cgroup_fj_stress_* in controllers from ubuntu_ltp failed with dir creation / fork on B-KVM

It looks like the test failed with cannot fork, and the following test will failed with mkdir.

Take this one for example:
 tag=cgroup_fj_stress_perf_event_2_9_each stime=1571098406 dur=7 exit=exited stat=2 core=no cu=451 cs=280
 ...
 ...
 cgroup_fj_stress_perf_event_2_9_each 1 TINFO: Attaching task 1326 to /sys/fs/cgroup/perf_event/ltp/2/1/1/2/1/2/1
 cgroup_fj_stress_perf_event_2_9_each 1 TINFO: Attaching task 1329 to /sys/fs/cgroup/perf_event/ltp/2/1/1/2/1/2/1/1
 /opt/ltp/testcases/bin/cgroup_fj_stress.sh: 53: /opt/ltp/testcases/bin/cgroup_fj_stress.sh: Cannot fork
 tag=cgroup_fj_stress_perf_event_10_3_each stime=1571098419 dur=2 exit=exited stat=2 core=no cu=134 cs=84
 startup='Tue Oct 15 00:13:39 2019'
 cgroup_fj_stress_perf_event_10_3_each 1 TINFO: Subsystem perf_event is mounted at /sys/fs/cgroup/perf_event
 cgroup_fj_stress_perf_event_10_3_each 1 TINFO: Creating subgroups ...
 mkdir: cannot create directory ‘/sys/fs/cgroup/perf_event/ltp’: File exists
 cgroup_fj_stress_perf_event_10_3_each 1 TBROK: mkdir /sys/fs/cgroup/perf_event/ltp failed
 cgroup_fj_stress_perf_event_10_3_each 1 TINFO: Removing all ltp subgroups...

Failed test cases sorted alphabetically:
cgroup_fj_stress_blkio_10_3_each -mkdir
cgroup_fj_stress_blkio_2_9_each -cannot fork
cgroup_fj_stress_cpu_10_3_each -mkdir
cgroup_fj_stress_cpu_2_9_each -cannot fork
cgroup_fj_stress_cpuacct_10_3_each -mkdir
cgroup_fj_stress_cpuacct_2_9_each -cannot fork
cgroup_fj_stress_cpuset_10_3_each -mkdir
cgroup_fj_stress_cpuset_2_9_each -cannot fork
cgroup_fj_stress_devices_10_3_each -mkdir
cgroup_fj_stress_devices_2_9_each -cannot fork
cgroup_fj_stress_freezer_10_3_each -mkdir
cgroup_fj_stress_freezer_2_9_each -cannot fork
cgroup_fj_stress_hugetlb_10_3_each -mkdir
cgroup_fj_stress_hugetlb_2_9_each -cannot fork
cgroup_fj_stress_memory_10_3_each -mkdir
cgroup_fj_stress_memory_2_9_each -cannot fork
cgroup_fj_stress_net_cls_10_3_each -mkdir
cgroup_fj_stress_net_cls_2_9_each -cannot fork
cgroup_fj_stress_net_prio_10_3_each -mkdir
cgroup_fj_stress_net_prio_2_9_each -cannot fork
cgroup_fj_stress_perf_event_10_3_each -mkdir
cgroup_fj_stress_perf_event_2_9_each -cannot fork