---
title:  "Labs of CS350"
date:   2022-02-22 17:08:17 -0400
tags: ["Xv6", "Teaching", "Operating system", "Binghamton university"]
author: Pengzhan Hao
cover: '/static/2022-02/BU.jpeg'
---

This will be a series regarding lab I gave during the spring 2022 semester.
  
The reason why I am writing this down is because it has been a week and no students ask for the solution of the last Lab.
I realise that learning gap between students are huge, especially when a non-profit university is admitting more and more students.
To help all students in understanding concepts of modern OS, I decided to write this post.
  
It starts with the past lab content I have (as the skelton), and will be amended with extra materials I think it helps.
Remember, it's for helping in learning. DON'T COPY & PASTE CODE!

## Index
[Lab1: Introduction of Makefile and Xv6.](#lab1-introduction)  
[Lab3: System calls for process management.](#lab3-process)  
[Lab4: Inter-processes communication.](#lab4-ipc)  
[Lab6/7: CPU scheduling.](#lab6-7-scheduling)

## Lab1-Introduction

## Lab3-Process

## Lab4-IPC

## Lab6-7-Scheduling

### First user process in xv6
#### Kernel works
In xv6, as the same as conventional linux OS, the very first user-level process is **init**.
Before **init**'s running, all the OS bootstraps happen in a highly privileged mode(kernel level).

Xv6's kernel has the entry point as the main function located in the file *main.c*.
The main function invokes 17 functions to set up kernel page tables, interrupt handlers, I/O devices and etc.
When all kernel preparations are done, by calling the function ***userinit()***, kernel will boot up process init.
~~~c
int
main(void)
{
  kinit1(end, P2V(4*1024*1024)); // phys page allocator
  kvmalloc();      // kernel page table
  mpinit();        // collect info about this machine
  lapicinit();
  seginit();       // set up segments
  cprintf("\ncpu%d: starting xv6\n\n", cpu->id);
  picinit();       // interrupt controller
  ioapicinit();    // another interrupt controller
  consoleinit();   // I/O devices & their interrupts
  uartinit();      // serial port
  pinit();         // process table
  tvinit();        // trap vectors
  binit();         // buffer cache
  fileinit();      // file table
  ideinit();       // disk
  if(!ismp)
    timerinit();   // uniprocessor timer
  startothers();   // start other processors
  kinit2(P2V(4*1024*1024), P2V(PHYSTOP)); // must come after startothers()
  userinit();      // first user process
  // Finish setting up this processor in mpmain.
  mpmain();
}
~~~  
It's tricky since that **init** is a user process, but kernel can't call any user-level system calls to create it.
Why? 1. Kernel has all privileges to create a user process. So it doesn't need to call system calls such as ***fork()***. 
And 2. All other user processes can be created by forking from its parent. 
Forking including clone the whole user virtual memory layout. However, the first process has no parent to fork from.
That's why it makes the creation of the first user process becomes so unique.
   
In *proc.c*, ***userinit()*** define there gives us the whole procedure of creating **init**.
Similar to the ***fork()***, but more simple.
Process control block(structures for storing the process status) was created at the very first by calling ***allocproc()***.
After then, by invoking ***setupkvm()***(defined in *vm.c*), kernel memory map was setup for the process.
During setting up kernel memory map, a page size virtual memory will be assigned to the process as ready. 
And later, this page size memory will be used to store instructions of **init**.
  
Followed by setup kernel stack for the **init** process, calling ***inituvm()*** will load **init**'s text into the page that is just being allocated.
***inituvm()*** takes 3 arguments: a pointer to the process's page directory (p->pgdir),
a char-type pointer declared from external which point to **init**'s text segment(_binary_initcode_start), and
a char-type pointer which points to an external integer as the size of the **init**'s text segment(_binary_initcode_size).
Simply put, it will load instructions of **init** into the memory.
  
So now, the problem becomes when and where did instructions for **init** have compiled into the kernel?
~~~c
void
userinit(void)
{
  struct proc *p;
  extern char _binary_initcode_start[], _binary_initcode_size[];
  
  p = allocproc();
  initproc = p;
  if((p->pgdir = setupkvm()) == 0)
    panic("userinit: out of memory?");
  inituvm(p->pgdir, _binary_initcode_start, (int)_binary_initcode_size);
  p->sz = PGSIZE;
  memset(p->tf, 0, sizeof(*p->tf));
  p->tf->cs = (SEG_UCODE << 3) | DPL_USER;
  p->tf->ds = (SEG_UDATA << 3) | DPL_USER;
  p->tf->es = p->tf->ds;
  p->tf->ss = p->tf->ds;
  p->tf->eflags = FL_IF;
  p->tf->esp = PGSIZE;
  p->tf->eip = 0;  // beginning of initcode.S

  safestrcpy(p->name, "initcode", sizeof(p->name));
  p->cwd = namei("/");

  p->state = RUNNABLE;
}
~~~  
#### Where the user-level code was integrated?
If you search the keyword "_binary_initcode_start" in the source code, you can't find any references.
The clue comes from the *Makefile*.
  
In the makefile, **initcode** is a prerequisites to compile the kernel image.
**Step 1**: Before kernel was compiled, *initcode.S* was first compiled to a runnable binary *initcode*.
This binary was very odd because it was not supposed to let any other OS to run it.
*Initcode.s* was first compiled without any standard including, and generating the intermediate file *initcode.o*.
  
**Step 2**: *Initcode.o* then linked to *Initcode.out* with two uncommon settings. 
First it specify the entry of this binary file as when "start" symbol points to.
This "start" symbol was declared in the assembly code.
Second it specify a absolute address(0) for the text segments. 
By doing this, text segments will be placed at the start of the binary file (except the header of the ELF)[^ldman].
  
**Step 3**: *Initcode.out* is already a minimized binary but it's not enough.
That's why when using **objcopy** to copy it to the file *initcode*, it further strip all headers and debug information[^objcopyman].
At this point, we have a minimal binary file *initcode*.
From the first byte of this file, it's only includes runnable instructions.
And the size of the file is only 44 bytes.
~~~bash
initcode: initcode.S
	$(CC) $(CFLAGS) -nostdinc -I. -c initcode.S                         # Step 1
	$(LD) $(LDFLAGS) -N -e start -Ttext 0 -o initcode.out initcode.o    # Step 2
	$(OBJCOPY) -S -O binary initcode.out initcode                       # Step 3
	$(OBJDUMP) -S initcode.o > initcode.asm
~~~
  
This binary later were appended to the kernel using following commands. 
And during this appending, 3 symbols were generated and added to the symbol table of the *kernel*[^ldman].
**"_binary_initcode_start"** contains the address of where the initcode segment was appended to.
**"_binary_initcode_end"** contains the address of where the initcode segment was ended at.
**"_binary_initcode_size"** is a \*ABS\* type symbol with value 0x2C(45) that specify the size of the initcode segment is 45 bytes.
~~~bash
kernel: $(OBJS) entry.o entryother initcode kernel.ld
	$(LD) $(LDFLAGS) -T kernel.ld -o kernel entry.o $(OBJS) -b binary initcode entryother # <- This Line
	$(OBJDUMP) -S kernel > kernel.asm
	$(OBJDUMP) -t kernel | sed '1,/SYMBOL TABLE/d; s/ .* / /; /^$$/d' > kernel.sym
~~~
**In short summary**,
using objdump, we can verify that source code *initcode.S* has been compiled and loaded into the kernel.
Also the segment of initcode's instructions was located by the pointer "_binary_initcode_start".
That's explain when calling ***inituvm(p->pgdir, _binary_initcode_start, (int)_binary_initcode_size);***, 
functionalities implemented in initcode.S will be loaded into the runtime of the first process within xv6.
~~~bash
# Header of the file kernel
kernel:     file format elf32-i386
kernel
architecture: i386, flags 0x00000112:
EXEC_P, HAS_SYMS, D_PAGED
start address 0x0010000c

Program Header:
    LOAD off    0x00001000 vaddr 0x80100000 paddr 0x00100000 align 2**12
         filesz 0x00008c6a memsz 0x00008c6a flags r-x
...
Sections:
Idx Name          Size      VMA       LMA       File off  Algn
  0 .text         00008586  80100000  00100000  00001000  2**2
                  CONTENTS, ALLOC, LOAD, READONLY, CODE
...
SYMBOL TABLE:
...
8010b50c g       .data	00000000 _binary_initcode_end
...
8010b4e0 g       .data	00000000 _binary_initcode_start
...
0000002c g       *ABS*	00000000 _binary_initcode_size
...
~~~

#### User-level code

Take a look of content in the *initcode.S*, you will find the code can explain itself well.
There are no other jobs but just calling system call **exec** to run a user-level binary **"init"**.
  
*Initcode.S*:
~~~bash
# Initial process execs /init.

#include "syscall.h"
#include "traps.h"


# exec(init, argv)
.globl start
start:
  pushl $argv
  pushl $init
  pushl $0  // where caller pc would be
  movl $SYS_exec, %eax
  int $T_SYSCALL

# for(;;) exit();
exit:
  movl $SYS_exit, %eax
  int $T_SYSCALL
  jmp exit

# char init[] = "/init\0";
init:
  .string "/init\0"

# char *argv[] = { init, 0 };
.p2align 2
argv:
  .long init
  .long 0
~~~

The **"init"** mentioned above is not a pure user-level binary executable that compiled from the source code *init.c*.
Within *init.c*, a file named *console* will be created at the runtime for saving standard outputs and errors.
Then it will forked a child process(the second user process), and let it run program **"sh"**.
  
**"sh"** is the xv6's default shell, a user-level program that generated from source *sh.c*.
After the shell boots up, you can interactive with the xv6. 
This's how first process (and second process) was started in the xv6.
  
*init.c*:
~~~c
// init: The initial user-level program

#include "types.h"
#include "stat.h"
#include "user.h"
#include "fcntl.h"

char *argv[] = { "sh", 0 };

int
main(void)
{
  int pid, wpid;

  if(open("console", O_RDWR) < 0){
    mknod("console", 1, 1);
    open("console", O_RDWR);
  }
  dup(0);  // stdout
  dup(0);  // stderr

  for(;;){
    printf(1, "init: starting sh\n");
    pid = fork();
    if(pid < 0){
      printf(1, "init: fork failed\n");
      exit();
    }
    if(pid == 0){
      exec("sh", argv);
      printf(1, "init: exec sh failed\n");
      exit();
    }
    while((wpid=wait()) >= 0 && wpid != pid)
      printf(1, "zombie!\n");
  }
}
~~~

[^ldman]: [ld\(1\) - Linux man page](https://linux.die.net/man/1/ld)
[^objcopyman]: [3 objcopy - binutils mannual](https://sourceware.org/binutils/docs/binutils/objcopy.html)

### Xv6's round robin schduler

The Scheduler is the core of an operating system. 
With the scheduling of processes, the kernel can achieve near-real-time execution of multiple workloads.
The scheduling problem is also an active aspect of computer science research. 
You can’t have one algorithm to fit all scenarios.
  
Xv6 by default has a round-robin scheduler. 
It’s controlled using two-level for-loops, where the top-level for-loop is an endless loop that will keep the scheduler busy running. 
The second-level nested for-loop will iterate a data structure named Ptable where all control information for processes is stored. 
Information including pid, process name, etc. is stored in a structure called proc. Ptable is an array of processes. 
Every runnable process in the Ptable will run strictly 1 time tick until the for-loop reached the last process in the Ptable. 
Then it will loop back to the top-level for-loop for the next iteration of processes. 
  

~~~c
// In file proc.c
struct {
  struct spinlock lock;
  struct proc proc[NPROC];
} ptable;

// In file proc.h
struct proc {
  uint sz;                     // Size of process memory (bytes)
  pde_t* pgdir;                // Page table
  char *kstack;                // Bottom of kernel stack for this process
  enum procstate state;        // Process state
  int pid;                     // Process ID
  struct proc *parent;         // Parent process
  struct trapframe *tf;        // Trap frame for current syscall
  struct context *context;     // swtch() here to run process
  void *chan;                  // If non-zero, sleeping on chan
  int killed;                  // If non-zero, have been killed
  struct file *ofile[NOFILE];  // Open files
  struct inode *cwd;           // Current directory
  char name[16];               // Process name (debugging)
};
~~~

~~~c
// In file proc.c
void
scheduler(void)
{
  struct proc *p;

  for(;;){
    // Enable interrupts on this processor.
    sti();

    // Loop over process table looking for process to run.
    acquire(&ptable.lock);
    for(p = ptable.proc; p < &ptable.proc[NPROC]; p++){
      if(p->state != RUNNABLE)
        continue;

      // Switch to chosen process.  It is the process's job
      // to release ptable.lock and then reacquire it
      // before jumping back to us.
      proc = p;
      switchuvm(p);
      p->state = RUNNING;
      swtch(&cpu->scheduler, proc->context);
      switchkvm();

      // Process is done running for now.
      // It should have changed its p->state before coming back.
      proc = 0;
    }
    release(&ptable.lock);

  }
}
~~~

It’s not hard to understand why this logic makes a round-robin manner. 
This is very important to understand how to pick a process to run because scheduling is about always picking the appropriate process to achieve higher performance. 
  
You can always come up with some new ideas for designing a good scheduler policy. 
Understanding how to switch from one process to another is equivalently important. 
  
Once the process for the next time tick is selected. 
It’s time to switch from the running scheduler to the selected process. Wait for a second, there are two questions we haven’t answered. 
1. What is the running scheduler? 
2. How did the last running process stop running and give the CPU back to the scheduler?




### Lab


