net: skb_segment() should preserve backpressure

This patch generalizes commit d6a4a1041176 ("tcp: GSO should be TSQ
friendly") to protocols using skb_set_owner_w()

TCP uses its own destructor (tcp_wfree) and needs a more complex scheme
as explained in commit 6ff50cd55545 ("tcp: gso: do not generate out of
order packets")

This allows UDP sockets using UFO to get proper backpressure,
thus avoiding qdisc drops and excessive cpu usage.

Here are performance test results (macvlan on vlan):

- Before
# netperf -t UDP_STREAM ...
Socket Message Elapsed Messages
Size Size Time Okay Errors Throughput
bytes bytes secs # # 10^6bits/sec

212992 65507 60.00 144096 1224195 1258.56
212992 60.00 51 0.45

Average: CPU %user %nice %system %iowait %steal %idle
Average: all 0.23 0.00 25.26 0.08 0.00 74.43

- After
# netperf -t UDP_STREAM ...
Socket Message Elapsed Messages
Size Size Time Okay Errors Throughput
bytes bytes secs # # 10^6bits/sec

212992 65507 60.00 109593 0 957.20
212992 60.00 109593 957.20

Average: CPU %user %nice %system %iowait %steal %idle
Average: all 0.18 0.00 8.38 0.02 0.00 91.43

[edumazet] Rewrote patch and changelog.

Signed-off-by: Toshiaki Makita <[e-mail]>
Signed-off-by: Eric Dumazet <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/core/skbuff.c

···
 	 * (see validate_xmit_skb_list() for example)
 	 */
 	segs->prev = tail;
+
+	/* Following permits correct backpressure, for protocols
+	 * using skb_set_owner_w().
+	 * Idea is to tranfert ownership from head_skb to last segment.
+	 */
+	if (head_skb->destructor == sock_wfree) {
+		swap(tail->truesize, head_skb->truesize);
+		swap(tail->destructor, head_skb->destructor);
+		swap(tail->sk, head_skb->sk);
+	}
 	return segs;
 
 err: