Why can't some Linux processes use pstree printing?

Question

I'm completing an assignment which is about realize a pstree command on Linux,my basic idea is to Traverse the files in the /proc directory and look through the /proc/pid/status and /proc/pid/task to see the relationships between different processes,however,when i traverse the files,i notice that some processes are not showing up on the shell after pstree command,i don't how to solve this

for example:/proc/2/status is:

Name:   kthreadd
Umask:  0000
State:  S (sleeping)
Tgid:   2
Ngid:   0
Pid:    2
PPid:   0
....

but i use command pstree -p|grep kthreadd ,the pstree command couldn't find the kthreadd processes

Answer 1

All kernel threads are descendants of kthreadd (pid 2), which is
spawned by the kernel (pid 0) during boot. The kthreadd enumerates
other kernel threads; it provides interface routines through which
other kernel threads can be dynamically spawned at runtime by kernel
services. Kernel threads can be viewed from the command line with the
ps -ef command--they are shown in [square brackets]:

Kernel threads

As PID 2 is not a descendent of PID 1, it's not in the display of pstree.