timer for measuring function execution time not working

I'm writing a cuda library and I need to check the differences in performance between the option CPU and GPU. So I created a simple class called Timer to measure the time required to execute first a GPU function and then the CPU version.

class Timer
{
public:
    Timer()
    {
        _StartTimepoint = std::chrono::steady_clock::now();
    }
 
    ~Timer() {}

    void Stop()
    {

        _stopped = true;
        using namespace std::chrono;
        auto endTimepoint = steady_clock::now();

        auto start = time_point_cast<milliseconds>(_StartTimepoint).time_since_epoch().count();
        auto end = time_point_cast<milliseconds>(endTimepoint).time_since_epoch().count();

        auto _ms = end - start;



        _secs   = _ms   / 1000;
        _ms    -= _secs * 1000;
        _mins   = _secs / 60;
        _secs  -= _mins * 60;
        _hour   = _mins / 60;
        _mins  -= _hour * 60;


    }


    double GetTime(){
        if(_stopped == true)
            return _ms;
        else{
            Stop();
            return _ms;
        }
    }

private:
    std::chrono::time_point< std::chrono::steady_clock> _StartTimepoint;
    double _secs,_ms,_mins,_hour;
    bool _stopped = false;
};

Since I need to check the performances for different values of a parameter m I just run both the functions inside a for loop as you can see:

for (size_t m = MIN_M; m < MAX_M; m+=M_STEP){
        m_array[m_cont] = m;
        //simulate
        double time_gpu,time_cpu;

        Timer timer_gpu;
        run_device(prcr_args,seeds,&m_array[m_cont]);
        timer_gpu.Stop();
        time_gpu = timer_gpu.GetTime();

        Timer timer_cpu;
        simulate_host(prcr_args,seeds,&m_array[m_cont]);
        timer_cpu.Stop();
        time_cpu = timer_cpu.GetTime();
        
        double g = time_cpu/time_gpu;
        
        ofs << m  //stream to print the results
            << "," << time_cpu
            << "," << time_gpu 
            << "," << g << "\n";
        m_cont ++;
    }

The problem is that the results i obtain are incredibly small and clearly wrong since they all are equal (the execution time should increase with m) and that my code requires a couple of minutes to run.

m,cpu_time,gpu_time,g
10,9.88131e-324,6.90979e-310,1.43004e-14
15,9.88131e-324,6.90979e-310,1.43004e-14
....
90,9.88131e-324,6.90979e-310,1.43004e-14
95,9.88131e-324,6.90979e-310,1.43004e-14
100,9.88131e-324,6.90979e-310,1.43004e-14

My guess is that the CPU doesn't execute the cycle sequentially and therefore starts and stops the clock immediately.