How to have the same routine executed sometimes by the CPU and sometimes by the GPU with OpenACC?

I'm dealing with a routine which I want the first time to be executed by the CPU and every other time by the GPU. This routine contains the loop:

for (k = kb; k <= ke; k++){
for (j = jb; j <= je; j++){
for (i = ib; i <= ie; i++){
  ...
}}}

I tried with adding #pragma acc loop collapse(3) to the loop and #pragma acc routine(routine) vector just before the calls where I want the GPU to execute the routine. -Minfo=accel doesn't report any message and with Nsight-System I see that the routine is always executed by the CPU so in this way it doesn't work. Why the compiler is reading neither of the two #pragma?

Solution

To follow on to Thomas' answer, here's an example of using the "if" clause:

% cat test.c
#include <stdlib.h>
#include <stdio.h>

void compute(int * Arr, int size, int use_gpu) {

#pragma acc parallel loop copyout(Arr[:size]) if(use_gpu)
    for (int i=0; i < size; ++i) {
        Arr[i] = i;
    }
}

int main() {

   int *Arr;
   int size;
   int use_gpu;

   size=1024;
   Arr = (int*) malloc(sizeof(int)*size);

// Run on the host
   use_gpu=0;
   compute(Arr,size,use_gpu);

// Run on the GPU
   use_gpu=1;
   compute(Arr,size,use_gpu);

   free(Arr);

}
% nvc -acc -Minfo=accel test.c
compute:
      4, Generating copyout(Arr[:size]) [if not already present]
         Generating NVIDIA GPU code
          7, #pragma acc loop gang, vector(128) /* blockIdx.x threadIdx.x */
% setenv NV_ACC_TIME 1
% a.out

Accelerator Kernel Timing data
test.c
  compute  NVIDIA  devicenum=0
    time(us): 48
    4: compute region reached 1 time
        4: kernel launched 1 time
            grid: [8]  block: [128]
             device time(us): total=5 max=5 min=5 avg=5
            elapsed time(us): total=331 max=331 min=331 avg=331
    4: data region reached 2 times
        9: data copyout transfers: 1
             device time(us): total=43 max=43 min=43 avg=43

I'm using nvc and set the compiler's runtime profiler (NV_ACC_TIME=1) to show that the kernel is launched only once.