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, 28 insertions, 1 deletions

diff --git a/net/core/dst.c b/net/core/dst.c
index 32e542d7f472..8abe628b79f1 100644
--- a/net/core/dst.c
+++ b/net/core/dst.c
@@ -271,13 +271,40 @@ void dst_release(struct dst_entry *dst)
         if (dst) {
                 int newrefcnt;
 
-                smp_mb__before_atomic_dec();
                 newrefcnt = atomic_dec_return(&dst->__refcnt);
                 WARN_ON(newrefcnt < 0);
+                if (unlikely(dst->flags & DST_NOCACHE) && !newrefcnt) {
+                        dst = dst_destroy(dst);
+                        if (dst)
+                                __dst_free(dst);
+                }
         }
 }
 EXPORT_SYMBOL(dst_release);
 
+/**
+ * skb_dst_set_noref - sets skb dst, without a reference
+ * @skb: buffer
+ * @dst: dst entry
+ *
+ * Sets skb dst, assuming a reference was not taken on dst
+ * skb_dst_drop() should not dst_release() this dst
+ */
+void skb_dst_set_noref(struct sk_buff *skb, struct dst_entry *dst)
+{
+        WARN_ON(!rcu_read_lock_held() && !rcu_read_lock_bh_held());
+        /* If dst not in cache, we must take a reference, because
+         * dst_release() will destroy dst as soon as its refcount becomes zero
+         */
+        if (unlikely(dst->flags & DST_NOCACHE)) {
+                dst_hold(dst);
+                skb_dst_set(skb, dst);
+        } else {
+                skb->_skb_refdst = (unsigned long)dst | SKB_DST_NOREF;
+        }
+}
+EXPORT_SYMBOL(skb_dst_set_noref);
+
 /* Dirty hack. We did it in 2.2 (in __dst_free),
  * we have _very_ good reasons not to repeat
  * this mistake in 2.3, but we have no choice