netem: Introduce skb_orphan_partial() helper

Commit 547669d483e578 ("tcp: xps: fix reordering issues") added
unexpected reorders in case netem is used in a MQ setup for high
performance test bed.

ETH=eth0
tc qd del dev $ETH root 2>/dev/null
tc qd add dev $ETH root handle 1: mq
for i in `seq 1 32`
do
 tc qd add dev $ETH parent 1:$i netem delay 100ms
done

As all tcp packets are orphaned by netem, TCP stack believes it can
set skb->ooo_okay on all packets.

In order to allow producers to send more packets, we want to
keep sk_wmem_alloc from reaching sk_sndbuf limit.

We can do that by accounting one byte per skb in netem queues,
so that TCP stack is not fooled too much.

Tested:

With above MQ/netem setup, scaling number of concurrent flows gives
linear results and no reorders/retransmits

lpq83:~# for n in 1 10 20 30 40 50 60 70 80 90 100
 do echo -n "n:$n " ; ./super_netperf $n -H 10.7.7.84; done
n:1 198.46
n:10 2002.69
n:20 4000.98
n:30 6006.35
n:40 8020.93
n:50 10032.3
n:60 12081.9
n:70 13971.3
n:80 16009.7
n:90 17117.3
n:100 17425.5

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

1 files changed, 19 insertions, 0 deletions

diff --git a/net/core/sock.c b/net/core/sock.c
index 85e8de1bc7fd..a753d97434dc 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -1576,6 +1576,25 @@ void sock_wfree(struct sk_buff *skb)
 }
 EXPORT_SYMBOL(sock_wfree);
 
+void skb_orphan_partial(struct sk_buff *skb)
+{
+	/* TCP stack sets skb->ooo_okay based on sk_wmem_alloc,
+	 * so we do not completely orphan skb, but transfert all
+	 * accounted bytes but one, to avoid unexpected reorders.
+	 */
+	if (skb->destructor == sock_wfree
+#ifdef CONFIG_INET
+	    || skb->destructor == tcp_wfree
+#endif
+		) {
+		atomic_sub(skb->truesize - 1, &skb->sk->sk_wmem_alloc);
+		skb->truesize = 1;
+	} else {
+		skb_orphan(skb);
+	}
+}
+EXPORT_SYMBOL(skb_orphan_partial);
+
 /*
  * Read buffer destructor automatically called from kfree_skb.
  */