Hello Tejun, I've discovered what looks to be a bug in the reporting of PIDs in the cgroup.procs file in the "domain threaded" node at the root of a threaded subtree. The following demo is on vanilla kernel 4.19. Suppose we have the following multithreaded process: $ ps -L 654 PID LWP TTY STAT TIME COMMAND 654 654 pts/12 Tl 0:00 ./cpu_multithread_burner 100 654 655 pts/12 Tl 0:01 ./cpu_multithread_burner 100 Now suppose we create a threaded subtree in the v2 hierarchy: # cd /sys/fs/cgroup/unified/ # mkdir -p x/a/b # echo 'threaded' > x/a/cgroup.type # echo 'threaded' > x/a/b/cgroup.type Then we move the multithreaded process into x/a in the threaded subtree: # echo 654 > x/a/cgroup.procs Now we visualize the set-up using my visualization program[1]: # go run ~mtk/lsp/cgroups/view_v2_cgroups.go x x [dt] PIDs: {654} a [t] TIDs: {654 655-[654]} b [t] The above is as I expect. Now, we move the thread group leader (it has to be the thread group leader to show the bug) to a x/a/b, and again use my visualization program: # echo 654 > x/a/b/cgroup.threads # go run ~mtk/lsp/cgroups/view_v2_cgroups.go x x [dt] PIDs: {654 655} a [t] TIDs: {655-[654]} b [t] TIDs: {654} Note how the *thread ID* of the non-thread-group-leader (655) is being reported in the x/cgroup.procs! And just to verify that this is not a bug in my visualization program: # cat x/cgroup.procs 655 654 Your thoughts? Thanks, Michael [1] https://www.spinics.net/lists/cgroups/msg20710.html -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/