28

I know there are two "levels" of programs: User space and kernel space.

My question is: I want to see only kernel programs,or better: programs on kernel space.

Is this approach correct?

ps -ef|grep "\["

root         1     0  0 20:23 ?        00:00:00 init [4]
root         2     0  0 20:23 ?        00:00:00 [kthreadd]
root         3     2  0 20:23 ?        00:00:00 [ksoftirqd/0]
root         5     2  0 20:23 ?        00:00:00 [kworker/0:0H]
root         7     2  0 20:23 ?        00:00:06 [rcu_sched]
root         8     2  0 20:23 ?        00:00:00 [rcu_bh]
root         9     2  0 20:23 ?        00:00:00 [migration/0]
root        10     2  0 20:23 ?        00:00:00 [migration/1]
root        11     2  0 20:23 ?        00:00:00 [ksoftirqd/1]
root        13     2  0 20:23 ?        00:00:00 [kworker/1:0H]
root        14     2  0 20:23 ?        00:00:00 [migration/2]
....
7
43

Kernel processes (or "kernel threads") are children of PID 2 (kthreadd), so this might be more accurate:

ps --ppid 2 -p 2 -o uname,pid,ppid,cmd,cls

Add --deselect to invert the selection and see only user-space processes.

(This question was pretty much an exact inverse of this one.)

In 2.4.* and older kernels, this PID 2 convention did not exist yet.

2
  • Is it always true ? See the fourth comment in this question: stackoverflow.com/q/12213445/1971003 – Guy Avraham Aug 19 '18 at 12:52
  • It is possible that in early 2.6.* kernels the conversion to the "child of PID 2" convention was not quite complete. As HighKing indicates there, the name of PID 2 had also not fully stabilized to kthreadd by 2.6.18. If you're interested in exact details, go to kernel.org and use the git browser interface to drill down into the early history of kernel/kthread.c file. – telcoM Aug 19 '18 at 13:13
4

Kernel threads do not use RAM at all (or at least are displayed not to use any):

ps -eo cmd,vsize,rss | grep -E ' 0 +0$'
1

If you have tuna installed you can list all kernel threads like this:

$ tuna -U -P
                      thread       ctxt_switches
    pid SCHED_ rtpri affinity voluntary nonvoluntary             cmd 
  2      OTHER     0     0xff       290            1        kthreadd  
  3      OTHER     0     0xff         2            0          rcu_gp  
  4      OTHER     0     0xff         2            0      rcu_par_gp  
  6      OTHER     0        0        13            0 kworker/0:0H-kblockd  
  9      OTHER     0     0xff         2            0    mm_percpu_wq  
  10     OTHER     0        0       448            0     ksoftirqd/0
[..]

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