rtapi -- uspace -- "Using POSIX non-realtime"

latheoperator@125cnc:~$ uname -a
Linux 125cnc 4.8.9-jbc01 #1 SMP PREEMPT Sun Nov 20 09:34:59 CET 2016 x86_64 GNU/Linux
latheoperator@125cnc:~$ !lat
latency-test 1000000 1000000
Waiting for component 'threads' to become ready....Waited 3 seconds for master.  giving up.
Note: Using POSIX non-realtime

Waiting for component 'lat' to become ready....
Note: Using POSIX non-realtime
latheoperator@125cnc:~$ ls -l /usr/bin/rtapi_app 
-rwsr-xr-x 1 root root 102072 Nov 21 09:08 /usr/bin/rtapi_app
latheoperator@125cnc:~$ 

2. Load the RTOS and RTAPI modules:

Note: for this and most of the following steps you will need
to be root, or use 'su -c "<command>"'

scripts/load_rtapi

See a number of messages pop up in the kernel log as components
of the RTOS are installed...
One of the last ones will be:

$ cd the_root_dir/src
$ ./autogen.sh
$ ./configure --with-realtime=uspace

Linux 125cnc 4.8.0-1-rt-amd64 #1 SMP PREEMPT RT Debian 4.8.5-1 (2016-10-28) x86_64 GNU/Linux

Note: Using POSIX non-realtime

$ ./configure --help|grep realtime
  --enable-simulator      Deprecated! Use --with-realtime=uspace instead.
  --with-realtime=PATH    Path where RTAI is installed, or "uspace" for POSIX

$ latency-test 
Note: Using POSIX realtime
...

Note: Using POSIX non-realtime

latheoperator@125cnc:~$ uname -a
Linux 125cnc 4.8.9-jbc01 #1 SMP PREEMPT Sun Nov 20 09:34:59 CET 2016 x86_64 GNU/

static int detect_realtime() {
    struct utsname u;
    int crit1, crit2 = 0;
    FILE *fd;

    uname(&u);
    crit1 = strcasestr (u.version, "PREEMPT RT") != 0;

    if ((fd = fopen("/sys/kernel/realtime","r")) != NULL) {
        int flag;
        crit2 = ((fscanf(fd, "%d", &flag) == 1) && (flag == 1));
        fclose(fd);
    }
    return crit1 && crit2;
}

Linux t520 4.1.0-0.bpo.2-rt-amd64 #1 SMP PREEMPT RT Debian 4.1.6-1~bpo8+1 (2015-09-09) x86_64 GNU/Linux