reintroduce tx mitigation

to quote the previous commit:

> this replaces ifq_start with code that waits until at least 4 packets
> have been queued on the ifq before calling the drivers start routine.
> if less than 4 packets get queued, the start routine is called from
> a task in a softnet tq.
>
> 4 packets was chosen this time based on testing sephe did in dfly
> which showed no real improvement when bundling more packets.  hrvoje
> popovski tested this on several nics and found an improvement of
> 10 to 20 percent when forwarding across the board.
>
> because some of the ifq's work could be sitting on a softnet tq,
> ifq_barrier now calls taskq_barrier to guarantee any work that was
> pending there has finished.
>
> ok mpi@ visa@

this was backed out because of a race in the net80211 stack that
anton@ hit. mpi@ committed a workaround for it in revision 1.30 of
src/sys/net80211/ieee80211_pae_output.c.

im putting this in again so we can see what breaks next.

2 files changed, 46 insertions, 8 deletions

diff --git a/sys/net/ifq.c b/sys/net/ifq.c
index add143c1acb..6b71376a1b4 100644
--- a/sys/net/ifq.c
+++ b/sys/net/ifq.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: ifq.c,v 1.19 2017/12/15 01:40:39 dlg Exp $ */
+/*	$OpenBSD: ifq.c,v 1.20 2018/01/02 07:08:10 dlg Exp $ */
 
 /*
  * Copyright (c) 2015 David Gwynne <dlg@openbsd.org>
@@ -70,9 +70,16 @@ struct priq {
 void	ifq_start_task(void *);
 void	ifq_restart_task(void *);
 void	ifq_barrier_task(void *);
+void	ifq_bundle_task(void *);
 
 #define TASK_ONQUEUE 0x1
 
+static inline void
+ifq_run_start(struct ifqueue *ifq)
+{
+	ifq_serialize(ifq, &ifq->ifq_start);
+}
+
 void
 ifq_serialize(struct ifqueue *ifq, struct task *t)
 {
@@ -114,6 +121,16 @@ ifq_is_serialized(struct ifqueue *ifq)
 }
 
 void
+ifq_start(struct ifqueue *ifq)
+{
+	if (ifq_len(ifq) >= min(4, ifq->ifq_maxlen)) {
+		task_del(ifq->ifq_softnet, &ifq->ifq_bundle);
+		ifq_run_start(ifq);
+	} else
+		task_add(ifq->ifq_softnet, &ifq->ifq_bundle);
+}
+
+void
 ifq_start_task(void *p)
 {
 	struct ifqueue *ifq = p;
@@ -137,11 +154,31 @@ ifq_restart_task(void *p)
 }
 
 void
+ifq_bundle_task(void *p)
+{
+	struct ifqueue *ifq = p;
+
+	ifq_run_start(ifq);
+}
+
+void
 ifq_barrier(struct ifqueue *ifq)
 {
 	struct cond c = COND_INITIALIZER();
 	struct task t = TASK_INITIALIZER(ifq_barrier_task, &c);
 
+	if (!task_del(ifq->ifq_softnet, &ifq->ifq_bundle)) {
+		int netlocked = (rw_status(&netlock) == RW_WRITE);
+
+		if (netlocked) /* XXXSMP breaks atomicity */
+			NET_UNLOCK();
+
+		taskq_barrier(ifq->ifq_softnet);
+
+		if (netlocked)
+			NET_LOCK();
+	}
+
 	if (ifq->ifq_serializer == NULL)
 		return;
 
@@ -166,6 +203,7 @@ void
 ifq_init(struct ifqueue *ifq, struct ifnet *ifp, unsigned int idx)
 {
 	ifq->ifq_if = ifp;
+	ifq->ifq_softnet = net_tq(ifp->if_index);
 	ifq->ifq_softc = NULL;
 
 	mtx_init(&ifq->ifq_mtx, IPL_NET);
@@ -187,6 +225,7 @@ ifq_init(struct ifqueue *ifq, struct ifnet *ifp, unsigned int idx)
 	mtx_init(&ifq->ifq_task_mtx, IPL_NET);
 	TAILQ_INIT(&ifq->ifq_task_list);
 	ifq->ifq_serializer = NULL;
+	task_set(&ifq->ifq_bundle, ifq_bundle_task, ifq);
 
 	task_set(&ifq->ifq_start, ifq_start_task, ifq);
 	task_set(&ifq->ifq_restart, ifq_restart_task, ifq);
@@ -238,6 +277,8 @@ ifq_destroy(struct ifqueue *ifq)
 {
 	struct mbuf_list ml = MBUF_LIST_INITIALIZER();
 
+	ifq_barrier(ifq); /* ensure nothing is running with the ifq */
+
 	/* don't need to lock because this is the last use of the ifq */
 
 	ifq->ifq_ops->ifqop_purge(ifq, &ml);
diff --git a/sys/net/ifq.h b/sys/net/ifq.h
index 8e0fcd8f01d..345d1068594 100644
--- a/sys/net/ifq.h
+++ b/sys/net/ifq.h
@@ -1,4 +1,4 @@
-/*	$OpenBSD: ifq.h,v 1.18 2017/12/21 03:59:32 dlg Exp $ */
+/*	$OpenBSD: ifq.h,v 1.19 2018/01/02 07:08:10 dlg Exp $ */
 
 /*
  * Copyright (c) 2015 David Gwynne <dlg@openbsd.org>
@@ -25,6 +25,7 @@ struct ifq_ops;
 
 struct ifqueue {
 	struct ifnet		*ifq_if;
+	struct taskq		*ifq_softnet;
 	union {
 		void			*_ifq_softc;
 		/*
@@ -57,6 +58,7 @@ struct ifqueue {
 	struct mutex		 ifq_task_mtx;
 	struct task_list	 ifq_task_list;
 	void			*ifq_serializer;
+	struct task		 ifq_bundle;
 
 	/* work to be serialised */
 	struct task		 ifq_start;
@@ -405,6 +407,7 @@ void		 ifq_attach(struct ifqueue *, const struct ifq_ops *, void *);
 void		 ifq_destroy(struct ifqueue *);
 void		 ifq_add_data(struct ifqueue *, struct if_data *);
 int		 ifq_enqueue(struct ifqueue *, struct mbuf *);
+void		 ifq_start(struct ifqueue *);
 struct mbuf	*ifq_deq_begin(struct ifqueue *);
 void		 ifq_deq_commit(struct ifqueue *, struct mbuf *);
 void		 ifq_deq_rollback(struct ifqueue *, struct mbuf *);
@@ -441,12 +444,6 @@ ifq_is_oactive(struct ifqueue *ifq)
 }
 
 static inline void
-ifq_start(struct ifqueue *ifq)
-{
-	ifq_serialize(ifq, &ifq->ifq_start);
-}
-
-static inline void
 ifq_restart(struct ifqueue *ifq)
 {
 	ifq_serialize(ifq, &ifq->ifq_restart);