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>

1 files changed, 3 insertions, 1 deletions

diff --git a/net/core/neighbour.c b/net/core/neighbour.c
index 96b1a749a..b142a0d 100644
--- a/net/core/neighbour.c
+++ b/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);