android doesn’t work for bg_non_interactive?

when I use cgroup for cpu limit in android, I found it doesn’t work, I write the test demo ,i start 4 thread ,and the cpu is 97% ,then I back to the background and my application is 4:cpu:/bg_non_interactive,but cpu is still 97% but not 10%….

    new Thread(new Runnable() {
        @Override
        public void run() {
            int i = 0;
            while(true) {
                i++;
            }
        }
    }).start();
    new Thread(new Runnable() {
        @Override
        public void run() {
            int i = 0;
            while(true) {
                i++;
            }
        }
    }).start();
    new Thread(new Runnable() {
        @Override
        public void run() {
            int i = 0;
            while(true) {
                i++;
            }
        }
    }).start();
    new Thread(new Runnable() {
        @Override
        public void run() {
            int i = 0;
            while(true) {
                i++;
            }
        }
    }).start();
  PID PR CPU% S  #THR     VSS     RSS PCY UID      Name
 1841  1  97% S    24 869604K  45368K  bg u0_a502  com.xxx.eventdemo
   34  2   0% S     1      0K      0K  fg root     kconsole
 4869  2   0% R     1   2184K    948K  fg root     top
  646  1   0% S     4   5716K   1424K  fg media_rw /system/bin/sdcard

130|[email protected]:/dev/cpuctl # cat /proc/1841/cgroup                            
4:cpu:/bg_non_interactive
2:memory:/sw
1:cpuacct:/uid_10502/pid_1841

Source: StackOverflow