net: sch_netem: Fix an inconsistency in ingress netem timestamps.

Alex Sidorenko reported:

"while experimenting with 'netem' we have found some strange behaviour. It
seemed that ingress delay as measured by 'ping' command shows up on some
hosts but not on others.

After some investigation I have found that the problem is that skbuff->tstamp
field value depends on whether there are any packet sniffers enabled. That
is:

- if any ptype_all handler is registered, the tstamp field is as expected
- if there are no ptype_all handlers, the tstamp field does not show the delay"

This patch prevents unnecessary update of tstamp in dev_queue_xmit_nit()
on ingress path (with act_mirred) adding a check, so minimal overhead on
the fast path, but only when sniffers etc. are active.

Since netem at ingress seems to logically emulate a network before a host,
tstamp is zeroed to trigger the update and pretend delays are from the
outside.
Reported-by: Alex Sidorenko <alexandre.sidorenko@hp.com>
Tested-by: Alex Sidorenko <alexandre.sidorenko@hp.com>
Signed-off-by: Jarek Poplawski <jarkao2@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -1336,7 +1336,12 @@ static void dev_queue_xmit_nit(struct sk_buff *skb, struct net_device *dev)
 {
 	struct packet_type *ptype;
 
+#ifdef CONFIG_NET_CLS_ACT
+	if (!(skb->tstamp.tv64 && (G_TC_FROM(skb->tc_verd) & AT_INGRESS)))
+		net_timestamp(skb);
+#else
 	net_timestamp(skb);
+#endif
 
 	rcu_read_lock();
 	list_for_each_entry_rcu(ptype, &ptype_all, list) {

net/sched/sch_netem.c

@@ -280,6 +280,14 @@ static struct sk_buff *netem_dequeue(struct Qdisc *sch)
 			if (unlikely(!skb))
 				return NULL;
 
+#ifdef CONFIG_NET_CLS_ACT
+			/*
+			 * If it's at ingress let's pretend the delay is
+			 * from the network (tstamp will be updated).
+			 */
+			if (G_TC_FROM(skb->tc_verd) & AT_INGRESS)
+				skb->tstamp.tv64 = 0;
+#endif
 			pr_debug("netem_dequeue: return skb=%p\n", skb);
 			sch->q.qlen--;
 			return skb;