ipv6: Avoid rt6_probe() taking writer lock in the fast path

The patch checks neigh->nud_state before acquiring the writer lock.
Note that rt6_probe() is only used in CONFIG_IPV6_ROUTER_PREF.

40 udpflood processes and a /64 gateway route are used.
The gateway has NUD_PERMANENT.  Each of them is run for 30s.
At the end, the total number of finished sendto():

Before: 55M
After: 95M

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>
CC: Julian Anastasov <ja@ssi.bg>
CC: YOSHIFUJI Hideaki <hideaki.yoshifuji@miraclelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 0ef52623fc7778b37a19bc4dc8f520381f2ea61f..54fccf0d705ddee83e3ba1e1b655fc58cd5c38a4 100644 (file)
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -560,6 +560,9 @@ static void rt6_probe(struct rt6_info *rt)
        rcu_read_lock_bh();
        neigh = __ipv6_neigh_lookup_noref(rt->dst.dev, &rt->rt6i_gateway);
        if (neigh) {
+               if (neigh->nud_state & NUD_VALID)
+                       goto out;
+
                work = NULL;
                write_lock(&neigh->lock);
                if (!(neigh->nud_state & NUD_VALID) &&
@@ -583,6 +586,7 @@ static void rt6_probe(struct rt6_info *rt)
                schedule_work(&work->work);
        }
 
+out:
        rcu_read_unlock_bh();
 }
 #else