Due: Sunday 09/27 11:59 am EDT (noon)
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 read all of the following sections: 1. Introduction, C. Coding Standards, Testing and Debugging. You should at least skim the material from A.1 Loading through A.5 Memory Allocation, especially A.3 Synchronization. To complete this project you will also need to read B. 4.4BSD Scheduler.
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 (see section 1. 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 threads/switch.S
, which is 80x86
assembly code. (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 section E.5 GDB). You can set a
breakpoint on schedule() to start out, and then
single-step from there.(1) 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 calls switch_threads(),
another thread starts running, and the first thing the new thread does
is to return from switch_threads(). You will understand the thread
system once you understand why and how the switch_threads() that
gets called is different from the switch_threads() that returns.
See section A.2.3 Thread Switching, for more information.
int buf[1000];. Alternatives to stack allocation include the page allocator and the block allocator (see section A.5 Memory Allocation).
Listing 1 provides a brief overview of
the files in the 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.
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 section A.3 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.
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 (see section F.3 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 section E. Debugging Tools). Be sure to read the section on backtraces (see section E.4 Backtraces), which will help you to get the most out of every kernel panic or assertion failure.
make check runs all tests. If you are debugging one test, you can run just that test
with make tests/threads/<test_case>.result. Read the Testing part for more details.
We suggest first implementing the following, which can happen in parallel:
timer_sleep() in Exercise 1.1.
thread_create().
Then you can add full support for priority scheduling in Exercise 2.1 by considering all other possible scenarios and the synchronization primitives.
Then you can tackle either Exercise 3.1 first or Exercise 2.2 first.
Before you turn in your project, you must copy the
project 1 design document template into your source tree under the name
pintos/src/threads/DESIGNDOC
and fill it in. We recommend that
you read the design document template before you start working on the
project. See section D. Project Documentation, for a sample design document
that goes along with a fictitious project.
timer_sleep(), defined in devices/timer.c. Although a working implementation is provided, it "busy waits," that is, it spins in a loop checking the current time and calling
thread_yield() until enough time has gone by. Reimplement it to
avoid busy waiting.
timer_sleep() is useful for threads that operate in real-time,
e.g. for blinking the cursor once per second.
The argument to timer_sleep() is expressed in timer ticks, not in
milliseconds or any another unit. There are TIMER_FREQ timer
ticks per second, where TIMER_FREQ is a macro defined in
devices/timer.h. The default value is 100. We don't recommend
changing this value, because any change is likely to cause many of
the tests to fail.
Separate functions timer_msleep(), timer_usleep(), and
timer_nsleep() do exist for sleeping a specific number of
milliseconds, microseconds, or nanoseconds, respectively, but these will
call timer_sleep() automatically when necessary. You do not need
to modify them.
If your delays seem too short or too long, reread the explanation of the
-r
option to pintos (see section 1.1.4 Debugging versus Testing).
struct list and the provided functions to be useful for this exercise.
Read the comments in lib/kernel/list.h carefully, since this list design/usage
is different from the typical linked list you are familiar with (actually, Linux kernel
uses
a similar design). Searching the Pintos codebase to see how struct list is used
may also give you some inspirations.
timer_sleep implementation calls timer_ticks,
which returns the current ticks. Check where the static ticks
variable is updated. You can search with grep or use
cscope to help you find this out: type make cscope,
then type cscope -d, in the Find this C symbol: type
ticks.
The alarm clock implementation is not needed for later projects, although it could be useful for project 4.
Thread priorities range from PRI_MIN (0) to PRI_MAX (63).
Lower numbers correspond to lower priorities, so that priority 0
is the lowest priority and priority 63 is the highest.
The initial thread priority is passed as an argument to
thread_create(). If there's no reason to choose another
priority, use PRI_DEFAULT (31). The PRI_ macros are
defined in threads/thread.h
, and you should not change their
values.
ready_list (directly and indirectly).
One issue with priority scheduling is "priority inversion". Consider high, medium, and low priority threads H, M, and L, respectively. If H needs to wait for L (for instance, for a lock held by L), and M is on the ready list, then H will never get the CPU because the low priority thread will not get any CPU time. A partial fix for this problem is for H to "donate" its priority to L while L is holding the lock, then recall the donation once L releases (and thus H acquires) the lock.
priority-donate-nest and priority-donate-chain tests.
threads/thread.c.
You need not provide any interface to allow a thread to directly modify other threads' priorities.
The priority scheduler is not used in any later project.
Like the priority scheduler, the advanced scheduler chooses the thread to run based on priorities. However, the advanced scheduler does not do priority donation. Thus, we recommend that you have the priority scheduler working, except possibly for priority donation, before you start work on the advanced scheduler.
You must write your code to allow us to choose a scheduling algorithm
policy at Pintos startup time. By default, the priority scheduler
must be active, but we must be able to choose the 4.4BSD
scheduler
with the -mlfqs
kernel option. Passing this
option sets thread_mlfqs, declared in threads/thread.h
, to
true when the options are parsed by parse_options(), which happens
early in main().
When the 4.4BSD scheduler is enabled, threads no longer
directly control their own priorities. The priority argument to
thread_create() should be ignored, as well as any calls to
thread_set_priority(), and thread_get_priority() should return
the thread's current priority as set by the scheduler.
mlfqs-load-avg.c.
In fact, the inefficiency in your alarm clock implementation can also
influence your MLFQS behavior. So double check if your implementation there
can be optimized.
The advanced scheduler is not used in any later project.
We will collect your solution automatically through GitHub by taking a snapshot
by the deadline. Thus, be sure to commit your changes and do a git push
to GitHub, especially in the last few minutes! Your
submission must reside in a branch called lab1-handin.
You can create this branch with git checkout -b lab1-handin.
You can use other branches (e.g., master or lab1-dev)
during development, but be sure to sync these changes to the submission
branch with git checkout lab1-handin then git merge <branch_name>.
Double check that your submission resides in the correct branch lab1-handin
(note the middle part is a dash -, not underscore) by the deadline.
Using a different branch name will result in failure to collect and
grade your submission in time.
Here's a summary of our reference solution, produced by the
diffstat program. The final row gives total lines inserted
and deleted; a changed line counts as both an insertion and a deletion.
The reference solution represents just one possible solution. Many other solutions are also possible and many of those differ greatly from the reference solution. Some excellent solutions may not modify all the files modified by the reference solution, and some may modify files not modified by the reference solution.
devices/timer.c | 42 +++++- threads/fixed-point.h | 120 ++++++++++++++++++ threads/synch.c | 88 ++++++++++++- threads/thread.c | 196 ++++++++++++++++++++++++++---- threads/thread.h | 23 +++ 5 files changed, 440 insertions(+), 29 deletions(-) |
fixed-point.h
is a new file added by the reference solution.
Makefiles when I add a new source file?
To add a .c
file, edit the top-level Makefile.build
.
Add the new file to variable dir_SRC
, where
dir is the directory where you added the file. For this
project, that means you should add it to threads_SRC or
devices_SRC. Then run make. If your new file
doesn't get
compiled, run make clean and then try again.
When you modify the top-level Makefile.build
and re-run
make, the modified
version should be automatically copied to
threads/build/Makefile
. The converse is
not true, so any changes will be lost the next time you run make
clean from the threads
directory. Unless your changes are
truly temporary, you should prefer to edit Makefile.build
.
A new .h
file does not require editing the Makefile
s.
warning: no previous prototype for `func' mean?
It means that you defined a non-static function without
preceding it by a prototype. Because non-static functions are
intended for use by other .c
files, for safety they should be
prototyped in a header file included before their definition. To fix
the problem, add a prototype in a header file that you include, or, if
the function isn't actually used by other .c
files, make it
static.
Timer interrupts occur TIMER_FREQ times per second. You can
adjust this value by editing devices/timer.h
. The default is
100 Hz.
We don't recommend changing this value, because any changes are likely to cause many of the tests to fail.
There are TIME_SLICE ticks per time slice. This macro is
declared in threads/thread.c
. The default is 4 ticks.
We don't recommend changing this value, because any changes are likely to cause many of the tests to fail.
See section Testing.
pass()?
You are probably looking at a backtrace that looks something like this:
0xc0108810: debug_panic (lib/kernel/debug.c:32) 0xc010a99f: pass (tests/threads/tests.c:93) 0xc010bdd3: test_mlfqs_load_1 (...threads/mlfqs-load-1.c:33) 0xc010a8cf: run_test (tests/threads/tests.c:51) 0xc0100452: run_task (threads/init.c:283) 0xc0100536: run_actions (threads/init.c:333) 0xc01000bb: main (threads/init.c:137) |
This is just confusing output from the backtrace program. It
does not actually mean that pass() called debug_panic(). In
fact, fail() called debug_panic() (via the PANIC()
macro). GCC knows that debug_panic() does not return, because it
is declared NO_RETURN (see section E.3 Function and Parameter Attributes), so it doesn't include any code in fail() to take
control when debug_panic() returns. This means that the return
address on the stack looks like it is at the beginning of the function
that happens to follow fail() in memory, which in this case happens
to be pass().
See section E.4 Backtraces, for more information.
schedule()?
Every path into schedule() disables interrupts. They eventually
get re-enabled by the next thread to be scheduled. Consider the
possibilities: the new thread is running in switch_thread() (but
see below), which is called by schedule(), which is called by one
of a few possible functions:
thread_exit(), but we'll never switch back into such a thread, so
it's uninteresting.
thread_yield(), which immediately restores the interrupt level upon
return from schedule().
thread_block(), which is called from multiple places:
sema_down(), which restores the interrupt level before returning.
idle(), which enables interrupts with an explicit assembly STI
instruction.
wait() in devices/intq.c, whose callers are responsible for re-enabling interrupts.
There is a special case when a newly created thread runs for the first
time. Such a thread calls intr_enable() as the first action in
kernel_thread(), which is at the bottom of the call stack for every
kernel thread but the first.
Don't worry about the possibility of timer values overflowing. Timer values are expressed as signed 64-bit numbers, which at 100 ticks per second should be good for almost 2,924,712,087 years. By then, we expect Pintos to have been phased out of the Computer Science curriculum.
Yes, strict priority scheduling can lead to starvation because a thread will not run if any higher-priority thread is runnable. The advanced scheduler introduces a mechanism for dynamically changing thread priorities.
Strict priority scheduling is valuable in real-time systems because it offers the programmer more control over which jobs get processing time. High priorities are generally reserved for time-critical tasks. It's not "fair," but it addresses other concerns not applicable to a general-purpose operating system.
When a lock is released, the highest priority thread waiting for that lock should be unblocked and put on the list of ready threads. The scheduler should then run the highest priority thread on the ready list.
Yes. If there is a single highest-priority thread, it continues
running until it blocks or finishes, even if it calls
thread_yield().
If multiple threads have the same highest priority,
thread_yield() should switch among them in "round robin" order.
Priority donation only changes the priority of the donee thread. The donor thread's priority is unchanged. Priority donation is not additive: if thread A (with priority 5) donates to thread B (with priority 3), then B's new priority is 5, not 8.
Yes. Consider a ready, low-priority thread L that holds a lock. High-priority thread H attempts to acquire the lock and blocks, thereby donating its priority to ready thread L.
Yes. While a thread that has acquired lock L is blocked for any
reason, its priority can increase by priority donation if a
higher-priority thread attempts to acquire L. This case is
checked by the priority-donate-sema test.
Yes. If a thread added to the ready list has higher priority than the running thread, the correct behavior is to immediately yield the processor. It is not acceptable to wait for the next timer interrupt. The highest priority thread should run as soon as it is runnable, preempting whatever thread is currently running.
thread_set_priority() affect a thread receiving donations?
It sets the thread's base priority. The thread's effective priority
becomes the higher of the newly set priority or the highest donated
priority. When the donations are released, the thread's priority
becomes the one set through the function call. This behavior is checked
by the priority-donate-lower test.
Suppose you are seeing output in which some test names are doubled, like this:
(alarm-priority) begin (alarm-priority) (alarm-priority) Thread priority 30 woke up. Thread priority 29 woke up. (alarm-priority) Thread priority 28 woke up. |
What is happening is that output from two threads is being
interleaved. That is, one thread is printing "(alarm-priority)
Thread priority 29 woke up.\n" and another thread is printing
"(alarm-priority) Thread priority 30 woke up.\n", but the first
thread is being preempted by the second in the middle of its output.
This problem indicates a bug in your priority scheduler. After all, a thread with priority 29 should not be able to run while a thread with priority 30 has work to do.
Normally, the implementation of the printf() function in the
Pintos kernel attempts to prevent such interleaved output by acquiring
a console lock during the duration of the printf call and
releasing it afterwards. However, the output of the test name,
e.g., (alarm-priority), and the message following it is output
using two calls to printf, resulting in the console lock being
acquired and released twice.
It doesn't have to. We won't test priority donation and the advanced scheduler at the same time.
Yes. In general, your implementation may differ from the description, as long as its behavior is the same.
If your implementation mysteriously fails some of the advanced scheduler tests, try the following: