cpu.rt_runtime_us behaviour is different in RHEL 7 Machine as compared to RHEL 8

Issue

• Differences in cpu.rt_runtime_us behavior between RHEL 7 and RHEL 8.
• Real-time scheduling time budget limit behavior differs between RHEL 7 and RHEL 8

RHEL 7:
=======

# cat /sys/fs/cgroup/cpuacct/user.slice/user-0.slice/session-1.scope/cpu.rt_runtime_us
100000

# perf stat  stress-ng -c 1 -t 10 --sched fifo --sched-prio 99 --taskset 2
...                                     
      10.007720112 seconds time elapsed

       4.381507000 seconds user   <=---- [ almost 4 times ]
       0.002936000 seconds sys


RHEL 8:
=======

# cat /sys/fs/cgroup/cpuacct/user.slice/cpu.rt_runtime_us
100000

# perf stat  stress-ng -c 1  -t 10 --sched fifo --sched-prio 99 --taskset 2
stress-ng: info:  [1814] setting to a 10 second run per stressor
...                                        
      10.007775763 seconds time elapsed

       1.000498000 seconds user    << user time showing 1 sec as expected
       0.006842000 seconds sys

• When setting cpu.rt_runtime_us globally and for a user.slice, the time command is showing more user+sys time in RHEL 7 and showing the value under limit in RHEL 8 for Real-time priority scheduling task.