net: avoid RCU for NOCACHE dst

There is no point using RCU for dst we allocate for a very short time
(used once).

Change dst_release() to take DST_NOCACHE into account, but also change
skb_dst_set_noref() to force a refcount increment for such dst.

This is a _huge_ gain, because we dont waste memory to store xx thousand
of dsts. Instead of queueing them to RCU, we can free them instantly.

CPU caches can stay hot, re-using same memory blocks to hold temporary
dsts.

Note : remove unneeded smp_mb__before_atomic_dec(); in dst_release(),
since atomic_dec_return() implies a full memory barrier.

Stress test, 160.000.000 udp frames sent, IP route cache disabled
(DDOS).

Before:

real    0m38.091s
user    0m13.189s
sys     7m53.018s

After:

real	0m29.946s
user	0m12.157s
sys	7m40.605s

For reference, if IP route cache was enabled :

real	0m32.030s
user	0m10.521s
sys	8m15.243s

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 4 insertions, 5 deletions

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index ff98983d2a45..d6cb2bfcd8e1 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -1105,9 +1105,9 @@ restart:
                  * Note that we do rt_free on this new route entry, so that
                  * once its refcount hits zero, we are still able to reap it
                  * (Thanks Alexey)
-                 * Note also the rt_free uses call_rcu.  We don't actually
-                 * need rcu protection here, this is just our path to get
-                 * on the route gc list.
+                 * Note: To avoid expensive rcu stuff for this uncached dst,
+                 * we set DST_NOCACHE so that dst_release() can free dst without
+                 * waiting a grace period.
                  */
 
                 rt->dst.flags |= DST_NOCACHE;
@@ -1117,12 +1117,11 @@ restart:
                                 if (net_ratelimit())
                                         printk(KERN_WARNING
                                             "Neighbour table failure & not caching routes.\n");
-                                rt_drop(rt);
+                                ip_rt_put(rt);
                                 return err;
                         }
                 }
 
-                rt_free(rt);
                 goto skip_hashing;
         }