498 lines
16 KiB
HTML
498 lines
16 KiB
HTML
<html>
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<head>
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<title>Lab: Alarm and uthread</title>
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<link rel="stylesheet" href="homework.css" type="text/css" />
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</head>
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<body>
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<h1>Lab: Alarm and uthread</h1>
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This lab will familiarize you with the implementation of system calls
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and switching between threads of execution. In particular, you will
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implement new system calls (<tt>sigalarm</tt> and <tt>sigreturn</tt>)
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and switching between threads in a user-level thread package.
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<h2>Warmup: RISC-V assembly</h2>
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<p>For this lab it will be important to understand a bit of RISC-V assembly.
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<p>Add a file user/call.c with the following content, modify the
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Makefile to add the program to the user programs, and compile (make
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fs.img). The Makefile also produces a binary and a readable
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assembly a version of the program in the file user/call.asm.
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<pre>
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#include "kernel/param.h"
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#include "kernel/types.h"
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#include "kernel/stat.h"
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#include "user/user.h"
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int g(int x) {
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return x+3;
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}
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int f(int x) {
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return g(x);
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}
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void main(void) {
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printf(1, "%d %d\n", f(8)+1, 13);
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exit();
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}
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</pre>
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<p>Read through user/call.asm and understand it. The instruction manual
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for RISC-V is in the doc directory (doc/riscv-spec-v2.2.pdf). Here
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are some questions that you should answer for yourself:
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<ul>
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<li>Which registers contain arguments to functions? Which
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register holds 13 in the call to <tt>printf</tt>? Which register
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holds the second argument? Which register holds the third one? Etc.
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<li>Where is the function call to <tt>f</tt> from main? Where
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is the call to <tt>g</tt>?
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(Hint: the compiler may inline functions.)
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<li>At what address is the function <tt>printf</tt> located?
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<li>What value is in the register <tt>ra</tt> just after the <tt>jalr</tt>
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to <tt>printf</tt> in <tt>main</tt>?
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</ul>
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<h2>Warmup: system call tracing</h2>
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<p>In this exercise you will modify the xv6 kernel to print out a line
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for each system call invocation. It is enough to print the name of the
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system call and the return value; you don't need to print the system
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call arguments.
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<p>
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When you're done, you should see output like this when booting
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xv6:
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<pre>
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...
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fork -> 2
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exec -> 0
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open -> 3
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close -> 0
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$write -> 1
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write -> 1
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</pre>
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<p>
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That's init forking and execing sh, sh making sure only two file descriptors are
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open, and sh writing the $ prompt. (Note: the output of the shell and the
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system call trace are intermixed, because the shell uses the write syscall to
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print its output.)
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<p> Hint: modify the syscall() function in kernel/syscall.c.
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<p>Run the xv6 programs you wrote in earlier labs and inspect the system call
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trace. Are there many system calls? Which system calls correspond
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to code in the applications you wrote?
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<p>Optional: print the system call arguments.
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<h2>Alarm</h2>
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<p>
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In this exercise you'll add a feature to xv6 that periodically alerts
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a process as it uses CPU time. This might be useful for compute-bound
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processes that want to limit how much CPU time they chew up, or for
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processes that want to compute but also want to take some periodic
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action. More generally, you'll be implementing a primitive form of
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user-level interrupt/fault handlers; you could use something similar
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to handle page faults in the application, for example.
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<p>
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You should add a new <tt>sigalarm(interval, handler)</tt> system call.
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If an application calls <tt>sigalarm(n, fn)</tt>, then after every
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<tt>n</tt> "ticks" of CPU time that the program consumes, the kernel
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should cause application function
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<tt>fn</tt> to be called. When <tt>fn</tt> returns, the application
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should resume where it left off. A tick is a fairly arbitrary unit of
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time in xv6, determined by how often a hardware timer generates
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interrupts.
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<p>
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You should put the following test program in <tt>user/alarmtest.c</tt>:
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<pre>
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#include "kernel/param.h"
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#include "kernel/types.h"
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#include "kernel/stat.h"
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#include "kernel/riscv.h"
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#include "user/user.h"
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void test0();
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void test1();
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void periodic();
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int
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main(int argc, char *argv[])
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{
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test0();
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test1();
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exit();
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}
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void test0()
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{
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int i;
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printf(1, "test0 start\n");
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sigalarm(2, periodic);
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for(i = 0; i < 1000*500000; i++){
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if((i % 250000) == 0)
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write(2, ".", 1);
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}
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sigalarm(0, 0);
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printf(1, "test0 done\n");
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}
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void
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periodic()
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{
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printf(1, "alarm!\n");
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}
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void __attribute__ ((noinline)) foo(int i, int *j) {
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if((i % 2500000) == 0) {
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write(2, ".", 1);
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}
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*j += 1;
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}
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void test1() {
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int i;
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int j;
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printf(1, "test1 start\n");
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j = 0;
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sigalarm(2, periodic);
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for(i = 0; i < 1000*500000; i++){
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foo(i, &j);
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}
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if(i != j) {
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printf(2, "i %d should = j %d\n", i, j);
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exit();
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}
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printf(1, "test1 done\n");
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}
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</pre>
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The program calls <tt>sigalarm(2, periodic1)</tt> in <tt>test0</tt> to
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ask the kernel to force a call to <tt>periodic()</tt> every 2 ticks,
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and then spins for a while.
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You can see the assembly
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code for alarmtest in user/alarmtest.asm, which may be handy
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for debugging.
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When you've finished the lab,
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<tt>alarmtest</tt> should produce output like this:
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<pre>
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$ alarmtest
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test0 start
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...................................................alarm!
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.............................................................alarm!
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(repeated many times)
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test0 done
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test1 start
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..alarm!
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..alarm!
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..alarm!
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(repeated many times)
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test1 done
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$
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</pre>
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<p>
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At first, however, you'll see that alarmtest only prints periods,
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and doesn't print "alarm!".
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<p>The main challenge will be to arrange that the handler is invoked
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when the process's alarm interval expires. You'll need to modify
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usertrap() in kernel/trap.c so that when a
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process's alarm interval expires, the process executes
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the handler. How can you do that? You will need to understand
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how system calls work (i.e., the code in kernel/trampoline.S
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and kernel/trap.c). Which register contains the address to which
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system calls return?
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<p>Your solution will be only a few lines of code, but it may be tricky to
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get it right.
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<h3>test0: invoke handler</h3>
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<p>Get started by modifying the kernel to jump to the alarm handler in
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user space, which will cause test0 to print "alarm!". Don't worry yet
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what happens after the "alarm!" output; it's OK for now if your
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program crashes after printing "alarm!". Here are some hints:
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<ul>
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<li>You'll need to modify the Makefile to cause <tt>alarmtest.c</tt>
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to be compiled as an xv6 user program.
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<li>The right declaration to put in <tt>user/user.h</tt> is:
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<pre>
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int sigalarm(int ticks, void (*handler)());
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</pre>
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<li>Update user/sys.pl (which generates user/usys.S),
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kernel/syscall.h, and kernel/syscall.c
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to allow <tt>alarmtest</tt> to invoke the sigalarm system
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call.
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<li>Your <tt>sys_sigalarm()</tt> should store the alarm interval and
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the pointer to the handler function in new fields in the <tt>proc</tt>
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structure, defined in <tt>kernel/proc.h</tt>.
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<li>You'll need to keep track of how many ticks have passed since the
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last call (or are left until the next call) to a process's alarm
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handler; you'll need a new field in <tt>struct proc</tt> for this
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too. You can initialize <tt>proc</tt> fields in <tt>allocproc()</tt>
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in <tt>proc.c</tt>.
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<li>Every tick, the hardware clock forces an interrupt, which is handled
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in <tt>usertrap()</tt>; you should add some code here.
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<li>You only want to manipulate a process's alarm ticks if there's a a
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timer interrupt; you want something like
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<pre>
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if(which_dev == 2) ...
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</pre>
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<li>Only invoke the alarm function if the process has a
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timer outstanding. Note that the address of the user's alarm
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function might be 0 (e.g., in alarmtest.asm, <tt>periodic</tt> is at
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address 0).
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<li>It will be easier to look at traps with gdb if you tell qemu to
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use only one CPU, which you can do by running
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<pre>
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make CPUS=1 qemu
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</pre>
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<li>You've succeeded if alarmtest prints "alarm!".
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</ul>
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<h3>test1(): resume interrupted code</h3>
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Chances are that alarmtest crashes at some point after it prints
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"alarm!". Depending on how your solution works, that point may be in
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test0, or it may be in test1. Crashes are likely caused
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by the alarm handler (<tt>periodic</tt> in alarmtest.c) returning
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to the wrong point in the user program.
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<p>
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Your job now is to ensure that, when the alarm handler is done,
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control returns to
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the instruction at which the user program was originally
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interrupted by the timer interrupt. You must also ensure that
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the register contents are restored to values they held
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at the time of the interrupt, so that the user program
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can continue undisturbed after the alarm.
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<p>Your solution is likely to require you to save and restore
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registers---what registers do you need to save and restore to resume
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the interrupted code correctly? (Hint: it will be many).
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Several approaches are possible; one convenient plan is to add another
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system call <tt>sigreturn</tt> that the user-space alarm handler calls when it is
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done, and which restores registers and returns to the original
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interrupted user instruction.
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Some hints:
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<ul>
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<li>Add a new <tt>sigreturn</tt> system call.
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<li>Have <tt>usertrap</tt> save enough state in
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<tt>struct proc</tt> when the timer goes off
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that <tt>sigreturn</tt> can correctly return to the
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interrupted user code.
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<li>Prevent re-entrant calls to the handler----if a handler hasn't
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returned yet, the kernel shouldn't call it again.
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</ul>
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<p>Once you pass <tt>test0</tt> and <tt>test1</tt>, run usertests to
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make sure you didn't break any other parts of the kernel.
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<h2>Uthread: switching between threads</h2>
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<p>Download <a href="uthread.c">uthread.c</a> and <a
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href="uthread_switch.S">uthread_switch.S</a> into your xv6 directory.
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Make sure <tt>uthread_switch.S</tt> ends with <tt>.S</tt>, not
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<tt>.s</tt>. Add the
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following rule to the xv6 Makefile after the _forktest rule:
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<pre>
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$U/_uthread: $U/uthread.o $U/uthread_switch.o
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$(LD) $(LDFLAGS) -N -e main -Ttext 0 -o $U/_uthread $U/uthread.o $U/uthread_switch.o $(ULIB)
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$(OBJDUMP) -S $U/_uthread > $U/uthread.asm
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</pre>
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Make sure that the blank space at the start of each line is a tab,
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not spaces.
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<p>
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Add <tt>_uthread</tt> in the Makefile to the list of user programs defined by UPROGS.
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<p>Run xv6, then run <tt>uthread</tt> from the xv6 shell. The xv6 kernel will print an error message about <tt>uthread</tt> encountering a page fault.
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<p>Your job is to complete <tt>uthread_switch.S</tt>, so that you see output similar to
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this (make sure to run with CPUS=1):
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<pre>
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~/classes/6828/xv6$ make CPUS=1 qemu
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...
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$ uthread
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my thread running
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my thread 0x0000000000002A30
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my thread running
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my thread 0x0000000000004A40
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my thread 0x0000000000002A30
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my thread 0x0000000000004A40
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my thread 0x0000000000002A30
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my thread 0x0000000000004A40
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my thread 0x0000000000002A30
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my thread 0x0000000000004A40
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my thread 0x0000000000002A30
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...
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my thread 0x0000000000002A88
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my thread 0x0000000000004A98
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my thread: exit
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my thread: exit
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thread_schedule: no runnable threads
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$
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</pre>
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<p><tt>uthread</tt> creates two threads and switches back and forth between
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them. Each thread prints "my thread ..." and then yields to give the other
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thread a chance to run.
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<p>To observe the above output, you need to complete <tt>uthread_switch.S</tt>, but before
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jumping into <tt>uthread_switch.S</tt>, first understand how <tt>uthread.c</tt>
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uses <tt>uthread_switch</tt>. <tt>uthread.c</tt> has two global variables
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<tt>current_thread</tt> and <tt>next_thread</tt>. Each is a pointer to a
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<tt>thread</tt> structure. The thread structure has a stack for a thread and a
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saved stack pointer (<tt>sp</tt>, which points into the thread's stack). The
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job of <tt>uthread_switch</tt> is to save the current thread state into the
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structure pointed to by <tt>current_thread</tt>, restore <tt>next_thread</tt>'s
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state, and make <tt>current_thread</tt> point to where <tt>next_thread</tt> was
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pointing to, so that when <tt>uthread_switch</tt> returns <tt>next_thread</tt>
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is running and is the <tt>current_thread</tt>.
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<p>You should study <tt>thread_create</tt>, which sets up the initial stack for
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a new thread. It provides hints about what <tt>uthread_switch</tt> should do.
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Note that <tt>thread_create</tt> simulates saving all callee-save registers
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on a new thread's stack.
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<p>To write the assembly in <tt>thread_switch</tt>, you need to know how the C
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compiler lays out <tt>struct thread</tt> in memory, which is as
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follows:
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<pre>
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--------------------
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| 4 bytes for state|
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--------------------
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| stack size bytes |
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| for stack |
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--------------------
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| 8 bytes for sp |
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-------------------- <--- current_thread
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......
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......
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--------------------
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| 4 bytes for state|
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--------------------
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| stack size bytes |
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| for stack |
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--------------------
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| 8 bytes for sp |
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-------------------- <--- next_thread
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</pre>
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The variables <tt>&next_thread</tt> and <tt>¤t_thread</tt> each
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contain the address of a pointer to <tt>struct thread</tt>, and are
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passed to <tt>thread_switch</tt>. The following fragment of assembly
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will be useful:
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<pre>
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ld t0, 0(a0)
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sd sp, 0(t0)
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</pre>
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This saves <tt>sp</tt> in <tt>current_thread->sp</tt>. This works because
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<tt>sp</tt> is at
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offset 0 in the struct.
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You can study the assembly the compiler generates for
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<tt>uthread.c</tt> by looking at <tt>uthread.asm</tt>.
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<p>To test your code it might be helpful to single step through your
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<tt>uthread_switch</tt> using <tt>riscv64-linux-gnu-gdb</tt>. You can get started in this way:
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<pre>
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(gdb) file user/_uthread
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Reading symbols from user/_uthread...
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(gdb) b *0x230
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</pre>
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0x230 is the address of uthread_switch (see uthread.asm). When you
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compile it may be at a different address, so check uthread_asm.
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You may also be able to type "b uthread_switch". <b>XXX This doesn't work
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for me; why?</b>
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<p>The breakpoint may (or may not) be triggered before you even run
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<tt>uthread</tt>. How could that happen?
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<p>Once your xv6 shell runs, type "uthread", and gdb will break at
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<tt>thread_switch</tt>. Now you can type commands like the following to inspect
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the state of <tt>uthread</tt>:
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<pre>
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(gdb) p/x *next_thread
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$1 = {sp = 0x4a28, stack = {0x0 (repeats 8088 times),
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0x68, 0x1, 0x0 <repeats 102 times>}, state = 0x1}
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</pre>
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What address is <tt>0x168</tt>, which sits on the bottom of the stack
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of <tt>next_thread</tt>?
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With "x", you can examine the content of a memory location
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<pre>
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(gdb) x/x next_thread->sp
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0x4a28 <all_thread+16304>: 0x00000168
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</pre>
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Why does that print <tt>0x168</tt>?
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<h3>Optional challenges</h3>
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<p>The user-level thread package interacts badly with the operating system in
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several ways. For example, if one user-level thread blocks in a system call,
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another user-level thread won't run, because the user-level threads scheduler
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doesn't know that one of its threads has been descheduled by the xv6 scheduler. As
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another example, two user-level threads will not run concurrently on different
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cores, because the xv6 scheduler isn't aware that there are multiple
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threads that could run in parallel. Note that if two user-level threads were to
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run truly in parallel, this implementation won't work because of several races
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(e.g., two threads on different processors could call <tt>thread_schedule</tt>
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concurrently, select the same runnable thread, and both run it on different
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processors.)
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<p>There are several ways of addressing these problems. One is
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using <a href="http://en.wikipedia.org/wiki/Scheduler_activations">scheduler
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activations</a> and another is to use one kernel thread per
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user-level thread (as Linux kernels do). Implement one of these ways
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in xv6. This is not easy to get right; for example, you will need to
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implement TLB shootdown when updating a page table for a
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multithreaded user process.
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<p>Add locks, condition variables, barriers,
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etc. to your thread package.
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</body>
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</html>
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