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<html>
<head>
<title>Lab: Alarm and uthread</title>
<link rel="stylesheet" href="homework.css" type="text/css" />
</head>
<body>
<h1>Lab: Alarm and uthread</h1>
This lab will familiarize you with the implementation of system calls
and switching between threads of execution. In particular, you will
implement new system calls (<tt>sigalarm</tt> and <tt>sigreturn</tt>)
and switching between threads in a user-level thread package.
<h2>Warmup: RISC-V assembly</h2>
<p>For this lab it will be important to understand a bit of RISC-V assembly.
<p>Add a file user/call.c with the following content, modify the
Makefile to add the program to the user programs, and compile (make
fs.img). The Makefile also produces a binary and a readable
assembly a version of the program in the file user/call.asm.
<pre>
#include "kernel/param.h"
#include "kernel/types.h"
#include "kernel/stat.h"
#include "user/user.h"
int g(int x) {
return x+3;
}
int f(int x) {
return g(x);
}
void main(void) {
printf(1, "%d %d\n", f(8)+1, 13);
exit();
}
</pre>
<p>Read through user/call.asm and understand it. The instruction manual
for RISC-V is in the doc directory (doc/riscv-spec-v2.2.pdf). Here
are some questions that you should answer for yourself:
<ul>
<li>Which registers contain arguments to functions? Which
register holds 13 in the call to <tt>printf</tt>? Which register
holds the second argument? Which register holds the third one? Etc.
<li>Where is the function call to <tt>f</tt> from main? Where
is the call to <tt>g</tt>?
(Hint: the compiler may inline functions.)
<li>At what address is the function <tt>printf</tt> located?
<li>What value is in the register <tt>ra</tt> just after the <tt>jalr</tt>
to <tt>printf</tt> in <tt>main</tt>?
</ul>
<h2>Warmup: system call tracing</h2>
<p>In this exercise you will modify the xv6 kernel to print out a line
for each system call invocation. It is enough to print the name of the
system call and the return value; you don't need to print the system
call arguments.
<p>
When you're done, you should see output like this when booting
xv6:
<pre>
...
fork -> 2
exec -> 0
open -> 3
close -> 0
$write -> 1
write -> 1
</pre>
<p>
That's init forking and execing sh, sh making sure only two file descriptors are
open, and sh writing the $ prompt. (Note: the output of the shell and the
system call trace are intermixed, because the shell uses the write syscall to
print its output.)
<p> Hint: modify the syscall() function in kernel/syscall.c.
<p>Run the xv6 programs you wrote in earlier labs and inspect the system call
trace. Are there many system calls? Which system calls correspond
to code in the applications you wrote?
<p>Optional: print the system call arguments.
<h2>Alarm</h2>
<p>
In this exercise you'll add a feature to xv6 that periodically alerts
a process as it uses CPU time. This might be useful for compute-bound
processes that want to limit how much CPU time they chew up, or for
processes that want to compute but also want to take some periodic
action. More generally, you'll be implementing a primitive form of
user-level interrupt/fault handlers; you could use something similar
to handle page faults in the application, for example.
<p>
You should add a new <tt>sigalarm(interval, handler)</tt> system call.
If an application calls <tt>sigalarm(n, fn)</tt>, then after every
<tt>n</tt> "ticks" of CPU time that the program consumes, the kernel
should cause application function
<tt>fn</tt> to be called. When <tt>fn</tt> returns, the application
should resume where it left off. A tick is a fairly arbitrary unit of
time in xv6, determined by how often a hardware timer generates
interrupts.
<p>
You'll find a file <tt>user/alarmtest.c</tt> in your xv6
repository. Add it to the Makefile. It won't compile correctly
until you've added <tt>sigalarm</tt> and <tt>sigreturn</tt>
system calls (see below).
<p>
<tt>alarmtest</tt> calls <tt>sigalarm(2, periodic)</tt> in <tt>test0</tt> to
ask the kernel to force a call to <tt>periodic()</tt> every 2 ticks,
and then spins for a while.
You can see the assembly
code for alarmtest in user/alarmtest.asm, which may be handy
for debugging.
When you've finished the lab,
<tt>alarmtest</tt> should produce output like this:
<pre>
$ alarmtest
test0 start
......................................alarm!
test0 passed
test1 start
..alarm!
..alarm!
..alarm!
.alarm!
..alarm!
..alarm!
..alarm!
..alarm!
..alarm!
..alarm!
test1 passed
$
</pre>
<p>The main challenge will be to arrange that the handler is invoked
when the process's alarm interval expires. You'll need to modify
usertrap() in kernel/trap.c so that when a
process's alarm interval expires, the process executes
the handler. How can you do that? You will need to understand
how system calls work (i.e., the code in kernel/trampoline.S
and kernel/trap.c). Which register contains the address to which
system calls return?
<p>Your solution will be only a few lines of code, but it may be tricky to
get it right.
We'll test your code with the version of alarmtest.c in the original
repository; if you modify alarmtest.c, make sure your kernel changes
cause the original alarmtest to pass the tests.
<h3>test0: invoke handler</h3>
<p>Get started by modifying the kernel to jump to the alarm handler in
user space, which will cause test0 to print "alarm!". Don't worry yet
what happens after the "alarm!" output; it's OK for now if your
program crashes after printing "alarm!". Here are some hints:
<ul>
<li>You'll need to modify the Makefile to cause <tt>alarmtest.c</tt>
to be compiled as an xv6 user program.
<li>The right declarations to put in <tt>user/user.h</tt> are:
<pre>
int sigalarm(int ticks, void (*handler)());
int sigreturn(void);
</pre>
<li>Update user/sys.pl (which generates user/usys.S),
kernel/syscall.h, and kernel/syscall.c
to allow <tt>alarmtest</tt> to invoke the sigalarm and
sigreturn system calls.
<li>For now, your <tt>sys_sigreturn</tt> should just return zero.
<li>Your <tt>sys_sigalarm()</tt> should store the alarm interval and
the pointer to the handler function in new fields in the <tt>proc</tt>
structure, defined in <tt>kernel/proc.h</tt>.
<li>You'll need to keep track of how many ticks have passed since the
last call (or are left until the next call) to a process's alarm
handler; you'll need a new field in <tt>struct proc</tt> for this
too. You can initialize <tt>proc</tt> fields in <tt>allocproc()</tt>
in <tt>proc.c</tt>.
<li>Every tick, the hardware clock forces an interrupt, which is handled
in <tt>usertrap()</tt>; you should add some code here.
<li>You only want to manipulate a process's alarm ticks if there's a a
timer interrupt; you want something like
<pre>
if(which_dev == 2) ...
</pre>
<li>Only invoke the alarm function if the process has a
timer outstanding. Note that the address of the user's alarm
function might be 0 (e.g., in alarmtest.asm, <tt>periodic</tt> is at
address 0).
<li>It will be easier to look at traps with gdb if you tell qemu to
use only one CPU, which you can do by running
<pre>
make CPUS=1 qemu
</pre>
<li>You've succeeded if alarmtest prints "alarm!".
</ul>
<h3>test1(): resume interrupted code</h3>
Chances are that alarmtest crashes at some point after it prints
"alarm!". Depending on how your solution works, that point may be in
test0, or it may be in test1. Crashes are likely caused
by the alarm handler (<tt>periodic</tt> in alarmtest.c) returning
to the wrong point in the user program.
<p>
Your job now is to ensure that, when the alarm handler is done,
control returns to
the instruction at which the user program was originally
interrupted by the timer interrupt. You must also ensure that
the register contents are restored to values they held
at the time of the interrupt, so that the user program
can continue undisturbed after the alarm.
<p>Your solution is likely to require you to save and restore
registers---what registers do you need to save and restore to resume
the interrupted code correctly? (Hint: it will be many).
Several approaches are possible; for this lab you should make
the <tt>sigreturn</tt> system call
restore registers and return to the original
interrupted user instruction.
The user-space alarm handler
calls sigreturn when it is done.
Some hints:
<ul>
<li>Have <tt>usertrap</tt> save enough state in
<tt>struct proc</tt> when the timer goes off
that <tt>sigreturn</tt> can correctly return to the
interrupted user code.
<li>Prevent re-entrant calls to the handler----if a handler hasn't
returned yet, the kernel shouldn't call it again.
</ul>
<p>Once you pass <tt>test0</tt> and <tt>test1</tt>, run usertests to
make sure you didn't break any other parts of the kernel.
<h2>Uthread: switching between threads</h2>
<p>Download <a href="uthread.c">uthread.c</a> and <a
href="uthread_switch.S">uthread_switch.S</a> into your xv6 directory.
Make sure <tt>uthread_switch.S</tt> ends with <tt>.S</tt>, not
<tt>.s</tt>. Add the
following rule to the xv6 Makefile after the _forktest rule:
<pre>
$U/_uthread: $U/uthread.o $U/uthread_switch.o
$(LD) $(LDFLAGS) -N -e main -Ttext 0 -o $U/_uthread $U/uthread.o $U/uthread_switch.o $(ULIB)
$(OBJDUMP) -S $U/_uthread > $U/uthread.asm
</pre>
Make sure that the blank space at the start of each line is a tab,
not spaces.
<p>
Add <tt>_uthread</tt> in the Makefile to the list of user programs defined by UPROGS.
<p>Run xv6, then run <tt>uthread</tt> from the xv6 shell. The xv6 kernel will print an error message about <tt>uthread</tt> encountering a page fault.
<p>Your job is to complete <tt>uthread_switch.S</tt>, so that you see output similar to
this (make sure to run with CPUS=1):
<pre>
~/classes/6828/xv6$ make CPUS=1 qemu
...
$ uthread
my thread running
my thread 0x0000000000002A30
my thread running
my thread 0x0000000000004A40
my thread 0x0000000000002A30
my thread 0x0000000000004A40
my thread 0x0000000000002A30
my thread 0x0000000000004A40
my thread 0x0000000000002A30
my thread 0x0000000000004A40
my thread 0x0000000000002A30
...
my thread 0x0000000000002A88
my thread 0x0000000000004A98
my thread: exit
my thread: exit
thread_schedule: no runnable threads
$
</pre>
<p><tt>uthread</tt> creates two threads and switches back and forth between
them. Each thread prints "my thread ..." and then yields to give the other
thread a chance to run.
<p>To observe the above output, you need to complete <tt>uthread_switch.S</tt>, but before
jumping into <tt>uthread_switch.S</tt>, first understand how <tt>uthread.c</tt>
uses <tt>uthread_switch</tt>. <tt>uthread.c</tt> has two global variables
<tt>current_thread</tt> and <tt>next_thread</tt>. Each is a pointer to a
<tt>thread</tt> structure. The thread structure has a stack for a thread and a
saved stack pointer (<tt>sp</tt>, which points into the thread's stack). The
job of <tt>uthread_switch</tt> is to save the current thread state into the
structure pointed to by <tt>current_thread</tt>, restore <tt>next_thread</tt>'s
state, and make <tt>current_thread</tt> point to where <tt>next_thread</tt> was
pointing to, so that when <tt>uthread_switch</tt> returns <tt>next_thread</tt>
is running and is the <tt>current_thread</tt>.
<p>You should study <tt>thread_create</tt>, which sets up the initial stack for
a new thread. It provides hints about what <tt>uthread_switch</tt> should do.
Note that <tt>thread_create</tt> simulates saving all callee-save registers
on a new thread's stack.
<p>To write the assembly in <tt>thread_switch</tt>, you need to know how the C
compiler lays out <tt>struct thread</tt> in memory, which is as
follows:
<pre>
--------------------
| 4 bytes for state|
--------------------
| stack size bytes |
| for stack |
--------------------
| 8 bytes for sp |
-------------------- <--- current_thread
......
......
--------------------
| 4 bytes for state|
--------------------
| stack size bytes |
| for stack |
--------------------
| 8 bytes for sp |
-------------------- <--- next_thread
</pre>
The variables <tt>&next_thread</tt> and <tt>¤t_thread</tt> each
contain the address of a pointer to <tt>struct thread</tt>, and are
passed to <tt>thread_switch</tt>. The following fragment of assembly
will be useful:
<pre>
ld t0, 0(a0)
sd sp, 0(t0)
</pre>
This saves <tt>sp</tt> in <tt>current_thread->sp</tt>. This works because
<tt>sp</tt> is at
offset 0 in the struct.
You can study the assembly the compiler generates for
<tt>uthread.c</tt> by looking at <tt>uthread.asm</tt>.
<p>To test your code it might be helpful to single step through your
<tt>uthread_switch</tt> using <tt>riscv64-linux-gnu-gdb</tt>. You can get started in this way:
<pre>
(gdb) file user/_uthread
Reading symbols from user/_uthread...
(gdb) b *0x230
</pre>
0x230 is the address of uthread_switch (see uthread.asm). When you
compile it may be at a different address, so check uthread_asm.
You may also be able to type "b uthread_switch". <b>XXX This doesn't work
for me; why?</b>
<p>The breakpoint may (or may not) be triggered before you even run
<tt>uthread</tt>. How could that happen?
<p>Once your xv6 shell runs, type "uthread", and gdb will break at
<tt>thread_switch</tt>. Now you can type commands like the following to inspect
the state of <tt>uthread</tt>:
<pre>
(gdb) p/x *next_thread
$1 = {sp = 0x4a28, stack = {0x0 (repeats 8088 times),
0x68, 0x1, 0x0 <repeats 102 times>}, state = 0x1}
</pre>
What address is <tt>0x168</tt>, which sits on the bottom of the stack
of <tt>next_thread</tt>?
With "x", you can examine the content of a memory location
<pre>
(gdb) x/x next_thread->sp
0x4a28 <all_thread+16304>: 0x00000168
</pre>
Why does that print <tt>0x168</tt>?
<h3>Optional challenges</h3>
<p>The user-level thread package interacts badly with the operating system in
several ways. For example, if one user-level thread blocks in a system call,
another user-level thread won't run, because the user-level threads scheduler
doesn't know that one of its threads has been descheduled by the xv6 scheduler. As
another example, two user-level threads will not run concurrently on different
cores, because the xv6 scheduler isn't aware that there are multiple
threads that could run in parallel. Note that if two user-level threads were to
run truly in parallel, this implementation won't work because of several races
(e.g., two threads on different processors could call <tt>thread_schedule</tt>
concurrently, select the same runnable thread, and both run it on different
processors.)
<p>There are several ways of addressing these problems. One is
using <a href="http://en.wikipedia.org/wiki/Scheduler_activations">scheduler
activations</a> and another is to use one kernel thread per
user-level thread (as Linux kernels do). Implement one of these ways
in xv6. This is not easy to get right; for example, you will need to
implement TLB shootdown when updating a page table for a
multithreaded user process.
<p>Add locks, condition variables, barriers,
etc. to your thread package.
</body>
</html>
|
