net: introduce DST_NOCACHE flag

While doing stress tests with IP route cache disabled, and multi queue
devices, I noticed a very high contention on one rwlock used in
neighbour code.

When many cpus are trying to send frames (possibly using a high
performance multiqueue device) to the same neighbour, they fight for the
neigh->lock rwlock in order to call neigh_hh_init(), and fight on
hh->hh_refcnt (a pair of atomic_inc/atomic_dec_and_test())

But we dont need to call neigh_hh_init() for dst that are used only
once. It costs four atomic operations at least, on two contended cache
lines, plus the high contention on neigh->lock rwlock.

Introduce a new dst flag, DST_NOCACHE, that is set when dst was not
inserted in route cache.

With the stress test bench, sending 160000000 frames on one neighbour,
results are :

Before patch:

real	2m28.406s
user	0m11.781s
sys	36m17.964s


After patch:

real	1m26.532s
user	0m12.185s
sys	20m3.903s
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/dst.h

@@ -43,10 +43,11 @@ struct dst_entry {
 	short			error;
 	short			obsolete;
 	int			flags;
-#define DST_HOST		1
-#define DST_NOXFRM		2
-#define DST_NOPOLICY		4
-#define DST_NOHASH		8
+#define DST_HOST		0x0001
+#define DST_NOXFRM		0x0002
+#define DST_NOPOLICY		0x0004
+#define DST_NOHASH		0x0008
+#define DST_NOCACHE		0x0010
 	unsigned long		expires;
 
 	unsigned short		header_len;	/* more space at head required */

net/core/neighbour.c

@@ -1210,7 +1210,9 @@ int neigh_resolve_output(struct sk_buff *skb)
 	if (!neigh_event_send(neigh, skb)) {
 		int err;
 		struct net_device *dev = neigh->dev;
-		if (dev->header_ops->cache && !dst->hh) {
+		if (dev->header_ops->cache &&
+		    !dst->hh &&
+		    !(dst->flags & DST_NOCACHE)) {
 			write_lock_bh(&neigh->lock);
 			if (!dst->hh)
 				neigh_hh_init(neigh, dst, dst->ops->protocol);

net/ipv4/route.c

@@ -1107,6 +1107,7 @@ static int rt_intern_hash(unsigned hash, struct rtable *rt,
 		 * on the route gc list.
 		 */
 
+		rt->dst.flags |= DST_NOCACHE;
 		if (rt->rt_type == RTN_UNICAST || rt->fl.iif == 0) {
 			int err = arp_bind_neighbour(&rt->dst);
 			if (err) {