Getting iOS CPU System Time

Question

I'm trying to get iOS CPU time programmatically.

I'm creating an application that prints the CPU time on iOS with Swift 3. I'm using two different functions:

var usage = rusage()
getrusage(RUSAGE_SELF, &usage
print(usage.ru_stime) //print system time`

and

  var info = mach_task_basic_info()
  var count = mach_msg_type_number_t(MemoryLayout<mach_task_basic_info>.size)/4

  let kerr: kern_return_t = withUnsafeMutablePointer(to: &info) {
     $0.withMemoryRebound(to: integer_t.self, capacity: 1) {
        task_info(mach_task_self_,
                  task_flavor_t(MACH_TASK_BASIC_INFO),
                  $0,
                  &count)
     }
  }

  if kerr == KERN_SUCCESS {
     print("System time: \(info.system_time)")
  }
  else {
     print("Error with task_info(): " +
        (String(cString: mach_error_string(kerr), encoding: String.Encoding.ascii) ?? "unknown error"))
  }

When testing on the Xcode simulator, both methods work, giving me the CPU system time, but when I test on an actual device, the time returned is always zero. Using both of these functions, I can get CPU user time, but the CPU system time returned is always zero.

Is there permission that I have to enable for my application? Why do these functions work on the simulator but not on a real device?

Thank you.