginning of
   ipv6_gro_complete calculates inner_network_header based on skb->data by
   calling skb_set_inner_network_header, and setting it to point to the beginning
   of the ip header.
 - If a packet is going to be handled by BIG TCP, the following code block is
   going to shift the packet header, and skb->data is going to be changed as
   well. 

When the two flows are combined, inner_network_header will point to the wrong
place.

The fix is to place the whole encapsulation branch after the BIG TCP code block.
This way, inner_network_header is calculated with a correct value of skb->data.
Also, by arranging the code that way, the optimisation does not add an additional
branch.

Signed-off-by: Richard Gobert <richardbgobert@xxxxxxxxx>
---
 include/net/gro.h      |  9 +++++++++
 net/ethernet/eth.c     | 14 +++++++++++---
 net/ipv6/ip6_offload.c | 20 +++++++++++++++-----
 3 files changed, 35 insertions(+), 8 deletions(-)

diff --git a/include/net/gro.h b/include/net/gro.h
index 7b47dd6ce94f..35f60ea99f6c 100644
--- a/include/net/gro.h
+++ b/include/net/gro.h
@@ -86,6 +86,15 @@ struct napi_gro_cb {
 
 	/* used to support CHECKSUM_COMPLETE for tunneling protocols */
 	__wsum	csum;
+
+	/* Used in ipv6_gro_receive() */
+	u16	network_len;
+
+	/* Used in eth_gro_receive() */
+	__be16	network_proto;
+
+	/* Used in ipv6_gro_receive() */
+	u8	transport_proto;
 };
 
 #define NAPI_GRO_CB(skb) ((struct napi_gro_cb *)(skb)->cb)
diff --git a/net/ethernet/eth.c b/net/ethernet/eth.c
index 2edc8b796a4e..c2b77d9401e4 100644
--- a/net/ethernet/eth.c
+++ b/net/ethernet/eth.c
@@ -439,6 +439,9 @@ struct sk_buff *eth_gro_receive(struct list_head *head, struct sk_buff *skb)
 		goto out;
 	}
 
+	if (!NAPI_GRO_CB(skb)->encap_mark)
+		NAPI_GRO_CB(skb)->network_proto = type;
+
 	skb_gro_pull(skb, sizeof(*eh));
 	skb_gro_postpull_rcsum(skb, eh, sizeof(*eh));
 
@@ -455,13 +458,18 @@ EXPORT_SYMBOL(eth_gro_receive);
 
 int eth_gro_complete(struct sk_buff *skb, int nhoff)
 {
-	struct ethhdr *eh = (struct ethhdr *)(skb->data + nhoff);
-	__be16 type = eh->h_proto;
 	struct packet_offload *ptype;
+	struct ethhdr *eh;
 	int err = -ENOSYS;
+	__be16 type;
 
-	if (skb->encapsulation)
+	if (skb->encapsulation) {
+		eh = (struct ethhdr *)(skb->data + nhoff);
 		skb_set_inner_mac_header(skb, nhoff);
+		type = eh->h_proto;
+	} else {
+		type = NAPI_GRO_CB(skb)->network_proto;
+	}
 
 	ptype = gro_find_complete_by_type(type);
 	if (ptype != NULL)
diff --git a/net/ipv6/ip6_offload.c b/net/ipv6/ip6_offload.c
index 00dc2e3b0184..6e3a923ad573 100644
--- a/net/ipv6/ip6_offload.c
+++ b/net/ipv6/ip6_offload.c
@@ -232,6 +232,11 @@ INDIRECT_CALLABLE_SCOPE struct sk_buff *ipv6_gro_receive(struct list_head *head,
 	flush--;
 	nlen = skb_network_header_len(skb);
 
+	if (!NAPI_GRO_CB(skb)->encap_mark) {
+		NAPI_GRO_CB(skb)->transport_proto = proto;
+		NAPI_GRO_CB(skb)->network_len = nlen;
+	}
+
 	list_for_each_entry(p, head, list) {
 		const struct ipv6hdr *iph2;
 		__be32 first_word; /* <Version:4><Traffic_Class:8><Flow_Label:20> */
@@ -324,10 +329,6 @@ INDIRECT_CALLABLE_SCOPE int ipv6_gro_complete(struct sk_buff *skb, int nhoff)
 	int err = -ENOSYS;
 	u32 payload_len;
 
-	if (skb->encapsulation) {
-		skb_set_inner_protocol(skb, cpu_to_be16(ETH_P_IPV6));
-		skb_set_inner_network_header(skb, nhoff);
-	}
 
 	payload_len = skb->len - nhoff - sizeof(*iph);
 	if (unlikely(payload_len > IPV6_MAXPLEN)) {
@@ -341,6 +342,7 @@ INDIRECT_CALLABLE_SCOPE int ipv6_gro_complete(struct sk_buff *skb, int nhoff)
 		skb->len += hoplen;
 		skb->mac_header -= hoplen;
 		skb->network_header -= hoplen;
+		NAPI_GRO_CB(skb)->network_len += hoplen;
 		iph = (struct ipv6hdr *)(skb->data + nhoff);
 		hop_jumbo = (struct hop_jumbo_hdr *)(iph + 1);
 
@@ -358,7 +360,15 @@ INDIRECT_CALLABLE_SCOPE int ipv6_gro_complete(struct sk_buff *skb, int nhoff)
 		iph->payload_len = htons(payload_len);
 	}
 
-	nhoff += sizeof(*iph) + ipv6_exthdrs_len(iph, &ops);
+	if (skb->encapsulation) {
+		skb_set_inner_protocol(skb, cpu_to_be16(ETH_P_IPV6));
+		skb_set_inner_network_header(skb, nhoff);
+		nhoff += sizeof(*iph) + ipv6_exthdrs_len(iph, &ops);
+	} else {
+		ops = rcu_dereference(inet6_offloads[NAPI_GRO_CB(skb)->transport_proto]);
+		nhoff += NAPI_GRO_CB(skb)->network_len;
+	}
+
 	if (WARN_ON(!ops || !ops->callbacks.gro_complete))
 		goto out;
 
-- 
2.36.1