Kernel Threads

• kthreadd (pid = 2) created by kernel (pid = 0) during boot
• kthreadd helps in creating other kernel threads which are required to run periodically/asynchronously
• "ps" shows kernel threads
• Everything under [square bracket] are kernel threads

sw0:FID128:root> ps -eaf

UID        PID  PPID  C STIME TTY          TIME CMD

root         1     0  0 01:21 ?        00:00:00 init [5]

root         2     0  0 01:21 ?        00:00:00 [kthreadd]

root         3     2  0 01:21 ?        00:00:00 [ksoftirqd/0]

root         4     2  0 01:21 ?        00:00:00 [kworker/0:0]

• All calls from user-space to create a new process (including "create_thread" in kthreadd), will end up in do_fork call which internally calls copy_process

Source: https://subscription.packtpub.com/book/application_development/9781785883057/1/ch01lvl1sec13/kernel-threads

Linux Kernel Important Sites

1. http://www.linuxdriver.co.il/ldd3/chp-10-sect-4
2. 

Important Kernel Structures and its sizes

NOTE: The following are found in 32-bit Ubuntu system (Kernel version:  3.2.0-29-generic-pae)

1. Process kernel stack = 8KB in 32 bit; 16KB in 64 bit
Basically, 2 pages as process kernel stack

2. task_struct - structure for each process
sizeof(task_struct) = 3236 bytes = 3.1 KB

3. thread_struct - structure of thread info in each task_struct
sizeof(thread_struct) = 128 bytes

/proc File system usages

NOTE: The following are found in 32-bit Ubuntu system (Kernel version:  3.2.0-29-generic-pae)

1. [PID_MAX]:  default - 32768 (short int)
"/proc/sys/kernel/pid_max"

2. [Loadable Modules]
"/proc/modules" Used by "lsmod"

3. [printk levels]: default 7
echo "/proc/sys/kernel/printk"