compute_etime
long double compute_etime(struct timeval cur_time, struct timeval prev_time);
busy_seconds = compute_etime(cur.busy_time, last.busy_time);
elapsed_time = compute_etime(now->busy_time, then ?
device_busy = compute_etime(now->dinfo->devices[di].busy_time, then ?
busy_seconds = compute_etime(cur.busy_time, last.busy_time);
busy_seconds = compute_etime(cur.busy_time, last.busy_time);