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>
diff --git a/include/net/sock.h b/include/net/sock.h
index b9f2b09..53d4714 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -1520,6 +1520,7 @@
unsigned long size, int force,
gfp_t priority);
extern void sock_wfree(struct sk_buff *skb);
+extern void skb_orphan_partial(struct sk_buff *skb);
extern void sock_rfree(struct sk_buff *skb);
extern void sock_edemux(struct sk_buff *skb);
diff --git a/net/core/sock.c b/net/core/sock.c
index 85e8de1..a753d97 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -1576,6 +1576,25 @@
}
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.
*/
diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
index 82f6016..a6d788d 100644
--- a/net/sched/sch_netem.c
+++ b/net/sched/sch_netem.c
@@ -412,12 +412,9 @@
/* If a delay is expected, orphan the skb. (orphaning usually takes
* place at TX completion time, so _before_ the link transit delay)
- * Ideally, this orphaning should be done after the rate limiting
- * module, because this breaks TCP Small Queue, and other mechanisms
- * based on socket sk_wmem_alloc.
*/
if (q->latency || q->jitter)
- skb_orphan(skb);
+ skb_orphan_partial(skb);
/*
* If we need to duplicate packet, then re-insert at top of the
