scoot pinning around

util/cpupin_linux.go

@@ -0,0 +1,23 @@
+//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 sendmmsg 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)
+}