Linux kernal some notes

1. What the Kernel Is

• Core of OS: manages CPU, memory, devices, filesystems, networking
• Runs in kernel space (privileged)
• Linux = monolithic kernel (modular via loadable modules)

2. Kernel Space vs User Space

• User space: apps, shells (no hardware access)
• Kernel space: drivers, scheduler, memory mgmt
• Transition via system calls (syscalls)

3. Kernel Architecture (Main Subsystems)

• Scheduler – CPU time allocation
• Memory Management – virtual memory, paging
• VFS – filesystem abstraction
• Block I/O – disks
• Network Stack – TCP/IP
• Device Drivers – hardware interface
• IPC – signals, pipes, shared memory

4. Process Management

• Process = program in execution
• Created by: fork() → exec()
• States: running, sleeping, stopped, zombie
• Scheduler: CFS (Completely Fair Scheduler)

5. Scheduling

• Preemptive, multitasking
• Priorities:
  • Normal (CFS)
  • Real-time: SCHED_FIFO, SCHED_RR
• Inspect: ps, top, htop

6. Memory Management

• Virtual memory per process
• Paging + demand paging
• Swap used when RAM is full
• Page cache for files
• OOM Killer kills processes on memory exhaustion

7. System Calls

• Controlled entry to kernel
• Examples:
  • read(), write()
  • open()
  • fork(), execve()
• Trace: strace

8. Kernel Modules

• Loadable code at runtime
• No reboot required

• Commands:

  lsmod modprobe <module> rmmod <module>

9. Device Model

• Everything as files: /dev
• Character vs block devices
• udev handles device nodes dynamically

10. Interrupts & Context Switching

• Interrupt: hardware signal to CPU
• Context switch: save/restore process state
• Expensive → minimize in performance-critical paths

11. Filesystems (VFS)

• VFS = abstraction layer
• Supports: ext4, xfs, btrfs, nfs
• Mounting connects FS to directory tree

12. Networking (Kernel Side)

• Full TCP/IP stack in kernel
• Netfilter handles firewall (iptables/nftables)
• Sockets implemented in kernel

13. Kernel Boot Flow

1. BIOS/UEFI
2. Bootloader (GRUB)
3. Kernel load
4. initramfs
5. systemd (PID 1)

14. Kernel Logs & Debugging

• View logs:

  dmesg journalctl -k

• Panic = fatal kernel error

15. Tuning & Parameters

• Runtime config via sysctl

  sysctl -a sysctl vm.swappiness=10

• Exposed in /proc and /sys

16. Security

• Capabilities (drop root privileges)
• LSM: SELinux, AppArmor
• Namespaces + cgroups (containers)

17. Kernel Versions

• LTS recommended for servers

• Check version:

  uname -r

Absolute must-know for interviews

• Syscalls vs library calls
• Fork–exec model
• Virtual memory & paging
• CFS scheduler
• Monolithic ≠ non-modular