have another go at tx mitigation

the idea is to call the hardware transmit routine less since in a
lot of cases posting a producer ring update to the chip is (very)
expensive. it's better to do it for several packets instead of each
packet, hence calling this tx mitigation.

this diff defers the call to the transmit routine to a network
taskq, or until a backlog of packets has built up. dragonflybsd
uses 16 as the size of it's backlog, so i'm copying them for now.

i've tried this before, but previous versions caused deadlocks. i
discovered that the deadlocks in the previous version was from
ifq_barrier calling taskq_barrier against the nettq. interfaces
generally hold NET_LOCK while calling ifq_barrier, but the tq might
already be waiting for the lock we hold.

this version just doesnt have ifq_barrier call taskq_barrier. it
instead relies on the IFF_RUNNING flag and normal ifq serialiser
barrier to guarantee the start routine wont be called when an
interface is going down. the taskq_barrier is only used during
interface destruction to make sure the task struct wont get used
in the future, which is already done without the NET_LOCK being
held.

tx mitigation provides a nice performanace bump in some setups. up
to 25% in some cases.

tested by tb@ and hrvoje popovski (who's running this in production).
ok visa@

1 files changed, 3 insertions, 1 deletions

diff --git a/sys/net/if.c b/sys/net/if.c
index 3289400486e..6534b5e9159 100644
--- a/sys/net/if.c
+++ b/sys/net/if.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: if.c,v 1.575 2019/04/14 06:57:00 dlg Exp $	*/
+/*	$OpenBSD: if.c,v 1.576 2019/04/16 04:04:19 dlg Exp $	*/
 /*	$NetBSD: if.c,v 1.35 1996/05/07 05:26:04 thorpej Exp $	*/
 
 /*
@@ -615,6 +615,8 @@ if_attach_common(struct ifnet *ifp)
 	ifp->if_snd.ifq_ifqs[0] = &ifp->if_snd;
 	ifp->if_ifqs = ifp->if_snd.ifq_ifqs;
 	ifp->if_nifqs = 1;
+	if (ifp->if_txmit == 0)
+		ifp->if_txmit = IF_TXMIT_DEFAULT;
 
 	ifiq_init(&ifp->if_rcv, ifp, 0);