net: Check skb->rxhash in gro_receive

When initializing a gro_list for a packet, first check the rxhash of
the incoming skb against that of the skb's in the list. This should be
a very strong inidicator of whether the flow is going to be matched,
and potentially allows a lot of other checks to be short circuited.
Use skb_hash_raw so that we don't force the hash to be calculated.

Tested by running netperf 200 TCP_STREAMs between two machines with
GRO, HW rxhash, and 1G. Saw no performance degration, slight reduction
of time in dev_gro_receive.
Signed-off-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -3821,10 +3821,18 @@ static void gro_list_prepare(struct napi_struct *napi, struct sk_buff *skb)
 {
 	struct sk_buff *p;
 	unsigned int maclen = skb->dev->hard_header_len;
+	u32 hash = skb_get_hash_raw(skb);
 
 	for (p = napi->gro_list; p; p = p->next) {
 		unsigned long diffs;
 
+		NAPI_GRO_CB(p)->flush = 0;
+
+		if (hash != skb_get_hash_raw(p)) {
+			NAPI_GRO_CB(p)->same_flow = 0;
+			continue;
+		}
+
 		diffs = (unsigned long)p->dev ^ (unsigned long)skb->dev;
 		diffs |= p->vlan_tci ^ skb->vlan_tci;
 		if (maclen == ETH_HLEN)
@@ -3835,7 +3843,6 @@ static void gro_list_prepare(struct napi_struct *napi, struct sk_buff *skb)
 				       skb_gro_mac_header(skb),
 				       maclen);
 		NAPI_GRO_CB(p)->same_flow = !diffs;
-		NAPI_GRO_CB(p)->flush = 0;
 	}
 }