device/telemetry: optimize TWAMP with higher kernel scheduling priority

[snormore]

An optimization we could make is to tune the TWAMP sender/reflector to have higher kernel scheduling priority.

This would require the TWAMP sender/reflector code in Go to be locked to the OS thread, which has some caveats/footguns, but it can work if careful. Alternatively, it could be in a separate process entirely.

Example

package main

import (
	"fmt"
	"runtime"
	"time"
	"unsafe"

	"golang.org/x/sys/unix"
)

func main() {
	runtime.LockOSThread()
	defer runtime.UnlockOSThread()

	if err := elevateThreadPriority(); err != nil {
		panic(fmt.Sprintf("failed to elevate thread priority: %v", err))
	}

	fmt.Println("Running high-priority work on a locked thread...")

	for i := 0; i < 5; i++ {
		fmt.Println("tick", i)
		time.Sleep(500 * time.Millisecond)
	}
}

func elevateThreadPriority() error {
	param := &unix.SchedParam{SchedPriority: 10} // range: 1–99 for SCHED_FIFO/RR
	tid := unix.Gettid()

	_, _, errno := unix.RawSyscall(
		unix.SYS_SCHED_SETSCHEDULER,
		uintptr(tid),
		uintptr(unix.SCHED_FIFO),
		uintptr(unsafe.Pointer(param)),
	)
	if errno != 0 {
		return errno
	}
	return nil
}