spread N routines over n lanes

(cherry picked from commit 7043baeb34)

interface.go

@@ -45,7 +45,11 @@ type InterfaceConfig struct {
 	routines           int
 	// Multiport means writers[i] is bound to listen.port+i (not a shared
 	// SO_REUSEPORT port) and lane tunnels are negotiated with capable peers.
-	Multiport      bool
+	Multiport bool
+	// LaneCount is the number of lanes counting the base tunnel as lane 0
+	// (multiport.lanes, clamped to routines). Routines at or beyond it share
+	// the configured lanes round-robin.
+	LaneCount      int
 	MessageMetrics *MessageMetrics
 	version        string
 	relayManager   *relayManager
@@ -91,6 +95,7 @@ type Interface struct {
 	dropMulticast         bool
 	routines              int
 	multiport             bool
+	laneCount             int
 	disconnectInvalid     atomic.Bool
 	closed                atomic.Bool
 	// cpuAffinity, when non-empty, names the CPUs each TUN reader goroutine
@@ -222,6 +227,7 @@ func NewInterface(ctx context.Context, c *InterfaceConfig) (*Interface, error) {
 		dropMulticast:         c.DropMulticast,
 		routines:              c.routines,
 		multiport:             c.Multiport,
+		laneCount:             c.LaneCount,
 		version:               c.version,
 		writers:               make([]udp.Conn, c.routines),
 		batchers:              make([]*batch.MultiCoalescer, c.routines),
@@ -444,16 +450,19 @@ func (f *Interface) pinThisThread(i int) {
 	}
 }
 
-// txQueue is the per-routine TX state owned by one listenIn goroutine. lane
-// is bound to the routine's own socket and carries lane-tunnel data; base is
-// bound to socket 0 and carries base-tunnel and relay data, which must keep
-// the base source port (a vanilla peer would otherwise see per-routine source
-// ports and roam-thrash). The two alias when multiport is off or on routine 0.
-// Concurrent sendmmsg on the shared socket-0 fd is safe: a flow is pinned to
-// one routine by tun steering, so per-flow wire order still holds.
+// txQueue is the per-routine TX state owned by one listenIn goroutine.
+// laneSlot is the lane this routine's traffic rides (laneSlotFor); lane is
+// bound to that slot's socket and carries lane-tunnel data; base is bound to
+// socket 0 and carries base-tunnel and relay data, which must keep the base
+// source port (a vanilla peer would otherwise see per-routine source ports
+// and roam-thrash). The two alias when multiport is off or laneSlot is 0.
+// Concurrent sendmmsg on a shared fd (socket 0, or a lane socket shared by
+// overflow routines) is safe: a flow is pinned to one routine by tun
+// steering, so per-flow wire order still holds.
 type txQueue struct {
-	lane *batch.SendBatch
-	base *batch.SendBatch
+	laneSlot int
+	lane     *batch.SendBatch
+	base     *batch.SendBatch
 }
 
 func (tx *txQueue) full() bool {
@@ -466,11 +475,24 @@ func (tx *txQueue) full() bool {
 // flush drains base before lane so that when a flow moves from the base
 // tunnel onto a freshly established lane mid-window, its packets still leave
 // this host in encryption order.
-func (tx *txQueue) flush(f *Interface, i int) {
+func (tx *txQueue) flush(f *Interface) {
 	if tx.base != tx.lane {
 		f.flushSendBatch(tx.base, 0)
 	}
-	f.flushSendBatch(tx.lane, i)
+	f.flushSendBatch(tx.lane, tx.laneSlot)
+}
+
+// laneSlotFor maps a routine index to the lane its traffic rides. When
+// multiport.lanes is below routines, overflow routines share the configured
+// lanes round-robin instead of all falling back onto the base tunnel's
+// single underlay flow. Sharers use the lane's own socket and 4-tuple, so
+// this is vanilla-style same-flow sharing: no cross-path replay skew, and
+// per-flow ordering still holds (a flow stays pinned to one routine).
+func (f *Interface) laneSlotFor(i int) int {
+	if f.multiport && f.laneCount > 0 {
+		return i % f.laneCount
+	}
+	return i
 }
 
 func (f *Interface) listenIn(queue tio.Queue, i int) {
@@ -482,9 +504,10 @@ func (f *Interface) listenIn(queue tio.Queue, i int) {
 
 	rejectBuf := make([]byte, mtu)
 	arenaSize := batch.SendBatchCap * (udp.MTU + 32)
-	sb := batch.NewSendBatch(f.writers[i], batch.SendBatchCap, arenaSize)
-	tx := &txQueue{lane: sb, base: sb}
-	if f.multiport && i != 0 {
+	laneSlot := f.laneSlotFor(i)
+	sb := batch.NewSendBatch(f.writers[laneSlot], batch.SendBatchCap, arenaSize)
+	tx := &txQueue{laneSlot: laneSlot, lane: sb, base: sb}
+	if f.multiport && laneSlot != 0 {
 		tx.base = batch.NewSendBatch(f.writers[0], batch.SendBatchCap, arenaSize)
 	}
 	fwPacket := &firewall.ParsedPacket{}
@@ -509,10 +532,10 @@ func (f *Interface) listenIn(queue tio.Queue, i int) {
 			// accumulated so the first packets of a deep read drain
 			// hit the wire while the rest are still being encrypted.
 			if tx.full() {
-				tx.flush(f, i)
+				tx.flush(f)
 			}
 		}
-		tx.flush(f, i)
+		tx.flush(f)
 	}
 
 	f.l.Debug("overlay reader is done", "reader", i)