Description of problem:

  ps is showing all threads in the same cgroup/cpuset when they are on different cgroup/cpuset.

Version-Release number of selected component (if applicable):
  procps-ng-3.3.10-3.el7.x86_64

How reproducible:
  Always

Steps to Reproduce:
  On a RHEL7 minimal install (I am using auditd just as an example of multi-thread app).

1. Create a cpuset:
  # cd /sys/fs/cgroup/cpuset
  # mkdir app
  # cd app/
  # echo 0 > cpuset.cpus 
  # echo 0 > cpuset.mems 

3. select a multi-threaded proc (e.g auditd):

  # ps -eLo pid,lwp,comm,cgroup | grep audit
    108   108 kauditd         -
    596   596 auditd          1:name=systemd:/system.slice/auditd.service
    596   602 auditd          1:name=systemd:/system.slice/auditd.service

4. Insert the thread with the same pid/lwp in this new cgroup/cpuset:
  # echo 596 > tasks
  # cat tasks 
  596

5. List the threads showing its cgroup/cpuset:
  # ps -eLo pid,lwp,comm,cgroup | grep audit
  108   108 kauditd         -
  596   596 auditd          2:cpuset:/app,1:name=systemd:/system.slice/auditd.service
  596   602 auditd          2:cpuset:/app,1:name=systemd:/system.slice/auditd.service

Actual results:
ps shows all threads in the same cgroup/cpuset, but they are not in the same cgroup/cpuset.

Expected results:
ps showing the correct cgroup/cpuset for threads.

Additional info:

  Stracing ps:

  It opens the proc/PID/ dir and checks the status of the proc.

	stat("/proc/596", {st_mode=S_IFDIR|0555, st_size=0, ...}) = 0
	open("/proc/596/stat", O_RDONLY)        = 6
	read(6, "596 (auditd) S 1 596 596 0 -1 42"..., 2048) = 359
	close(6)                                = 0
	open("/proc/596/status", O_RDONLY)      = 6
	read(6, "Name:\tauditd\nState:\tS (sleeping)"..., 2048) = 1039
	close(6)                                = 0
  It reads the proc cgroup (but never the threads cgroup file):

	open("/proc/596/cgroup", O_RDONLY)      = 6
	read(6, "10:hugetlb:/\n9:perf_event:/\n8:bl"..., 131072) = 159
	read(6, "", 130913)                     = 0
	close(6)                                = 0
	openat(AT_FDCWD, "/proc/596/task", O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC) = 6
	getdents(6, /* 4 entries */, 32768)     = 96
	stat("/proc/596/task/596", {st_mode=S_IFDIR|0555, st_size=0, ...}) = 0
	open("/proc/596/task/596/stat", O_RDONLY) = 7
	read(7, "596 (auditd) S 1 596 596 0 -1 42"..., 1024) = 359
	close(7)                                = 0
	open("/proc/596/task/596/status", O_RDONLY) = 7
	read(7, "Name:\tauditd\nState:\tS (sleeping)"..., 1024) = 1024
	read(7, "t_switches:\t15\n", 1024)      = 15
	close(7)                                = 0
	write(1, "  596   596 auditd          2:cp"..., 86) = 86
	stat("/proc/596/task/602", {st_mode=S_IFDIR|0555, st_size=0, ...}) = 0
	open("/proc/596/task/602/stat", O_RDONLY) = 7
	read(7, "602 (auditd) S 1 596 596 0 -1 10"..., 2048) = 364
	close(7)                                = 0
	open("/proc/596/task/602/status", O_RDONLY) = 7
	read(7, "Name:\tauditd\nState:\tS (sleeping)"..., 2048) = 1040
	close(7)                                = 0
	write(1, "  596   602 auditd          2:cp"..., 86) = 86
	getdents(6, /* 0 entries */, 32768)     = 0

Additional info 2:
	I opened a similar BUG Report for RHEL6 (BZ1284076), but ps
	behaves differently.