Homework: Threads and Context Switching

Read: setjmp.S and proc.c (focus on the code that switches between processes, specifically scheduler and sched).

Hand-In Procedure

You are to turn in this homework during lecture. Please write up your answers to the exercises below and hand them in to a 6.828 staff member at the beginning of lecture.

Introduction

In this homework you will investigate how the kernel switches between two processes.

Assignment:

Turn in: Where does the scheduler's stack sit in memory? Is it the same stack that a running process uses when executing in the kernel? (The scheduler is in proc.c.)

Turn in: What value does setjmp return first to its caller? (Setjmp is in setjmp.S.)

Turn in: What value does setjmp return when it returns the second time to its caller?

Turn in:: If we look at a setjmp on a particular jump buffer followed by a longjmp to that same buffer (that is, both functions are called once), setjmp returns twice! and longjmp never returns. How is this accomplished by setjmp.S? (A sentence or two will suffice.)

In proc.c's scheduler there are two lines:

      if(setjmp(&cpus[cpu()].jmpbuf) == 0)
        longjmp(&p->jmpbuf);

      cprintf("setjmp called in scheduler\n");
      if(setjmp(&cpus[cpu()].jmpbuf) == 0){
        cprintf("setjmp in scheduler returned 0; longjmp\n");
        longjmp(&p->jmpbuf);
      }else
        cprintf("setjmp in scheduler returned 1\n");

Similarly, in sched, replace:

  if(setjmp(&p->jmpbuf) == 0)
    longjmp(&cpus[cpu()].jmpbuf);

  cprintf("setjmp called in sched\n");
  if(setjmp(&p->jmpbuf) == 0){
    cprintf("setjmp in sched returned 0; longjmp\n");
    longjmp(&cpus[cpu()].jmpbuf);
  }else
    cprintf("setjmp in sched returned 1\n");

Rebuild your kernel and boot it on bochs. You will get around 150 printed lines about setjmp as the kernel boots.

For now, ignore the first few lines of output. You should see a regular six-line pattern repeated over and over in the printed lines.

Turn in: What is the six-line pattern? (The first line is setjmp called in scheduler.)

Turn in: Why are there two setjmp returned prints for each setjmp called print?

Now look at the first three lines of output about setjmp. You will notice that they do not follow the pattern. The first two lines are:

  setjmp called in scheduler
  setjmp in scheduler returned 0; longjmp

Turn in: In the repeated six-line pattern, where does the longjmp in the scheduler return to (which line number in the printout)? Where does the very first longjmp in the scheduler return to (which line number in the printout)?

(Feel free to set a breakpoint at the return statement in longjmp to answer this question.)

This completes the homework.