Project1: Threads

In this assignment, we give you a minimally functional thread system. Your job is to extend the functionality of this system to gain a better understanding of synchronization problems. You will be working primarily in the threads directory for this assignment, with some work in the devices directory on the side. Compilation should be done in the threads directory. Before you read the description of this project, you should at least skim the material Synchronization.

Background

Understanding Threads

The first step is to read and understand the code for the initial thread system. Pintos already implements thread creation and thread completion, a simple scheduler to switch between threads, and synchronization primitives (semaphores, locks, condition variables, and optimization barriers).

Some of this code might seem slightly mysterious. If you haven't already compiled and run the base system, as described in the introduction you should do so now. You can read through parts of the source code to see what's going on. If you like, you can add calls to printf() almost anywhere, then recompile and run to see what happens and in what order. You can also run the kernel in a debugger and set breakpoints at interesting spots, single-step through code and examine data, and so on.

When a thread is created, you are creating a new context to be scheduled. You provide a function to be run in this context as an argument to thread_create(). The first time the thread is scheduled and runs, it starts from the beginning of that function and executes in that context. When the function returns, the thread terminates. Each thread, therefore, acts like a mini-program running inside Pintos, with the function passed to thread_create() acting like main().

At any given time, exactly one thread runs and the rest, if any, become inactive. The scheduler decides which thread to run next. (If no thread is ready to run at any given time, then the special idle thread, implemented in idle(), runs.) Synchronization primitives can force context switches when one thread needs to wait for another thread to do something.

The mechanics of a context switch are in thread_launch() in threads/thread.c (You don't have to understand it.) It saves the state of the currently running thread and restores the state of the thread we're switching to.

Using the GDB debugger, slowly trace through a context switch to see what happens (see GDB). You can set a breakpoint on schedule() to start out, and then single-step from there. Be sure to keep track of each thread's address and state, and what procedures are on the call stack for each thread. You will notice that when one thread executes iret in do_iret(), another thread starts running.

Warning: In Pintos, each thread is assigned a small, fixed-size execution stack just under 4 kB in size. The kernel tries to detect stack overflow, but it cannot do so perfectly. You may cause bizarre problems, such as mysterious kernel panics, if you declare large data structures as non-static local variables, e.g. int buf[1000];. Alternatives to stack allocation include the page allocator and the block allocator (see Memory Allocation.)

Source Files

Here is a brief overview of the files in the threads, and include/threads directory. You will not need to modify most of this code, but the hope is that presenting this overview will give you a start on what code to look at.

threads codes

• loader.S, loader.h

  The kernel loader. Assembles to 512 bytes of code and data that the PC BIOS loads into memory and which in turn finds the kernel on disk, loads it into memory, and jumps to bootstrap() in start.S. You should not need to look at this code or modify it. start.S does basic setup needed for memory protection and jump into the 64bit long mode. Unlike the loader, this code is actually part of the kernel.

• kernel.lds.S

  The linker script used to link the kernel. Sets the load address of the kernel and arranges for start.S to be near the beginning of the kernel image. Again, you should not need to look at this code or modify it, but it's here in case you're curious.

• init.c, init.h

  Kernel initialization, including main(), the kernel's main program. You should look over main() at least to see what gets initialized. You might want to add your own initialization code here.

• thread.c, thread.h

  Basic thread support. Much of your work will take place in these files. thread.h defines struct thread, which you are likely to modify in all four projects. See Threads for more information.

• palloc.c, palloc.h

  Page allocator, which hands out system memory in multiples of 4 kB pages. See Page Allocator for more information.

• malloc.c, malloc.h

  A simple implementation of malloc() and free() for the kernel. See Block Allocator for more information.

• interrupt.c, interrupt.h

  Basic interrupt handling and functions for turning interrupts on and off.

• intr-stubs.S, intr-stubs.h

  Assembly code for low-level interrupt handling.

• synch.c, synch.h

  Basic synchronization primitives: semaphores, locks, condition variables, and optimization barriers. You will need to use these for synchronization in all four projects. See Synchronization for more information.

• mmu.c, mmu.h

  Functions for x86-64 page table operations. You will look this file closer after the lab1.

• io.h

  Functions for I/O port access. This is mostly used by source code in the devices directory that you won't have to touch.

• vaddr.h, pte.h

  Functions and macros for working with virtual addresses and page table entries. These will be more important to you in project 3. For now, you can ignore them.

• flags.h

  Macros that define a few bits in the x86-64 flags register. Probably of no interest.

devices codes

The basic threaded kernel also includes these files in the devices directory:

• timer.c, timer.h

  System timer that ticks, by default, 100 times per second. You will modify this code in this project.

• vga.c, vga.h

  VGA display driver. Responsible for writing text to the screen. You should have no need to look at this code. printf() calls into the VGA display driver for you, so there's little reason to call this code yourself.

• serial.c, serial.h

  Serial port driver. Again, printf() calls this code for you, so you don't need to do so yourself. It handles serial input by passing it to the input layer (see below).

• block.c, block.h

  An abstraction layer for block devices, that is, random-access, disk-like devices that are organized as arrays of fixed-size blocks. Out of the box, Pintos supports two types of block devices: IDE disks and partitions. Block devices, regardless of type, won't actually be used until project 2.

• ide.c, ide.h

  Supports reading and writing sectors on up to 4 IDE disks.

• partition.c, partition.h

  Understands the structure of partitions on disks, allowing a single disk to be carved up into multiple regions (partitions) for independent use.

• kbd.c, kbd.h

  Keyboard driver. Handles keystrokes passing them to the input layer (see below).

• input.c, input.h

  Input layer. Queues input characters passed along by the keyboard or serial drivers.

• intq.c, intq.h

  Interrupt queue, for managing a circular queue that both kernel threads and interrupt handlers want to access. Used by the keyboard and serial drivers.

• rtc.c, rtc.h

  Real-time clock driver, to enable the kernel to determine the current date and time. By default, this is only used by thread/init.c to choose an initial seed for the random number generator.

• speaker.c, speaker.h

  Driver that can produce tones on the PC speaker.

• pit.c, pit.h

  Code to configure the 8254 Programmable Interrupt Timer. This code is used by both devices/timer.c and devices/speaker.c because each device uses one of the PIT's output channel.

lib codes

Finally, lib and lib/kernel contain useful library routines. (lib/user will be used by user programs, starting in project 2, but it is not part of the kernel.) Here's a few more details:

• ctype.h, inttypes.h, limits.h, stdarg.h, stdbool.h, stddef.h, stdint.h, stdio.c, stdio.h, stdlib.c, stdlib.h, string.c, string.h

  A subset of the standard C library.

• debug.c, debug.h

  Functions and macros to aid debugging. See Debugging Tools for more information.

• random.c, random.h

  Pseudo-random number generator. The actual sequence of random values will not vary from one Pintos run to another.

• round.h

  Macros for rounding.

• syscall-nr.h

  System call numbers. Not used until project 2.

• kernel/list.c, kernel/list.h

  Doubly linked list implementation. Used all over the Pintos code, and you'll probably want to use it a few places yourself in project 1. We recommand you to skim this code before you start (especially comments in the header file.)

• kernel/bitmap.c, kernel/bitmap.h

  Bitmap implementation. You can use this in your code if you like, but you probably won't have any need for it in project 1.

• kernel/hash.c, kernel/hash.h

  Hash table implementation. Likely to come in handy for project 3.

• kernel/console.c, kernel/console.h, kernel/stdio.h

  Implements printf() and a few other functions.

Synchronization

Proper synchronization is an important part of the solutions to these problems. Any synchronization problem can be easily solved by turning interrupts off: while interrupts are off, there is no concurrency, so there's no possibility for race conditions. Therefore, it's tempting to solve all synchronization problems this way, but don't. Instead, use semaphores, locks, and condition variables to solve the bulk of your synchronization problems. Read the tour section on synchronization (see Synchronization) or the comments in threads/synch.c if you're unsure what synchronization primitives may be used in what situations.

In the Pintos projects, the only class of problem best solved by disabling interrupts is coordinating data shared between a kernel thread and an interrupt handler. Because interrupt handlers can't sleep, they can't acquire locks. This means that data shared between kernel threads and an interrupt handler must be protected within a kernel thread by turning off interrupts.

This project only requires accessing a little bit of thread state from interrupt handlers. For the alarm clock, the timer interrupt needs to wake up sleeping threads. In the advanced scheduler, the timer interrupt needs to access a few global and per-thread variables. When you access these variables from kernel threads, you will need to disable interrupts to prevent the timer interrupt from interfering.

When you do turn off interrupts, take care to do so for the least amount of code possible, or you can end up losing important things such as timer ticks or input events. Turning off interrupts also increases the interrupt handling latency, which can make a machine feel sluggish if taken too far.

The synchronization primitives themselves in synch.c are implemented by disabling interrupts. You may need to increase the amount of code that runs with interrupts disabled here, but you should still try to keep it to a minimum.

Disabling interrupts can be useful for debugging, if you want to make sure that a section of code is not interrupted. You should remove debugging code before turning in your project. (Don't just comment it out, because that can make the code difficult to read.)

There should be no busy waiting in your submission. A tight loop that calls thread_yield() is one form of busy waiting.

Development Suggestions

In the past, many groups divided the assignment into pieces, then each group member worked on his or her piece until just before the deadline, at which time the group reconvened to combine their code and submit. This is a bad idea. We do not recommend this approach. Groups that do this often find that two changes conflict with each other, requiring lots of last-minute debugging. Some groups who have done this have turned in code that did not even compile or boot, much less pass any tests.

Instead, we recommend integrating your team's changes early and often, using a source code control system such as git. This is less likely to produce surprises, because everyone can see everyone else's code as it is written, instead of just when it is finished. These systems also make it possible to review changes and, when a change introduces a bug, drop back to working versions of code.

You should expect to run into bugs that you simply don't understand while working on this and subsequent projects. When you do, reread the appendix on debugging tools, which is filled with useful debugging tips that should help you to get back up to speed (see Debugging Tools). Be sure to read the section on backtraces (see Backtraces), which will help you to get the most out of every kernel panic or assertion failure.