udp: use a separate rx queue for packet reception

under udp flood the sk_receive_queue spinlock is heavily contended.
This patch try to reduce the contention on such lock adding a
second receive queue to the udp sockets; recvmsg() looks first
in such queue and, only if empty, tries to fetch the data from
sk_receive_queue. The latter is spliced into the newly added
queue every time the receive path has to acquire the
sk_receive_queue lock.

The accounting of forward allocated memory is still protected with
the sk_receive_queue lock, so udp_rmem_release() needs to acquire
both locks when the forward deficit is flushed.

On specific scenarios we can end up acquiring and releasing the
sk_receive_queue lock multiple times; that will be covered by
the next patch

Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/udp.h

@@ -80,6 +80,9 @@ struct udp_sock {
 						struct sk_buff *skb,
 						int nhoff);
 
+	/* udp_recvmsg try to use this before splicing sk_receive_queue */
+	struct sk_buff_head	reader_queue ____cacheline_aligned_in_smp;
+
 	/* This field is dirtied by udp_recvmsg() */
 	int		forward_deficit;
 };