netlink: wake up netlink listeners sooner

A netlink listening task (such as Quagga/zebra) can easily get overrun when lots of events happen such as a link state transition with full BGP route table. This happens because the sender does not yield to the receiver (unless socket queue is full). The problem is exacerbated because it is typical for listeners to set large receive buffer to attempt to keep up.

This patch changes it to yield sooner at halfway instead. Still not a cure-all for listener overrun if listener is slow, but works much reliably.

@@ -960,7 +960,7 @@ static int netlink_broadcast_deliver(str
 		skb_set_owner_r(skb, sk);
 		skb_queue_tail(&sk->sk_receive_queue, skb);
 		sk->sk_data_ready(sk, skb->len);
-		return atomic_read(&sk->sk_rmem_alloc) > sk->sk_rcvbuf;
+		return atomic_read(&sk->sk_rmem_alloc) > sk->sk_rcvbuf / 2;
 	}
 	return -1;
 }

From: Stephen Hemminger <[email protected]>
Date: Wed, 21 Dec 2011 13:49:44 -0800

		return (atomic_read(&sk->sk_rmem_alloc) << 1) > sk->sk_rcvbuf;