Add uthread
This commit is contained in:
Frans Kaashoek
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<html>
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<head>
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<title>Lab: system calls</title>
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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: system calls</h1>
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<h1>Lab: Alarm and uthread</h1>
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This lab makes you familiar with the implementation of system calls.
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In particular, you will implement a new system
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calls: <tt>sigalarm</tt> and <tt>sigreturn</tt>.
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<b>Note: before this lab, it would be good to have recitation section on gdb and understanding assembly</b>
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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 programs you wrote in the lab and inspect the system call
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trace. Are there many system calls? Which systems calls correspond
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to code in the applications you wrote above?
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<p>Optional: print the system call arguments.
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This lab makes you familiar 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 of a user-level thread package.
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<h2>RISC-V assembly</h2>
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<p>For the alarm system call it will be important to understand RISC-V
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assembly. Since in later labs you will also read and write assembly,
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it is important that you familiarize yourself with RISC_V assembly.
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<p>For this lab it will be important to understand 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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@ -96,8 +58,43 @@ void main(void) {
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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 programs you wrote in the lab and inspect the system call
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trace. Are there many system calls? Which systems calls correspond
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to code in the applications you wrote above?
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<p>Optional: print the system call arguments.
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<h2>alarm</h2>
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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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@ -227,7 +224,7 @@ alarmtest starting
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code for the alarmtest program in alarmtest.asm, which will be handy
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for debugging.
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<h2>Test0: invoke handler</h2>
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<h3>Test0: invoke handler</h3>
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<p>To get started, the best strategy is to first pass test0, which
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will force you to handle the main challenge above. Here are some
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@ -279,7 +276,7 @@ use only one CPU, which you can do by running
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</ul>
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<h2>test1(): resume interrupted code</h2>
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<h3>test1(): resume interrupted code</h3>
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<p>Test0 doesn't tests whether the handler returns correctly to
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interrupted instruction in test0. If you didn't get this right, it
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@ -311,16 +308,182 @@ use only one CPU, which you can do by running
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<li>Prevent re-entrant calls to the handler----if a handler hasn't
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returned yet, don't call it again.
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<ul>
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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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