crazy core pinning junk

cpupick/cpupick.go

@@ -0,0 +1,187 @@
+// Package cpupick chooses which CPUs the tun reader threads pin to when the
+// operator has not chosen for us (tun.cpu_affinity). The stock spread —
+// allowed[i] for routine i — has two failure modes this package exists to fix:
+//
+//   - every co-located nebula starts its spread at allowed[0], so N instances
+//     on one box stack their readers onto the same cores, and allowed[0] is
+//     usually CPU 0, the core housekeeping and default IRQ affinity already
+//     favor;
+//   - on heterogeneous CPUs (ARM big.LITTLE, Intel P/E hybrids, AMD compact
+//     cores) low IDs are not necessarily fast cores, and pinning an encrypt
+//     thread to an efficiency core caps that queue's throughput.
+//
+// Default instead returns a preference-ordered pin list: the allowed set
+// filtered to performance cores (when the platform distinguishes them and
+// enough remain for every routine), confined to a single NUMA node and spread
+// across distinct physical cores when the topology permits, CPU 0's physical
+// core demoted to last resort, and the order rotated by a stable per-instance
+// key so co-located instances spread instead of stacking.
+package cpupick
+
+import (
+	"log/slog"
+
+	"github.com/slackhq/nebula/util"
+)
+
+// topology is the slice of machine layout arrange consults: the NUMA node
+// and the physical core behind each candidate CPU, plus which core CPU 0
+// lives on (zeroCore, -1 when unknown — tracked separately because CPU 0's
+// SMT sibling deserves demotion even when CPU 0 itself isn't a candidate).
+// Probed from sysfs on Linux; flatTopology stands in when the platform can't
+// say, which turns every topology rule into a no-op rather than a wrong
+// answer.
+type topology struct {
+	nodeOf   map[int]int
+	coreOf   map[int]int
+	zeroCore int
+}
+
+// flatTopology places every CPU on node 0 and on a physical core of its own.
+func flatTopology(cpus []int) topology {
+	t := topology{
+		nodeOf:   make(map[int]int, len(cpus)),
+		coreOf:   make(map[int]int, len(cpus)),
+		zeroCore: -1,
+	}
+	for i, c := range cpus {
+		t.nodeOf[c] = 0
+		t.coreOf[c] = i
+		if c == 0 {
+			t.zeroCore = i
+		}
+	}
+	return t
+}
+
+// Default computes the pin order for `routines` tun readers. key is any
+// stable per-instance value; the bound UDP port is ideal — distinct across
+// co-located instances, stable across restarts so benchmark runs stay
+// comparable. Returns nil when there is nothing useful to say (no affinity
+// support on this platform, lookup failure); callers keep their existing
+// fallback spread.
+func Default(routines int, key uint64, l *slog.Logger) []int {
+	allowed, err := util.AllowedCPUs()
+	if err != nil || len(allowed) == 0 {
+		return nil
+	}
+	perf, signal := perfCPUs(allowed)
+	cands := pickCandidates(allowed, perf, routines)
+	if len(cands) == 0 {
+		return nil
+	}
+	if len(perf) < routines {
+		signal = ""
+	}
+	cpus := arrange(cands, readTopology(cands), routines, splitmix64(key))
+	if l != nil {
+		l.Info("chose default pin CPUs for tun readers",
+			"cpus", cpus[:min(routines, len(cpus))],
+			"perfSignal", signal)
+	}
+	return cpus
+}
+
+// pickCandidates applies the enough-for-everyone guard: a perf filter that
+// leaves fewer candidates than routines is discarded — giving every reader
+// its own (possibly slow) core beats stacking two readers on a fast one.
+func pickCandidates(allowed, perf []int, routines int) []int {
+	if len(perf) < routines {
+		return allowed
+	}
+	return perf
+}
+
+// arrange turns the candidate set into the final pin order:
+//
+//  1. NUMA: when at least one node holds enough candidates for every
+//     routine, confine to one such node, chosen by the instance hash. The
+//     readers share hostmap and cipher state, so splitting one instance
+//     across nodes taxes every packet — and co-located instances that hash
+//     to different nodes stop competing entirely. When no node is big
+//     enough, span nodes rather than stack readers.
+//  2. Rotate the preferred candidates by the hash so instances spread.
+//  3. SMT: emit one thread per physical core before any of their siblings —
+//     two encrypt threads on one core split its execution units. Siblings
+//     still follow for the routines > cores case.
+//  4. CPU 0's whole physical core goes last: housekeeping and default IRQ
+//     noise on CPU 0 bleeds into its SMT sibling too. Within that tail the
+//     sibling precedes CPU 0 itself, which only catches the bleed-through.
+//
+// The rotation happens before the SMT pass so each instance's one-per-core
+// walk also starts at a different core, and CPU 0's core is excluded from
+// the rotation so no hash value can put it back at the front.
+func arrange(cands []int, topo topology, routines int, h uint64) []int {
+	byNode := map[int][]int{}
+	var nodes []int
+	for _, c := range cands {
+		n := topo.nodeOf[c]
+		if _, ok := byNode[n]; !ok {
+			nodes = append(nodes, n)
+		}
+		byNode[n] = append(byNode[n], c)
+	}
+	var eligible []int
+	for _, n := range nodes {
+		if len(byNode[n]) >= routines {
+			eligible = append(eligible, n)
+		}
+	}
+	if len(eligible) > 0 {
+		cands = byNode[eligible[int(h%uint64(len(eligible)))]]
+	}
+
+	// Split off CPU 0's core: its siblings tail the list, CPU 0 tails them.
+	preferred := make([]int, 0, len(cands))
+	var zeroTail []int
+	hasZero := false
+	for _, c := range cands {
+		switch {
+		case c == 0:
+			hasZero = true
+		case topo.zeroCore >= 0 && topo.coreOf[c] == topo.zeroCore:
+			zeroTail = append(zeroTail, c)
+		default:
+			preferred = append(preferred, c)
+		}
+	}
+	if hasZero {
+		zeroTail = append(zeroTail, 0)
+	}
+	if len(preferred) == 0 {
+		return zeroTail // CPU 0's core is all we have
+	}
+
+	// The node pick consumed the low hash bits; rotate by the high ones so
+	// the two choices stay independent.
+	off := int((h >> 32) % uint64(len(preferred)))
+	rot := make([]int, 0, len(preferred))
+	rot = append(rot, preferred[off:]...)
+	rot = append(rot, preferred[:off]...)
+
+	seenCore := make(map[int]bool, len(rot))
+	out := make([]int, 0, len(cands))
+	var siblings []int
+	for _, c := range rot {
+		g := topo.coreOf[c]
+		if seenCore[g] {
+			siblings = append(siblings, c)
+			continue
+		}
+		seenCore[g] = true
+		out = append(out, c)
+	}
+	out = append(out, siblings...)
+	out = append(out, zeroTail...)
+	return out
+}
+
+// splitmix64 decorrelates instance keys before the selection modulos: ports
+// on one box often share spacing (4242/4243, or round steps like +1000) that
+// raw key%len arithmetic would fold onto the same offset.
+func splitmix64(x uint64) uint64 {
+	x += 0x9e3779b97f4a7c15
+	x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9
+	x = (x ^ (x >> 27)) * 0x94d049bb133111eb
+	return x ^ (x >> 31)
+}