CS 423 Operating System Design: The Kernel Abstraction Tian anyin - - PowerPoint PPT Presentation

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CS 423 Operating System Design: The Kernel Abstraction Tian anyin - - PowerPoint PPT Presentation

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CS 423 Operating System Design: The Kernel Abstraction Tian anyin yin Xu Xu * Thanks for Prof. Adam Bates for the slides. CS423: Operating Systems Design Logistics MP 0 due is postponed to next Tue. C4 Paper Summary submitted to

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CS423: Operating Systems Design

Tian anyin yin Xu Xu

CS 423 Operating System Design: The Kernel Abstraction

* Thanks for Prof. Adam Bates for the slides.

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CS423: Operating Systems Design

Logistics

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  • MP 0 due is postponed to next Tue.
  • C4 Paper Summary submitted to

cs423sp20@gmail.com

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SLIDE 3

CS423: Operating Systems Design

Let’s do something fun.

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Let’s start with some questions.

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SLIDE 4

CS423: Operating Systems Design

Overview

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Process concept

  • A process is the OS abstraction for executing a

program with limited privileges Dual-mode operation: user vs. kernel

  • Kernel-mode: execute with complete privileges
  • User-mode: execute with fewer privileges

Safe control transfer

  • How do we switch from one mode to the other?
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CS423: Operating Systems Design

Process Abstraction

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Process: an instance of a program that runs with limited rights on the machine

  • Thread: a sequence of instructions within a process
  • Potentially many threads per process (for now,

assume 1:1)

  • Address space: set of rights of a process
  • Memory that the process can access
  • Other permissions the process has (e.g., which

system calls it can make, what files it can access)

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SLIDE 6

CS 423: Operating Systems Design

Thought Experiment

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How can we permit a process to execute with

  • nly limited privileges?
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SLIDE 7

CS423: Operating Systems Design

Thought Experiment

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How can we implement execution with limited privilege?

  • Execute each program instruction in a simulator
  • If the instruction is permitted, do the instruction
  • Otherwise, stop the process
  • Basic model in Javascript and other interpreted

languages

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SLIDE 8

CS423: Operating Systems Design

Thought Experiment

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How can we implement execution with limited privilege?

  • Execute each program instruction in a simulator
  • If the instruction is permitted, do the instruction
  • Otherwise, stop the process
  • Basic model in Javascript and other interpreted

languages Ok… but how do we go faster?

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SLIDE 9

CS423: Operating Systems Design

Thought Experiment

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How can we implement execution with limited privilege?

  • Execute each program instruction in a simulator
  • If the instruction is permitted, do the instruction
  • Otherwise, stop the process
  • Basic model in Javascript and other interpreted

languages Ok… but how do we go faster?

  • Run the unprivileged code directly on the CPU!
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SLIDE 10

CS423: Operating Systems Design

A Model of a CPU

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SLIDE 11

CS423: Operating Systems Design

A CPU with Dual-Mode Operation

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SLIDE 12

CS423: Operating Systems Design

HW Support for Dual-Mode

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Privileged instructions

  • Available to kernel
  • Not available to user code

Limits on memory accesses

  • To prevent user code from overwriting the kernel

Timer

  • To regain control from a user program in a loop

Safe way to switch from user mode to kernel mode, and vice versa

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SLIDE 13

CS423: Operating Systems Design

Privileged Instructions

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Examples? What should happen if a user program attempts to execute a privileged instruction?

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SLIDE 14

CS423: Operating Systems Design

User->Kernel Switches

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How/when do we switch from user to kernel mode?

  • 1. Interrupts
  • Triggered by timer and I/O devices
  • 2. Exceptions
  • Triggered by unexpected program behavior
  • Or malicious behavior!
  • 3. System calls (aka protected procedure call)
  • Request by program for kernel to do some
  • peration on its behalf
  • Only limited # of very carefully coded entry points
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SLIDE 15

CS 423: Operating Systems Design

Question

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How does the OS know when a process is in an infinite loop?

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SLIDE 16

CS423: Operating Systems Design

Hardware Timer

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Hardware device that periodically interrupts the processor

  • Returns control to the kernel handler
  • Interrupt frequency set by the kernel

Not by user code!

  • Interrupts can be temporarily deferred

Not by user code! Interrupt deferral crucial for implementing mutual exclusion

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SLIDE 17

CS423: Operating Systems Design

Kernel->User Switches

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How/when do we switch from kernel to user mode?

  • 1. New process/new thread start
  • Jump to first instruction in program/thread
  • 2. Return from interrupt, exception, system call
  • Resume suspended execution (return to PC)
  • 3. Process/thread context switch
  • Resume some other process (return to PC)
  • 4. User-level upcall (UNIX signal)
  • Asynchronous notification to user program
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CS 423: Operating Systems Design

CPU State

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What is the CPU’s behavior defined by at any given moment?

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CS 423: Operating Systems Design 19

Program Counter Code Segment Offset Program instructions

What is the CPU’s behavior defined by at any given moment?

CPU State

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CS 423: Operating Systems Design 20

Program Counter Code Segment Offset Data Segment Offset

Operand

Data Operand Current Instruction

OpCode

Program instructions Heap

What is the CPU’s behavior defined by at any given moment?

CPU State

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CS 423: Operating Systems Design 21

Program Counter Code Segment Offset Data Segment Offset

Operand

Data Operand Current Instruction

OpCode

Stack Segment Offset Stack Pointer Program instructions Heap Stack

What is the CPU’s behavior defined by at any given moment?

CPU State

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SLIDE 22

CS 423: Operating Systems Design 22

Program Counter Code Segment Offset Data Segment Offset

Operand

Data Operand Current Instruction

OpCode

Stack Segment Offset Stack Pointer Registers Program instructions Heap Stack

What is the CPU’s behavior defined by at any given moment?

CPU State

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CS 423: Operating Systems Design 23

Program Counter Code Segment Offset Data Segment Offset

Operand

Data Operand Current Instruction

OpCode

Stack Segment Offset Stack Pointer Registers

What defines the STATE of the CPU?

Program instructions Heap Stack

CPU State

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CS 423: Operating Systems Design

What’s a ‘real’ CPU?

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Program Counter Program instructions Code Segment Offset Heap Data Segment Offset

Operand

Data Operand Current Instruction

OpCode

Stack Segment Offset Stack Pointer Stack Registers

What’s the STATE of a real CPU?

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SLIDE 25

CS 423: Operating Systems Design

The Context Switch

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Program Counter Program instructions Code Segment Offset Heap Data Segment

Operand

Data Operand

OpCode

Stack Segment Stack Pointer Registers Stack Program Counter Program instructions Code Segment Offset Heap Data Segment

Operand

Data Operand

OpCode

Stack Segment Stack Pointer Stack

Save State (Context) Load State (Context)

Registers

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CS 423: Operating Systems Design

Process Control Block

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The state for processes that are not running on the CPU are maintained in the Process Control Block (PCB) data structure

Updated during context switch

An alternate PCB diagram

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SLIDE 27

CS 423: Operating Systems Design

The Context Switch

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Program Counter Program instructions Code Segment Offset Heap Data Segment

Operand

Data Operand

OpCode

Stack Segment Stack Pointer Registers Stack Program Counter Program instructions Code Segment Offset Heap Data Segment

Operand

Data Operand

OpCode

Stack Segment Stack Pointer Stack

Save State (Context) Load State (Context)

Registers

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SLIDE 28

CS 423: Operating Systems Design

The Context Switch

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Note: In thread context switches, heap is not switched!

Program Counter Program instructions Code Segment Offset Heap Data Segment

Operand

Data Operand

OpCode

Stack Segment Stack Pointer Stack Program Counter Program instructions Code Segment Offset

Operand OpCode

Stack Segment Stack Pointer Stack

Save State (Context) Load State (Context)

Registers Registers

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SLIDE 29

CS 423: Operating Systems Design

The Context Switch

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Note: In thread context switches, heap is not switched!

Program Counter Program instructions Code Segment Offset Heap Data Segment

Operand

Data Operand

OpCode

Stack Segment Stack Pointer Stack Program Counter Program instructions Code Segment Offset

Operand OpCode

Stack Segment Stack Pointer Stack

Save State (Context) Load State (Context)

Registers Registers

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SLIDE 30

CS 423: Operating Systems Design

Thread Context Switch

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Note: In thread context switches, heap is not switched!

Program Counter Program instructions Code Segment Offset Heap Data Segment

Operand

Data Operand

OpCode

Stack Segment Stack Pointer Stack Program Counter Program instructions Code Segment Offset

Operand OpCode

Stack Segment Stack Pointer Stack

Save State (Context) Load State (Context)

So who does the context switch, and when???

Registers Registers

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CS 423: Operating Systems Design

Thread Context Switch

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Note: In thread context switches, heap is not switched!

Program Counter Program instructions Code Segment Offset Heap Data Segment

Operand

Data Operand

OpCode

Stack Segment Stack Pointer Stack Program Counter Program instructions Code Segment Offset

Operand OpCode

Stack Segment Stack Pointer Stack

Save State (Context) Load State (Context)

Solution 1: An Interrupt

Registers Registers

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CS 423: Operating Systems Design

CTX Switch: Interrupt

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Program Counter Program instructions Code Segment Offset Stack Segment Stack Pointer Stack Program Counter Program instructions Code Segment Offset Stack Segment Stack Pointer Stack

Running Thread

Registers Registers

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CS 423: Operating Systems Design

CTX Switch: Interrupt

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Program Counter Program instructions Code Segment Offset Stack Segment Stack Pointer Stack Program Counter Program instructions Code Segment Offset Stack Segment Stack Pointer Stack

Save PC on thread stack Jump to Interrupt handler

Interrupt

Registers Registers

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CS 423: Operating Systems Design

CTX Switch: Interrupt

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Program Counter Program instructions Code Segment Offset Stack Segment Stack Pointer Registers Stack Program Counter Program instructions Code Segment Offset Stack Segment Stack Pointer Registers Stack

Save PC on thread stack Jump to Interrupt handler Handler

  • Save thread state in thread control block

(SP, registers, segment pointers, …)

Thread Control Block

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CS 423: Operating Systems Design

CTX Switch: Interrupt

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Program Counter Program instructions Code Segment Offset Stack Segment Stack Pointer Stack Program Counter Program instructions Code Segment Offset Stack Segment Stack Pointer Stack

Save PC on thread stack Jump to Interrupt handler Handler

  • Save thread state in thread control block

(SP, registers, segment pointers, …)

  • Choose next thread
  • Load thread state from control block

Thread Control Block Thread Control Block

Registers Registers

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CS 423: Operating Systems Design

CTX Switch: Interrupt

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Program Counter Program instructions Code Segment Offset Stack Segment Stack Pointer Stack Program Counter Program instructions Code Segment Offset Stack Segment Stack Pointer Stack

Save PC on thread stack Jump to Interrupt handler Handler

  • Save thread state in thread control block

(SP, registers, segment pointers, …)

  • Choose next thread
  • Load thread state from control block
  • Pop PC from thread stack (return from handler)

Thread Control Block Thread Control Block

Registers Registers

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CS 423: Operating Systems Design

CTX Switch: Interrupt

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Program Counter Program instructions Code Segment Offset Stack Segment Stack Pointer Stack Program Counter Program instructions Code Segment Offset Stack Segment Stack Pointer Stack

Save PC on thread stack Jump to Interrupt handler Handler

  • Save thread state in thread control block

(SP, registers, segment pointers, …)

  • Choose next thread
  • Load thread state from control block
  • Pop PC from thread stack (return from handler)

Thread Control Block Thread Control Block

Where does it return?

Registers Registers

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CS 423: Operating Systems Design

CTX Switch: Interrupt

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Program Counter Program instructions Code Segment Offset Stack Segment Stack Pointer Stack Program Counter Program instructions Code Segment Offset Stack Segment Stack Pointer Stack

Save PC on thread stack Jump to Interrupt handler Handler

  • Save thread state in thread control block

(SP, registers, segment pointers, …)

  • Choose next thread
  • Load thread state from control block
  • Pop PC from thread stack (return from handler)

Thread Control Block Thread Control Block

Where does it return?

Registers Registers

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CS423: Operating Systems Design

CTX Switch: Interrupt

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What are some examples of context switches due to interrupts?

  • Clock Interrupt: Task exceeds its time slice
  • I/O Interrupt: Waiting processes may be preempted
  • Memory Fault: CPU attempts to access a virtual

memory address that is not in main memory. OS may resume execution of another process while retrieving the block, then moves process to ready state.

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CS 423: Operating Systems Design

Thread Context Switch

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Note: In thread context switches, heap is not switched!

Program Counter Program instructions Code Segment Offset Heap Data Segment

Operand

Data Operand

OpCode

Stack Segment Stack Pointer Stack Program Counter Program instructions Code Segment Offset

Operand OpCode

Stack Segment Stack Pointer Stack

Save State (Context) Load State (Context)

Registers Registers

Solution 2: Voluntary yield()

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CS 423: Operating Systems Design

CTX Switch: Yield

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Program Counter Program instructions Code Segment Offset Stack Segment Stack Pointer Registers Stack Program Counter Program instructions Code Segment Offset Stack Segment Stack Pointer Registers Stack

Running Thread

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CS 423: Operating Systems Design

CTX Switch: Yield

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Program Counter Program instructions Code Segment Offset Stack Segment Stack Pointer Registers Stack Program Counter Program instructions Code Segment Offset Stack Segment Stack Pointer Registers Stack

Save PC on thread stack Jump to yield() function

yield()

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CS 423: Operating Systems Design

CTX Switch: Yield

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Program Counter Program instructions Code Segment Offset Stack Segment Stack Pointer Registers Stack Program Counter Program instructions Code Segment Offset Stack Segment Stack Pointer Registers Stack

Save PC on thread stack Jump to yield() function yield()

  • Save thread state in thread control block

(SP, registers, segment pointers, …)

Thread Control Block

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CS 423: Operating Systems Design

CTX Switch: Yield

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Program Counter Program instructions Code Segment Offset Stack Segment Stack Pointer Registers Stack Program Counter Program instructions Code Segment Offset Stack Segment Stack Pointer Registers Stack

Save PC on thread stack Jump to yield() function yield()

  • Save thread state in thread control block

(SP, registers, segment pointers, …)

  • Choose next thread

Thread Control Block

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CS 423: Operating Systems Design

CTX Switch: Yield

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Program Counter Program instructions Code Segment Offset Stack Segment Stack Pointer Registers Stack Program Counter Program instructions Code Segment Offset Stack Segment Stack Pointer Registers Stack

Save PC on thread stack Jump to yield() function yield()

  • Save thread state in thread control block

(SP, registers, segment pointers, …)

  • Choose next thread
  • Load thread state from control block

Thread Control Block Thread Control Block

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CS 423: Operating Systems Design 47

Program Counter Program instructions Code Segment Offset Stack Segment Stack Pointer Registers Stack Program Counter Program instructions Code Segment Offset Stack Segment Stack Pointer Registers Stack

Save PC on thread stack Jump to yield() function yield()

  • Save thread state in thread control block

(SP, registers, segment pointers, …)

  • Choose next thread
  • Load thread state from control block
  • Pop PC from thread stack (return from handler)

Thread Control Block Thread Control Block

CTX Switch: Yield

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CS 423: Operating Systems Design 48

CTX Switch: Yield

Program Counter Program instructions Code Segment Offset Stack Segment Stack Pointer Registers Stack Program Counter Program instructions Code Segment Offset Stack Segment Stack Pointer Registers Stack

Save PC on thread stack Jump to yield() function yield()

  • Choose next thread
  • swapcontext()

Thread Control Block Thread Control Block

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CS 423: Operating Systems Design 49

Scheduler

Program Counter Program instructions Code Segment Offset Stack Segment Stack Pointer Registers Stack Program Counter Program instructions Code Segment Offset Stack Segment Stack Pointer Registers Stack

Save PC on thread stack Jump to yield() function yield()

  • Choose next thread
  • swapcontext()

Thread Control Block Thread Control Block

Where is the Scheduling Policy?

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CS 423: Operating Systems Design 50

Scheduler

Program Counter Program instructions Code Segment Offset Stack Segment Stack Pointer Registers Stack Program Counter Program instructions Code Segment Offset Stack Segment Stack Pointer Registers Stack

Save PC on thread stack Jump to yield() function yield()

  • NextThreadID = scheduler()
  • swapcontext()

Thread Control Block Thread Control Block

Where is the Scheduling Policy?

Maintains a sorted queue of ready threads