ipv4: udp: Optimise multicast reception

UDP multicast rx path is a bit complex and can hold a spinlock
for a long time.

Using a small (32 or 64 entries) stack of socket pointers can help
to perform expensive operations (skb_clone(), udp_queue_rcv_skb())
outside of the lock, in most cases.

It's also a base for a future RCU conversion of multicast recption.

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

1 files changed, 50 insertions, 26 deletions

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index dd7f3d20989a..9d9072c6cce7 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -1329,49 +1329,73 @@ drop:
         return -1;
 }
 
+
+static void flush_stack(struct sock **stack, unsigned int count,
+                        struct sk_buff *skb, unsigned int final)
+{
+        unsigned int i;
+        struct sk_buff *skb1 = NULL;
+
+        for (i = 0; i < count; i++) {
+                if (likely(skb1 == NULL))
+                        skb1 = (i == final) ? skb : skb_clone(skb, GFP_ATOMIC);
+
+                if (skb1 && udp_queue_rcv_skb(stack[i], skb1) <= 0)
+                        skb1 = NULL;
+        }
+        if (unlikely(skb1))
+                kfree_skb(skb1);
+}
+
 /*
  *      Multicasts and broadcasts go to each listener.
  *
- *      Note: called only from the BH handler context,
- *      so we don't need to lock the hashes.
+ *      Note: called only from the BH handler context.
  */
 static int __udp4_lib_mcast_deliver(struct net *net, struct sk_buff *skb,
                                     struct udphdr  *uh,
                                     __be32 saddr, __be32 daddr,
                                     struct udp_table *udptable)
 {
-        struct sock *sk;
+        struct sock *sk, *stack[256 / sizeof(struct sock *)];
         struct udp_hslot *hslot = udp_hashslot(udptable, net, ntohs(uh->dest));
         int dif;
+        unsigned int i, count = 0;
 
         spin_lock(&hslot->lock);
         sk = sk_nulls_head(&hslot->head);
         dif = skb->dev->ifindex;
         sk = udp_v4_mcast_next(net, sk, uh->dest, daddr, uh->source, saddr, dif);
-        if (sk) {
-                struct sock *sknext = NULL;
-
-                do {
-                        struct sk_buff *skb1 = skb;
-
-                        sknext = udp_v4_mcast_next(net, sk_nulls_next(sk), uh->dest,
-                                                   daddr, uh->source, saddr,
-                                                   dif);
-                        if (sknext)
-                                skb1 = skb_clone(skb, GFP_ATOMIC);
-
-                        if (skb1) {
-                                int ret = udp_queue_rcv_skb(sk, skb1);
-                                if (ret > 0)
-                                        /* we should probably re-process instead
-                                         * of dropping packets here. */
-                                        kfree_skb(skb1);
-                        }
-                        sk = sknext;
-                } while (sknext);
-        } else
-                consume_skb(skb);
+        while (sk) {
+                stack[count++] = sk;
+                sk = udp_v4_mcast_next(net, sk_nulls_next(sk), uh->dest,
+                                       daddr, uh->source, saddr, dif);
+                if (unlikely(count == ARRAY_SIZE(stack))) {
+                        if (!sk)
+                                break;
+                        flush_stack(stack, count, skb, ~0);
+                        count = 0;
+                }
+        }
+        /*
+         * before releasing chain lock, we must take a reference on sockets
+         */
+        for (i = 0; i < count; i++)
+                sock_hold(stack[i]);
+
         spin_unlock(&hslot->lock);
+
+        /*
+         * do the slow work with no lock held
+         */
+        if (count) {
+                flush_stack(stack, count, skb, count - 1);
+
+                for (i = 0; i < count; i++)
+                        sock_put(stack[i]);
+        } else {
+                kfree_skb(skb);
+        }
         return 0;
 }