pin tun reader threads to CPUs so per-flow packets keep wire order

Each listenIn goroutine locks its OS thread and pins it to one CPU
(sched_setaffinity), so every UDP send from that goroutine leaves
through the same XPS-selected NIC TX ring instead of being sprayed
across rings and reordered. On by default via tun.pin_threads; queue i
pins to the i-th entry of the process's allowed CPU set (respecting
cpuset/taskset masks, whose IDs are often not 0..NumCPU-1), or to an
explicit tun.cpu_affinity list, validated against that same allowed
set. Linux only; pinning is a no-op elsewhere.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

examples/config.yml

@@ -254,6 +254,20 @@ tun:
   # Default MTU for every packet, safe setting is (and the default) 1300 for internet based traffic
   mtu: 1300
 
+  # Linux only. pin_threads pins each tun reader/encrypt OS thread to a single CPU. This keeps every goroutine's
+  # sends flowing through one XPS-selected NIC TX ring, so packets within a flow stay ordered on the wire
+  # instead of being sprayed across multiple TX rings and reordered. Not reloadable.
+  #pin_threads: true
+
+  # Linux only. cpu_affinity overrides which CPUs the tun reader threads pin to: a list of CPU IDs, one per routine
+  # (see the top-level `routines` setting). Lists shorter than `routines` are modulo-cycled across the queues; extra
+  # entries are ignored. IDs must be within the process's allowed CPU set, so this respects taskset / cgroup cpusets;
+  # a non-integer or not-allowed entry disables the override and falls back to spreading queues across the allowed
+  # CPUs. Only meaningful while pin_threads is true. Not reloadable.
+  #cpu_affinity:
+  #  - 2
+  #  - 4
+
   # Route based MTU overrides, you have known vpn ip paths that can support larger MTUs you can increase/decrease them here
   routes:
     #- mtu: 8800

interface.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"log/slog"
 	"net/netip"
+	"runtime"
 	"slices"
 	"sync"
 	"sync/atomic"
@@ -21,6 +22,7 @@ import (
 	"github.com/slackhq/nebula/overlay"
 	"github.com/slackhq/nebula/overlay/tio"
 	"github.com/slackhq/nebula/udp"
+	"github.com/slackhq/nebula/util"
 )
 
 const mtu = 9001
@@ -49,6 +51,18 @@ type InterfaceConfig struct {
 	reQueryWait     time.Duration
 
 	ConntrackCacheTimeout time.Duration
+
+	// CpuAffinity, when non-empty, names the CPUs each TUN reader goroutine
+	// should pin to. Queue i pins to CpuAffinity[i % len(CpuAffinity)] —
+	// shorter lists than `routines` cycle. Empty list keeps the default
+	// pin-to-(i % NumCPU) behavior. Only consulted when PinThreads is true.
+	CpuAffinity []int
+	// PinThreads controls whether each TUN reader OS thread is pinned to a
+	// single CPU (via tun.pin_threads, default true). Pinning keeps each
+	// goroutine's UDP sends on one XPS-selected NIC TX ring so per-flow
+	// packets stay ordered on the wire.
+	PinThreads bool
+
 	l *slog.Logger
 }
 
@@ -73,6 +87,15 @@ type Interface struct {
 	routines              int
 	disconnectInvalid     atomic.Bool
 	closed                atomic.Bool
+	// cpuAffinity, when non-empty, names the CPUs each TUN reader goroutine
+	// should pin to. Queue i pins to cpuAffinity[i % len(cpuAffinity)].
+	// Empty falls back to the default pin-to-(allowed CPU) behavior.
+	// Only consulted when pinThreads is true.
+	cpuAffinity []int
+	// pinThreads controls whether listenIn pins each TUN reader OS thread to
+	// a CPU at all (tun.pin_threads, default true). When false, threads are
+	// left free to migrate as on stock nebula.
+	pinThreads   bool
 	relayManager *relayManager
 
 	tryPromoteEvery atomic.Uint32
@@ -197,6 +220,8 @@ func NewInterface(ctx context.Context, c *InterfaceConfig) (*Interface, error) {
 		relayManager:          c.relayManager,
 		connectionManager:     c.connectionManager,
 		conntrackCacheTimeout: c.ConntrackCacheTimeout,
+		cpuAffinity:           c.CpuAffinity,
+		pinThreads:            c.PinThreads,
 
 		metricHandshakes: metrics.GetOrRegisterHistogram("handshakes", nil, metrics.NewExpDecaySample(1028, 0.015)),
 		messageMetrics:   c.MessageMetrics,
@@ -336,6 +361,28 @@ func (f *Interface) listenOut(i int) {
 }
 
 func (f *Interface) listenIn(queue tio.Queue, i int) {
+	// Pinning this thread (and goroutine) to a single CPU keeps every UDP send from this goroutine going through
+	// the same TX ring on the nic (XPS selects the ring by CPU), so the wire sees per-flow order. Skip entirely
+	// when tun.pin_threads is false.
+	if f.pinThreads {
+		var cpu int
+		if n := len(f.cpuAffinity); n > 0 {
+			// Explicit tun.cpu_affinity list wins; parseCpuAffinity already
+			// validated the entries against the allowed CPU set.
+			cpu = f.cpuAffinity[i%n]
+		} else if allowed, err := util.AllowedCPUs(); err == nil && len(allowed) > 0 {
+			// Default: spread queues across the CPUs we're actually allowed to
+			// run on. Under a cpuset/taskset mask these aren't 0..NumCPU-1, so
+			// i % NumCPU would pick unrunnable IDs and every pin would fail.
+			cpu = allowed[i%len(allowed)]
+		} else {
+			cpu = i % runtime.NumCPU()
+		}
+		if err := util.PinThreadToCPU(cpu); err != nil {
+			f.l.Warn("failed to pin tun reader to CPU", "queue", i, "cpu", cpu, "err", err)
+		}
+	}
+
 	out := make([]byte, mtu)
 	fwPacket := &firewall.Packet{}
 	nb := make([]byte, 12, 12)

main.go

@@ -7,6 +7,7 @@ import (
 	"net"
 	"net/netip"
 	"runtime/debug"
+	"slices"
 	"strings"
 	"time"
 
@@ -231,6 +232,8 @@ func Main(c *config.C, configTest bool, buildVersion string, l *slog.Logger, dev
 		relayManager:          NewRelayManager(ctx, l, hostMap, c),
 		punchy:                punchy,
 		ConntrackCacheTimeout: conntrackCacheTimeout,
+		CpuAffinity:           parseCpuAffinity(c, l, routines),
+		PinThreads:            c.GetBool("tun.pin_threads", true),
 		l:                     l,
 	}
 
@@ -282,6 +285,70 @@ func Main(c *config.C, configTest bool, buildVersion string, l *slog.Logger, dev
 	}, nil
 }
 
+// parseCpuAffinity reads `tun.cpu_affinity` from the config — a list of
+// integer CPU IDs, one per TUN reader goroutine. Empty / unset returns nil
+// (listenIn falls back to spreading queues across the allowed CPU set).
+// Length mismatch with `routines` is a warning, not an error: shorter lists
+// are modulo-cycled across queues, longer lists' tail is ignored. Invalid
+// entries (non-integer, or a CPU ID we're not allowed to run on) are also a
+// warning and disable the override entirely so we don't silently pin to the
+// wrong CPU. Entries are validated against the process's current affinity
+// mask (util.AllowedCPUs) rather than 0..NumCPU-1: under a cgroup cpuset or
+// taskset the runnable IDs are frequently not that contiguous range, and
+// pinning to an unrunnable ID always fails. If the allowed set can't be
+// determined we fall back to a plain non-negative check.
+func parseCpuAffinity(c *config.C, l *slog.Logger, routines int) []int {
+	raw := c.Get("tun.cpu_affinity")
+	if raw == nil {
+		return nil
+	}
+	rv, ok := raw.([]any)
+	if !ok {
+		l.Warn("tun.cpu_affinity must be a list of integers; ignoring", "value", raw)
+		return nil
+	}
+	// allowed is the set of CPU IDs we're actually permitted to run on. A nil
+	// slice (unsupported platform or lookup error) means "can't tell", so we
+	// only apply the weaker non-negative check in that case.
+	allowed, err := util.AllowedCPUs()
+	if err != nil {
+		l.Warn("could not determine allowed CPUs; validating tun.cpu_affinity against non-negative only", "error", err)
+		allowed = nil
+	}
+	cpus := make([]int, 0, len(rv))
+	for i, e := range rv {
+		var cpu int
+		switch v := e.(type) {
+		case int:
+			cpu = v
+		case int64:
+			cpu = int(v)
+		case float64:
+			cpu = int(v)
+		default:
+			l.Warn("tun.cpu_affinity entry not an integer; ignoring affinity",
+				"index", i, "value", e)
+			return nil
+		}
+		if cpu < 0 {
+			l.Warn("tun.cpu_affinity entry out of range; ignoring affinity",
+				"index", i, "cpu", cpu)
+			return nil
+		}
+		if len(allowed) > 0 && !slices.Contains(allowed, cpu) {
+			l.Warn("tun.cpu_affinity entry not in allowed CPU set; ignoring affinity",
+				"index", i, "cpu", cpu, "allowed", allowed)
+			return nil
+		}
+		cpus = append(cpus, cpu)
+	}
+	if len(cpus) != routines {
+		l.Warn("tun.cpu_affinity length doesn't match routines; queues will modulo-cycle through the list",
+			"affinity_len", len(cpus), "routines", routines)
+	}
+	return cpus
+}
+
 func moduleVersion() string {
 	info, ok := debug.ReadBuildInfo()
 	if !ok {

main_test.go

@@ -0,0 +1,51 @@
+package nebula
+
+import (
+	"testing"
+
+	"github.com/slackhq/nebula/config"
+	"github.com/slackhq/nebula/test"
+	"github.com/slackhq/nebula/util"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestParseCpuAffinity(t *testing.T) {
+	l := test.NewLogger()
+
+	// newConfig returns a config.C with tun.cpu_affinity set to v. A nil v
+	// leaves the key unset.
+	newConfig := func(v any) *config.C {
+		c := config.NewC(l)
+		if v != nil {
+			c.Settings["tun"] = map[string]any{"cpu_affinity": v}
+		}
+		return c
+	}
+
+	// unset -> nil (listenIn falls back to spreading across the allowed set)
+	assert.Nil(t, parseCpuAffinity(newConfig(nil), l, 1))
+
+	// Pick a CPU we're actually allowed to run on so a valid list survives
+	// validation regardless of the host's affinity mask.
+	allowed, _ := util.AllowedCPUs()
+	validCPU := 0
+	if len(allowed) > 0 {
+		validCPU = allowed[0]
+	}
+
+	// valid list -> parsed through unchanged
+	assert.Equal(t, []int{validCPU, validCPU}, parseCpuAffinity(newConfig([]any{validCPU, validCPU}), l, 2))
+
+	// a negative entry is out of range on every platform -> disables the override
+	assert.Nil(t, parseCpuAffinity(newConfig([]any{validCPU, -1}), l, 2))
+
+	// a non-integer entry -> disables the override
+	assert.Nil(t, parseCpuAffinity(newConfig([]any{validCPU, "not-a-cpu"}), l, 2))
+
+	// a CPU id outside the allowed set -> disables the override. Only assertable
+	// where we can enumerate the allowed set (e.g. linux); 1<<20 is far beyond
+	// any representable CPU id so it can never be in the mask.
+	if len(allowed) > 0 {
+		assert.Nil(t, parseCpuAffinity(newConfig([]any{1 << 20}), l, 1))
+	}
+}

util/cpupin_linux.go

@@ -0,0 +1,43 @@
+//go:build linux && !android && !e2e_testing
+
+package util
+
+import (
+	"runtime"
+
+	"golang.org/x/sys/unix"
+)
+
+// PinThreadToCPU restricts the calling OS thread to the given CPU via
+// sched_setaffinity(2). Combined with runtime.LockOSThread on the
+// goroutine, this prevents the kernel from migrating us across CPUs and
+// in turn keeps every UDP send from this goroutine going through the
+// same XPS-selected TX ring, eliminating the wire-side reorder that
+// otherwise fragments one nebula flow across multiple rings.
+func PinThreadToCPU(cpu int) error {
+	runtime.LockOSThread()
+	var set unix.CPUSet
+	set.Zero()
+	set.Set(cpu)
+	return unix.SchedSetaffinity(0, &set)
+}
+
+// AllowedCPUs returns the CPU IDs the calling process is currently allowed to
+// run on, as reported by sched_getaffinity(2). Under a cgroup cpuset or a
+// `taskset` mask the allowed IDs are frequently not the contiguous range
+// 0..NumCPU-1 (e.g. pinned to CPUs 4-7: NumCPU reports 4 while the valid IDs
+// are 4,5,6,7). Callers that need a real CPU to pin to must choose from this
+// set rather than assuming i % NumCPU is runnable, or every pin fails.
+func AllowedCPUs() ([]int, error) {
+	var set unix.CPUSet
+	if err := unix.SchedGetaffinity(0, &set); err != nil {
+		return nil, err
+	}
+	cpus := make([]int, 0, set.Count())
+	for cpu := 0; cpu < len(set)*64; cpu++ {
+		if set.IsSet(cpu) {
+			cpus = append(cpus, cpu)
+		}
+	}
+	return cpus, nil
+}

util/cpupin_other.go

@@ -0,0 +1,18 @@
+//go:build !linux || android || e2e_testing
+
+package util
+
+// PinThreadToCPU is a no-op outside Linux: only Linux exposes a stable
+// per-thread CPU affinity API and only Linux has XPS-driven TX ring
+// selection in the first place. On every other platform there's nothing
+// to fix here.
+func PinThreadToCPU(_ int) error {
+	return nil
+}
+
+// AllowedCPUs has no meaningful answer off Linux (no sched_getaffinity), so it
+// reports "unknown" by returning a nil slice and nil error. Callers treat an
+// empty result as "fall back to the default CPU choice".
+func AllowedCPUs() ([]int, error) {
+	return nil, nil
+}