Update post of os lab of scheduler.

_posts/2022-02-22-cs350-labs.md

@@ -31,8 +31,8 @@ Remember, it's for helping in learning. DON'T COPY & PASTE CODE!
 
 ### First user process in xv6
 #### Kernel works
-In xv6, as the same as conventional linux OS, the very first user level process is **init**.
+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 are happened in a high privileged mode(kernel level).
+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.
@@ -65,26 +65,26 @@ main(void)
   mpmain();
 }
 ~~~  
-It's tricky since that **init** is a user process, but kernel can't call any user level system calls to create it.
+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, first process have no parent to fork from.
+Forking including clone the whole user virtual memory layout. However, the first process has no parent to fork from.
-That's why its makes the creation of the first user process becomes so unique.
+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 assigned to the process as ready. 
+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 just being allocated.
+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 point to an external integer as the size of the **init**'s text segment(_binary_initcode_size).
+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** has compiled into the kernel?
+So now, the problem becomes when and where did instructions for **init** have compiled into the kernel?
 ~~~c
 void
 userinit(void)
@@ -276,3 +276,90 @@ main(void)
 [^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?
+
+