Commit a21bba94 authored Nov 24, 2008 by Eric Dumazet Committed by David S. Miller Nov 24, 2008

net: avoid a pair of dst_hold()/dst_release() in ip_push_pending_frames()



We can reduce pressure on dst entry refcount that slowdown UDP transmit
path on SMP machines. This pressure is visible on RTP servers when
delivering content to mediagateways, especially big ones, handling
thousand of streams. Several cpus send UDP frames to the same
destination, hence use the same dst entry.

This patch makes ip_push_pending_frames() steal the refcount its
callers had to take when filling inet->cork.dst.

This doesnt avoid all refcounting, but still gives speedups on SMP,
on UDP/RAW transmit path.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent 2e77d89b

net/ipv4/ip_output.c

+6 −1

Original line number	Original line	Diff line number	Diff line
	@@ -1284,7 +1284,12 @@ int ip_push_pending_frames(struct sock *sk)

	skb->priority = sk->sk_priority;		skb->priority = sk->sk_priority;
	skb->mark = sk->sk_mark;		skb->mark = sk->sk_mark;
	skb->dst = dst_clone(&rt->u.dst);		/*
			* Steal rt from cork.dst to avoid a pair of atomic_inc/atomic_dec
			* on dst refcount
			*/
			inet->cork.dst = NULL;
			skb->dst = &rt->u.dst;

	if (iph->protocol == IPPROTO_ICMP)		if (iph->protocol == IPPROTO_ICMP)
	icmp_out_count(net, ((struct icmphdr *)		icmp_out_count(net, ((struct icmphdr *)

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