Linux Kernel introduction

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Linux Kernel introduction
COSC 513 Xiaoping Yang
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What is Linux

• A clone of Unix operating system
• Provides for the efficient management of system
  resources
• Linux is almost a freeware
• First developed for 32-bit x86-based PCs (386 or
  higher).
• Three major components
• Kernel
• Shell Environment
• File structure

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What is the Linux Kernel?

• The Heart of the Linux Operating System
• Provides for the efficient management of system
  resources
• CPU
• Interprocess Communication
• Memory
• Devices
• Disks
• Terminals
• Networks
• Printers

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What inside Linux Kernel
Linux Kernel is a monolithic kernel

• Linux kernel Kernel contains
• System Call interface
• Memory Manager
• File System
• Network support

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Linux Processes and Tasks

• Linux Process Executing Program Linux Task is
  a generalization of a Thread
• Single threaded process is represented as a task
• Multi-threaded process is multiple tasks
• Only visible to the programmer and the kernel.
• The Scheduler decides which task(s) gets to use
  the CPU(s)

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Linux Scheduler

• Multi-level queue scheduler
• Three queues
• SCHED_FIFO - standard priority.
• Highest priority.
• No time-slice
• "supervisor" processes only
• SCHED_RR - priority round robin.
• Highest priority RR runs for 1 time slice.
• "supervisor" processes only
• SCHED_OTHER - time-sharing
• All Linux user processes

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Process Tree

• There is a tree like relationship among
  processes Given the following
• netscape
• emacs
• xfig
• bash is the parent process
• netscape, emacs and xfig are children of bash and
  are siblings
• netscape is the oldest sibling
• xfig is the youngest sibling
• Linux stores this structure as linked lists
• Linux will show you the tree (using ps f )

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Process Tree (cont.)
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Task Tree

• For every task the kernel maintains a task_struct
  task_struct contains all relevant information
  about a task.
• General
• PID,Program name
• Parent, youngest child, next sibling, previous
  sibling
• Process Times (start_time, utime, stime, cutime,
  cstime)
• Scheduling algorithm, priority, nice value, errno
  
• Process state
• Owner
• UID
• GID

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Task tree (Cont.)

• Files
• Information on all files opened by the process.
• Stored in the fs_struct sub-structure
• Memory
• Information about the memory used by a process.
• IPC / Synchronization
• Pointers to acquired Semaphores
• Bitmask of received Signals and associated Signal
  handlers
• Some other stuff...
• Linux stores all task_structs in a doubly linked
  list.

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Linux Process State
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Interrupts and System Calls

• An interrupt is a request from the hardware for
  immediate attention.
• Two types of interrupts in Linux
• Fast Interrupts
• Suspend current task, process interrupt, resume
  task
• All other interrupts are generally disabled
• Keyboard interrupt
• Mouse interrupt

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Interrupts and System Calls (cont.)

• Slow Interrupts
• All registers are saved.
• Only interrupts of same type are disabled.
• Scheduler is called when ISR exits.
• Timer interrupt
• Disk Drive interrupt
• System calls

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Fast Interrupts in Linux

• Suspend current task
• Save CPU registers that might be altered
• Switch CPU to kernel (system/supervisor/etc...)
  mode
• Block all interrupts
• Call Interrupt Service Routine (ISR)
• Unblock interrupts
• Switch CPU to user mode
• Resume suspended task
• Restore saved CPU Registers

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