derp: add TCP RTT metric on Linux (#5949)

Periodically poll the TCP RTT metric from all open TCP connections and
update a (bucketed) histogram metric.

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I6214902196b05bf7829c9d0ea501ce0e13d984cf

derp/derp_server.go

@@ -137,6 +137,7 @@ type Server struct {
 	multiForwarderDeleted        expvar.Int
 	removePktForwardOther        expvar.Int
 	avgQueueDuration             *uint64          // In milliseconds; accessed atomically
+	tcpRtt                       metrics.LabelMap // histogram
 
 	// verifyClients only accepts client connections to the DERP server if the clientKey is a
 	// known peer in the network, as specified by a running tailscaled's client's local api.
@@ -312,6 +313,7 @@ func NewServer(privateKey key.NodePrivate, logf logger.Logf) *Server {
 		watchers:             map[*sclient]bool{},
 		sentTo:               map[key.NodePublic]map[key.NodePublic]int64{},
 		avgQueueDuration:     new(uint64),
+		tcpRtt:               metrics.LabelMap{Label: "le"},
 		keyOfAddr:            map[netip.AddrPort]key.NodePublic{},
 	}
 	s.initMetacert()
@@ -713,6 +715,7 @@ func (c *sclient) run(ctx context.Context) error {
 	var grp errgroup.Group
 	sendCtx, cancelSender := context.WithCancel(ctx)
 	grp.Go(func() error { return c.sendLoop(sendCtx) })
+	grp.Go(func() error { return c.statsLoop(sendCtx) })
 	defer func() {
 		cancelSender()
 		if err := grp.Wait(); err != nil && !c.s.isClosed() {
@@ -1699,6 +1702,7 @@ func (s *Server) ExpVar() expvar.Var {
 	m.Set("average_queue_duration_ms", expvar.Func(func() any {
 		return math.Float64frombits(atomic.LoadUint64(s.avgQueueDuration))
 	}))
+	m.Set("counter_tcp_rtt", &s.tcpRtt)
 	var expvarVersion expvar.String
 	expvarVersion.Set(version.Long)
 	m.Set("version", &expvarVersion)

derp/derp_server_default.go

@@ -0,0 +1,14 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !linux
+// +build !linux
+
+package derp
+
+import "context"
+
+func (c *sclient) statsLoop(ctx context.Context) error {
+	return nil
+}

derp/derp_server_linux.go

@@ -0,0 +1,95 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package derp
+
+import (
+	"context"
+	"crypto/tls"
+	"net"
+	"time"
+
+	"golang.org/x/sys/unix"
+)
+
+func (c *sclient) statsLoop(ctx context.Context) error {
+	// If we can't get a TCP socket, then we can't send stats.
+	tcpConn := c.tcpConn()
+	if tcpConn == nil {
+		c.s.tcpRtt.Add("non-tcp", 1)
+		return nil
+	}
+	rawConn, err := tcpConn.SyscallConn()
+	if err != nil {
+		c.logf("error getting SyscallConn: %v", err)
+		c.s.tcpRtt.Add("error", 1)
+		return nil
+	}
+
+	const statsInterval = 10 * time.Second
+
+	ticker := time.NewTicker(statsInterval)
+	defer ticker.Stop()
+
+	var (
+		tcpInfo *unix.TCPInfo
+		sysErr  error
+	)
+statsLoop:
+	for {
+		select {
+		case <-ticker.C:
+			err = rawConn.Control(func(fd uintptr) {
+				tcpInfo, sysErr = unix.GetsockoptTCPInfo(int(fd), unix.IPPROTO_TCP, unix.TCP_INFO)
+			})
+			if err != nil || sysErr != nil {
+				continue statsLoop
+			}
+
+			// TODO(andrew): more metrics?
+			rtt := time.Duration(tcpInfo.Rtt) * time.Microsecond
+			c.s.tcpRtt.Add(durationToLabel(rtt), 1)
+
+		case <-ctx.Done():
+			return ctx.Err()
+		}
+	}
+}
+
+// tcpConn attempts to get the underlying *net.TCPConn from this client's
+// Conn; if it cannot, then it will return nil.
+func (c *sclient) tcpConn() *net.TCPConn {
+	nc := c.nc
+	for {
+		switch v := nc.(type) {
+		case *net.TCPConn:
+			return v
+		case *tls.Conn:
+			nc = v.NetConn()
+		default:
+			return nil
+		}
+	}
+}
+
+func durationToLabel(dur time.Duration) string {
+	switch {
+	case dur <= 10*time.Millisecond:
+		return "10ms"
+	case dur <= 20*time.Millisecond:
+		return "20ms"
+	case dur <= 50*time.Millisecond:
+		return "50ms"
+	case dur <= 100*time.Millisecond:
+		return "100ms"
+	case dur <= 150*time.Millisecond:
+		return "150ms"
+	case dur <= 250*time.Millisecond:
+		return "250ms"
+	case dur <= 500*time.Millisecond:
+		return "500ms"
+	default:
+		return "inf"
+	}
+}