carnegie mellon exceptions and processes slides adapted from: gregory kesden and markus püschel of...

Carnegie Mellon

Exceptions and Processes

Slides adapted from: Gregory Kesden and Markus Püschel of Carnegie Mellon University

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Control Flow

<startup>inst1

inst2

inst3

…instn

<shutdown>

Processors do only one thing: From startup to shutdown, a CPU simply reads and executes

(interprets) a sequence of instructions, one at a time This sequence is the CPU’s control flow (or flow of control)

Physical control flow

Time

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Altering the Control Flow Up to now: two mechanisms for changing control flow:

Jumps and branches Call and returnBoth react to changes in program state

Insufficient for a useful system: Difficult to react to changes in system state data arrives from a disk or a network adapter instruction divides by zero user hits Ctrl-C at the keyboard System timer expires

System needs mechanisms for “exceptional control flow”

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Exceptional Control Flow Exists at all levels of a computer system Low level mechanisms

Exceptions change in control flow in response to a system event

(i.e., change in system state) Combination of hardware and OS software

Higher level mechanisms Process context switch Signals Nonlocal jumps: setjmp()/longjmp() Implemented by either:

OS software (context switch and signals) C language runtime library (nonlocal jumps)

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Exceptions An exception is a transfer of control to the OS in response to

some event (i.e., change in processor state)

Examples: div by 0, arithmetic overflow, page fault, I/O request completes, Ctrl-C

User Process OS

exceptionexception processingby exception handler

• return to I_current• return to I_next•abort

event I_currentI_next

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01

2...

n-1

Interrupt Vectors

Each type of event has a unique exception number k

k = index into exception table (a.k.a. interrupt vector)

Handler k is called each time exception k occurs

ExceptionTable

code for exception handler 0

code for exception handler 1

code forexception handler 2

code for exception handler n-1

...

Exception numbers

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Asynchronous Exceptions (Interrupts) Caused by events external to the processor

Indicated by setting the processor’s interrupt pin Handler returns to “next” instruction

Examples: I/O interrupts

hitting Ctrl-C at the keyboard arrival of a packet from a network arrival of data from a disk

Hard reset interrupt hitting the reset button

Soft reset interrupt hitting Ctrl-Alt-Delete on a PC

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Synchronous Exceptions Caused by events that occur as a result of executing an

instruction: Traps

Intentional Examples: system calls, breakpoint traps, special instructions Returns control to “next” instruction

Faults Unintentional but possibly recoverable Examples: page faults (recoverable), protection faults

(unrecoverable), floating point exceptions Either re-executes faulting (“current”) instruction or aborts

Aborts unintentional and unrecoverable Examples: parity error, machine check Aborts current program

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Trap Example: Opening File User calls: open(filename, options) Function open executes system call instruction int

OS must find or create file, get it ready for reading or writing Returns integer file descriptor

0804d070 <__libc_open>: . . . 804d082: cd 80 int $0x80 804d084: 5b pop %ebx . . .

User Process OS

exception

open filereturns

intpop

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Fault Example: Page Fault User writes to memory location That portion (page) of user’s memory

is currently on disk

Page handler must load page into physical memory Returns to faulting instruction Successful on second try

int a[1000];main (){ a[500] = 13;}

80483b7: c7 05 10 9d 04 08 0d movl $0xd,0x8049d10

User Process OS

exception: page faultCreate page and load into memoryreturns

movl

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Fault Example: Invalid Memory Reference

Page handler detects invalid address Sends SIGSEGV signal to user process User process exits with “segmentation fault”

int a[1000];main (){ a[5000] = 13;}

80483b7: c7 05 60 e3 04 08 0d movl $0xd,0x804e360

User Process OS

exception: page fault

detect invalid addressmovl

signal process

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User mode vs. Kernel mode Priviliged instructions

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Exception handlers Return adress

Depends on the class of exception Current instruction (page fault) Next instruction

Push some additional processor state Necessary to restart the interrupted program when the handler

returns If the control is being transferred from user to kernel

All these items are pushed on the kernel stack Exception handlers run in kernel mode

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Exception Table IA32 (Excerpt)

Exception Number Description Exception Class

0 Divide error Fault

13 General protection fault Fault

14 Page fault Fault

18 Machine check Abort

32-127 OS-defined Interrupt or trap

128 (0x80) System call Trap

129-255 OS-defined Interrupt or trap

Check pp. 183:http://download.intel.com/design/processor/manuals/253665.pdf

http://download.intel.com/design/processor/manuals/253665.pdf

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# write our string to stdout

movl $len,%edx # third argument: message length movl $msg,%ecx # second argument: message to write movl $1,%ebx # first argument: file handle (stdout) movl $4,%eax # system call number (sys_write) int $0x80 # call kernel

# and exit

movl $0,%ebx # first argument: exit code movl $1,%eax # system call number (sys_exit) int $0x80 # call kernel

.data # section declaration

msg: .ascii "Hello, world!\n" # our dear string len = . - msg # length of our dear string

int main(){

write(1, “hello, world\n”, 13);exit(0);

}

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Today Exceptional Control Flow Processes

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Processes Definition: A process is an instance of a running program.

One of the most profound ideas in computer science Not the same as “program” or “processor”

Process provides each program with two key abstractions: Logical control flow

Each program seems to have exclusive use of the CPU Private virtual address space

Each program seems to have exclusive use of main memory

How are these Illusions maintained? Process executions interleaved (multitasking) Address spaces managed by virtual memory system

we’ll talk about this in a couple of weeks

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What is a process? A process is the OS's abstraction for execution

A process represents a single running application on the system Process has three main components: 1) Address space

The memory that the process can access Consists of various pieces: the program code, static variables, heap,

stack, etc. 2) Processor state

The CPU registers associated with the running process Includes general purpose registers, program counter, stack pointer,

etc. 3) OS resources

Various OS state associated with the process Examples: open files, network sockets, etc.

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Process Address Space

The range of virtual memory addresses that the process can access

Includes the code of the running program

The data of the running program (static variables and heap)

An execution stack Local variables

and saved registers for each procedure call

Stack

Heap

Initialized vars(data

segment)Code

(text segment)

Address space

0x00000000

0xFFFFFFFF

Stack pointer

Program counter

Uninitialized vars(BSS segment)

(Reserved for OS)

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Process Address Space Note!!! This is the

process's own view of the address space ---

physical memory may not be laid out this way at all. In fact, on systems

that support multiple running processes, it's pretty muchguaranteed to look different.

The virtual memory system provides this illusion to each process.

Stack

Heap


segment)Code

(text segment)

Address space

0x00000000

0xFFFFFFFF

Stack pointer

Program counter

Uninitialized vars(BSS segment)

(Reserved for OS)

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Execution State of a Process Each process has an execution state

Indicates what the process is currently doing Running:

Process is currently using the CPU Ready:

Currently waiting to be assigned to a CPU That is, the process could be running, but another process is using

the CPU Waiting (or sleeping):

Process is waiting for an event Such as completion of an I/O, a timer to go off, etc. Why is this different than “ready” ?

As the process executes, it moves between these states What state is the process in most of the time?

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Process State Transitions What causes schedule and unschedule transitions?

New

Terminated

Ready

Running

Waiting

create

kill or exit I/O, page fault,

etc.

I/O done

scheduleunschedule

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Process Control Block OS maintains a Process Control Block (PCB) for each process The PCB is a big data structure with many fields:

Process ID User ID Execution state

ready, running, or waiting Saved CPU state

CPU registers saved the last time the process was suspended. OS resources

Open files, network sockets, etc. Memory management info Scheduling priority

Give some processes higher priority than others Accounting information

Total CPU time, memory usage, etc.

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Context Switching Processes are managed by a shared chunk of OS code

called the kernel Important: the kernel is not a separate process, but rather runs as part

of some user process Control flow passes from one process to another via a

context switch

Process A Process B

user code

kernel code

user code

kernel code

user code

context switch

context switch

Time

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Context Switching The act of swapping a process state on or off the CPU is a context

switch

PC

Registers

PC

Registers

PID 1342State:

RunningPC

Registers

PID 4277State: Ready

PC

Registers


Save current CPU state

Currently running process

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switch

PC

Registers

PC

Registers


PC

Registers


PC

Registers


Suspend process

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switch

PC

Registers

PC

Registers


PC

Registers

PID 4277State:

RunningPC

Registers


Restore CPU state of new process

Pick next process

PC

Registers

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Context Switch Overhead Context switches are not cheap

Generally have a lot of CPU state to save and restore Also must update various flags in the PCB Picking the next process to run – scheduling – is also expensive

Context switch overhead in Linux 2.4.21 About 5.4 usec on a 2.4 GHz Pentium 4 This is equivalent to about 13,200 CPU cycles!

Not quite that many instructions since CPI > 1

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Context Switching in Linux

Process A

time

Process A is happily running along...

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Process A

time

Timer interrupt handler

1) Timer interrupt fires

2) PC saved on stackUserKernel

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Process A


time


2) PC saved on stack

Scheduler

4) Call schedule() routine

3) Rest of CPU statesaved in PCB

UserKernel

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Process A


time


2) PC saved on stack

Scheduler5) Decide next process to run

4) Call schedule() routine

3) Rest of CPU statesaved in PCB

Timer interrupthandler

6) Resume Process B(suspended withintimer interrupt handler!)

UserKernel

Process B

7) Return from interrupthandler – process CPUstate restored

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State Queues The OS maintains a set of state queues for each process state

Separate queues for ready and waiting states Generally separate queues for each kind of waiting process

e.g., One queue for processes waiting for disk I/O Another queue for processes waiting for network I/O, etc.

PC

Registers


PC

Registers

PID 4110State:

WaitingPC

Registers

PID 4002State:

WaitingPC

Registers

PID 4923State:

Waiting

PC

Registers


Ready queue

Disk I/O queue

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State Queue Transitions PCBs move between these queues as their state changes

When scheduling a process, pop the head off of the ready queue When I/O has completed, move PCB from waiting queue to ready

queue

PC

Registers


PC

Registers

PID 4110State:

WaitingPC

Registers

PID 4002State:

Waiting

PC

Registers


PC

Registers

PID 4923State:

Waiting

Ready queue

Disk I/O queue

PC

Registers


Disk I/O completes

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Process Creation One process can create, or fork, another process

The original process is the parent The new process is the child

What creates the first process in the system, and when? Parent process defines resources and access rights of

children Just like real life ... e.g., child process inherits parent's user ID

% pstree -p

init(1)-+-apmd(687) |-atd(847) |-crond(793) |-rxvt(2700)---bash(2702)---ooffice(2853) `-rxvt(2752)---bash(2754)

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PC

Registers


UNIX fork mechanism In UNIX, use the fork() system call to create a new process

This creates an exact duplicate of the parent process!! Creates and initializes a new PCB Creates a new address space Copies entire contents of parent's address space into the child Initializes CPU and OS resources to a copy of the parent's Places new PCB on ready queue

PC

Registers

PID 4109State:

Running

PC

Registers


PC

Registers


Ready queue

Process calls fork()

PC

Registers

PID 4110State: ReadyCopy state

PC

Registers


Add to end ofready queue

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fork: Creating New Processes int fork(void)

creates a new process (child process) that is identical to the calling process (parent process)

returns 0 to the child process returns child’s pid to the parent process

Fork is interesting (and often confusing) because it is called once but returns twice

pid_t pid = fork();if (pid == 0) { printf("hello from child\n");} else { printf("hello from parent\n");}

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Understanding fork


Process npid_t pid = fork();if (pid == 0) { printf("hello from child\n");} else { printf("hello from parent\n");}

Child Process m


pid = m


pid = 0



hello from parent hello from childWhich one is first?

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Fork Example #1

void fork1(){ int x = 1; pid_t pid = fork(); if (pid == 0) {

printf("Child has x = %d\n", ++x); } else {

printf("Parent has x = %d\n", --x); } printf("Bye from process %d with x = %d\n", getpid(), x);}

Parent and child both run same code Distinguish parent from child by return value from fork

Start with same state, but each has private copy Including shared output file descriptor Relative ordering of their print statements undefined

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Fork Example #2

void fork2(){ printf("L0\n"); fork(); printf("L1\n"); fork(); printf("Bye\n");}

Both parent and child can continue forking

L0 L1

L1

Bye

Bye

Bye

Bye

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Fork Example #3 Both parent and child can continue forking

void fork3(){ printf("L0\n"); fork(); printf("L1\n"); fork(); printf("L2\n"); fork(); printf("Bye\n");} L1 L2

L2

Bye

Bye

Bye

Bye

L1 L2

L2

Bye

Bye

Bye

Bye

L0

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void fork4(){ printf("L0\n"); if (fork() != 0) {

printf("L1\n"); if (fork() != 0) { printf("L2\n"); fork();}

} printf("Bye\n");}

L0 L1

Bye

L2

Bye

Bye

Bye

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void fork5(){ printf("L0\n"); if (fork() == 0) {

printf("L1\n"); if (fork() == 0) { printf("L2\n"); fork();}

} printf("Bye\n");}

L0 Bye

L1

Bye

Bye

Bye

L2

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Why have fork() at all? Why make a copy of the parent process? Don't you usually want to start a new program instead? Where might “cloning” the parent be useful?

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Why have fork() at all? Why make a copy of the parent process? Don't you usually want to start a new program instead? Where might “cloning” the parent be useful?

Web server – make a copy for each incoming connection Parallel processing – set up initial state, fork off multiple copies to

do work UNIX philosophy: System calls should be minimal.

Don't overload system calls with extra functionality if it is not always needed.

Better to provide a flexible set of simple primitives and let programmerscombine them in useful ways.

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Memory concerns So fork makes a copy of a process. What about memory usage?

Stack

Heap


segment)Code

(text segment)

Uninitialized vars

(BSS segment)

(Reserved for OS)

Parent

Stack

Heap


segment)Code

(text segment)

Uninitialized vars

(BSS segment)

(Reserved for OS)

Child #1

Stack

Heap


segment)Code

(text segment)

Uninitialized vars

(BSS segment)

(Reserved for OS)

Child #2

Stack

Heap


segment)Code

(text segment)

Uninitialized vars

(BSS segment)

(Reserved for OS)

Child #3Stack

Heap


segment)Code

(text segment)

Uninitialized vars

(BSS segment)

(Reserved for OS)

Child #4Stack

Heap


segment)Code

(text segment)

Uninitialized vars

(BSS segment)

(Reserved for OS)

Child #5

Stack

Heap


segment)Code

(text segment)

Uninitialized vars

(BSS segment)

(Reserved for OS)

Child #6

Stack

Heap


segment)Code

(text segment)

Uninitialized vars

(BSS segment)

(Reserved for OS)

Child #7

Stack

Heap


segment)Code

(text segment)

Uninitialized vars

(BSS segment)

(Reserved for OS)

Child #8

Stack

Heap


segment)Code

(text segment)

Uninitialized vars

(BSS segment)

(Reserved for OS)

Child #10

Stack

Heap


segment)Code

(text segment)

Uninitialized vars

(BSS segment)

(Reserved for OS)

Child #9

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Problematic?

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Memory concerns OS aggressively tries to share memory between processes.

Especially processes that are fork()'d copies of each other Copies of a parent process do not actually get a private copy

of the address space... ... Though that is the illusion that each process gets. Instead, they share the same physical memory, until one of them makes a

change. The virtual memory system is behind these shenanigans.

We will discuss this in much detail later in the course

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exit: Ending a process void exit(int status)

exits a process Normally return with status 0

atexit() registers functions to be executed upon exit

void cleanup(void) { printf("cleaning up\n");}

void fork6() { atexit(cleanup); fork(); exit(0);}

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Zombies Idea

When process terminates, still consumes system resources Various tables maintained by OS

Called a “zombie” Living corpse, half alive and half dead

Reaping Performed by parent on terminated child Parent is given exit status information Kernel discards process

What if parent doesn’t reap? If any parent terminates without reaping a child, then child will be

reaped by init process So, only need explicit reaping in long-running processes

e.g., shells and servers

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linux> ./forks 7 &[1] 6639Running Parent, PID = 6639Terminating Child, PID = 6640linux> ps PID TTY TIME CMD 6585 ttyp9 00:00:00 tcsh 6639 ttyp9 00:00:03 forks 6640 ttyp9 00:00:00 forks <defunct> 6641 ttyp9 00:00:00 pslinux> kill 6639[1] Terminatedlinux> ps PID TTY TIME CMD 6585 ttyp9 00:00:00 tcsh 6642 ttyp9 00:00:00 ps

ZombieExample

ps shows child process as “defunct”

Killing parent allows child to be reaped by init

void fork7(){ if (fork() == 0) {

/* Child */printf("Terminating Child, PID = %d\n", getpid());exit(0);

} else {printf("Running Parent, PID = %d\n", getpid());while (1) ; /* Infinite loop */

}}

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linux> ./forks 8Terminating Parent, PID = 6675Running Child, PID = 6676linux> ps PID TTY TIME CMD 6585 ttyp9 00:00:00 tcsh 6676 ttyp9 00:00:06 forks 6677 ttyp9 00:00:00 pslinux> kill 6676linux> ps PID TTY TIME CMD 6585 ttyp9 00:00:00 tcsh 6678 ttyp9 00:00:00 ps

NonterminatingChild Example

Child process still active even though parent has terminated

Must kill explicitly, or else will keep running indefinitely

void fork8(){ if (fork() == 0) {

/* Child */printf("Running Child, PID = %d\n", getpid());while (1) ; /* Infinite loop */

} else {printf("Terminating Parent, PID = %d\n", getpid());exit(0);

}}

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wait: Synchronizing with Children int wait(int *child_status)

suspends current process until one of its children terminates return value is the pid of the child process that terminated if child_status != NULL, then the object it points to will be set

to a status indicating why the child process terminated

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wait: Synchronizing with Childrenvoid fork9() { int child_status;

if (fork() == 0) { printf("HC: hello from child\n"); } else { printf("HP: hello from parent\n"); wait(&child_status); printf("CT: child has terminated\n"); } printf("Bye\n"); exit();}

HP

HC Bye

CT Bye

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wait() Example If multiple children completed, will take in arbitrary order Can use macros WIFEXITED and WEXITSTATUS to get information about exit

status

void fork10(){ pid_t pid[N]; int i; int child_status; for (i = 0; i < N; i++)

if ((pid[i] = fork()) == 0) exit(100+i); /* Child */

for (i = 0; i < N; i++) {pid_t wpid = wait(&child_status);if (WIFEXITED(child_status)) printf("Child %d terminated with exit status %d\n",

wpid, WEXITSTATUS(child_status));else printf("Child %d terminate abnormally\n", wpid);

}}

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waitpid(): Waiting for a Specific Process waitpid(pid, &status, options)

suspends current process until specific process terminates various options (that we won’t talk about)

void fork11(){ pid_t pid[N]; int i; int child_status; for (i = 0; i < N; i++)

if ((pid[i] = fork()) == 0) exit(100+i); /* Child */

for (i = 0; i < N; i++) {pid_t wpid = waitpid(pid[i], &child_status, 0);if (WIFEXITED(child_status)) printf("Child %d terminated with exit status %d\n",

wpid, WEXITSTATUS(child_status));else printf("Child %d terminated abnormally\n", wpid);

}

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fork() and execve() How do we start a new program, instead of just a copy of

the old program? Use the UNIX execve() system call

int execve(const char *filename, char *const argv [], char *const envp[]);

filename: name of executable file to run argv: Command line arguments envp: environment variable settings (e.g., $PATH, $HOME, etc.)

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fork() and execve() execve() does not fork a new process!

Rather, it replaces the address space and CPU state of the current process

Loads the new address space from the executable file and starts it from main()

So, to start a new program, use fork() followed by execve()

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execl and exec Family int execl(char *path, char *arg0, char *arg1, …, 0) Loads and runs executable at path with args arg0, arg1, …

path is the complete path of an executable object file By convention, arg0 is the name of the executable object file “Real” arguments to the program start with arg1, etc. List of args is terminated by a (char *)0 argument Environment taken from char **environ, which points to an array

of “name=value” strings: USER=ganger LOGNAME=ganger HOME=/afs/cs.cmu.edu/user/ganger

Returns -1 if error, otherwise doesn’t return! Family of functions includes execv, execve (base

function), execvp, execl, execle, and execlp

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exec: Loading and Running Programs

main() { if (fork() == 0) { execl("/usr/bin/cp", "cp", "foo", "bar", 0); } wait(NULL); printf("copy completed\n"); exit();}

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Summary Exceptions

Events that require nonstandard control flow Generated externally (interrupts) or internally (traps and faults)

Processes At any given time, system has multiple active processes Only one can execute at a time, though Each process appears to have total control of

processor + private memory space

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Summary (cont.) Spawning processes

Call to fork One call, two returns

Process completion Call exit One call, no return

Reaping and waiting for Processes Call wait or waitpid

Loading and running Programs Call execl (or variant) One call, (normally) no return

carnegie mellon exceptions and processes slides adapted from: gregory kesden and markus püschel of...

Documents

carnegie mellon control

carnegie mellon exceptions

time exception

exceptional control

exception handler n

cpus control flow

changing control flow

exception numbers