Linux Kernel introduction COSC 513 Xiaoping Yang

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

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

What inside Linux Kernel Linux kernel Kernel contains:  System Call interface  Memory Manager  File System  Network support Linux Kernel is a monolithic kernel

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)Scheduler

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

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 )

Process Tree (cont.)

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

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

Linux Process State

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

 Slow Interrupts 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 Interrupts and System Calls (cont.)

 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 Fast Interrupts in Linux

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