用户态线程

本章节我们将通过实现代码来执行线程之间的上下文切换,从而完成一个简单的用户级线程包. 相关链接

线程切换

下载编译uthread相关代码

下载uthread.c和uthread_switch.S到xv6目录,确保uthread_switch.S后缀为.S.

将如下规则添加到xv6 Makefile _forktest之后,Makefile命令起始不是空格,而是tab.

_uthread: uthread.o uthread_switch.o
     $(LD) $(LDFLAGS) -N -e main -Ttext 0 -o _uthread uthread.o uthread_switch.o $(ULIB)
     $(OBJDUMP) -S _uthread > uthread.asm

将_uthread添加到xv6 Makefile中的UPROGS.
运行xv6,在xv6 shell中执行uthread.xv6 内核将会打印出page fault.

目标

我们的目标是通过完成thread_switch.S,来使得在单核情况下,打印信息如下:

~/classes/6828/xv6$ make CPUS=1 qemu-nox
dd if=/dev/zero of=xv6.img count=10000
10000+0 records in
10000+0 records out
5120000 bytes transferred in 0.037167 secs (137756344 bytes/sec)
dd if=bootblock of=xv6.img conv=notrunc
1+0 records in
1+0 records out
512 bytes transferred in 0.000026 secs (19701685 bytes/sec)
dd if=kernel of=xv6.img seek=1 conv=notrunc
307+1 records in
307+1 records out
157319 bytes transferred in 0.003590 secs (43820143 bytes/sec)
qemu -nographic -hdb fs.img xv6.img -smp 1 -m 512 
Could not open option rom 'sgabios.bin': No such file or directory
xv6...
cpu0: starting
init: starting sh
$ uthread
my thread running
my thread 0x2A30
my thread running
my thread 0x4A40
my thread 0x2A30
my thread 0x4A40
my thread 0x2A30
my thread 0x4A40
....

分析

uthread.c创建了两个线程,并在他们之间来回切换.每个线程都会打印my thtread……,然后让出CPU给其他线程运行机会.
为了观察到上述现象,我们需要完成uthread_switch.S.当然在此之前,我们还需要了解uthread.c是如何使用uthread_switch的.
uthread.c中,有两个重要的全局变量: current_thread和next_thread.它们各自指向一个thread结构体.thread结构体从高到底,依次为线程状态,线程栈和栈顶指针保存区.
uthread_switch的任务是:
- 保存当前线程状态到current_thread.
- 从next_thread中恢复新的线程状态
- 将current_thread指向next_thread所指向的位置.
- 这样当uthread_switch返回后,将执行next_thread,且为current_thread.
thread_create创建了一个新的线程,它提供了uthread_switch应该如何实现的提示.
uthread_switch使用pushal和popal来一次性push或restore全部的8个寄存器.
uthread_create模拟了全部8个寄存器的值,即32个字节,全部为0.

如下是struct thread在内存中的布局:

 --------------------
 | 4 bytes for state|
 --------------------
 | stack size bytes |
 | for stack        |
 --------------------
 | 4 bytes for sp   |
 --------------------  <--- current_thread
      ......

      ......
 --------------------
 | 4 bytes for state|
 --------------------
 | stack size bytes |
 | for stack        |
 --------------------
 | 4 bytes for sp   |
 --------------------  <--- next_thread

current_thread和next_thread即指向struct thread的首地址.
如果我们希望修改current_thread指向的struct thread中的sp的值,汇编代码如下:
```
movl current_thread, %eax
movl %esp, (%eax)
```
以上汇编代码会将%esp寄存器中的值赋给current_thread->sp,这是因为sp的起始地址就是current_thread,还记得上面struct thread所展示的内存布局图么?
在编译完成后,我们可以通过uthread.asm来学习相关的汇编代码.

为了开发调试,我们还可以单步调试uthread_switch,以下是操作方法:

(gdb) symbol-file _uthread
Load new symbol table from "/Users/kaashoek/classes/6828/xv6/_uthread"? (y or n) y
Reading symbols from /Users/kaashoek/classes/6828/xv6/_uthread...done.
(gdb) b thread_switch
Breakpoint 1 at 0x204: file uthread_switch.S, line 9.
(gdb)

以上添加的断点,可能在运行uthread之前就被触发,请问是为什么?

当我们添加断点之后,运行uthread,gdb将会在uthread_switch处停止,我们可以通过如下方法查看uthread的状态.

(gdb) p/x next_thread->sp
$4 = 0x4ae8
(gdb) x/9x next_thread->sp
0x4ae8 :      0x00000000      0x00000000      0x00000000      0x00000000
0x4af8 :      0x00000000      0x00000000      0x00000000      0x00000000
0x4b08 :      0x000000d8

以上0xd8的地址是什么?是否位于next_thread的栈顶?

课后作业

用户态线程

线程切换

下载编译uthread相关代码

目标

分析

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