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2024-07-15 20:39| 来源: 网络整理| 查看: 265

目录隐藏 1 主动调度与上下文切换 1.1 代码相关 2 整体处理流程： 3 参考：主动调度与上下文切换

在学习极客时间刘超《趣谈Linux操作系统》 16 节时，讲到了主动调度，通过__schedule() 函数让出cpu三个步骤：

1. 取出任务队列；

2. 从队列取下一个任务；

3. 进行上下文切换；

其中第三步中上下文切换中调用switch_to函数，通过调用switch_to_asm函数完成栈顶执政寄存器的交换。代码如下：

/arch/x86/include/asm/switch_to.h #define switch_to(prev, next, last) \ do { \ prepare_switch_to(next); \ \ ((last) = __switch_to_asm((prev), (next))); \ } while (0) /arch/x86/entry/entry_64.S ENTRY(__switch_to_asm) UNWIND_HINT_FUNC 。。。 /* switch stack */ movq %rsp, TASK_threadsp(%rdi) movq TASK_threadsp(%rsi), %rsp 。。。。 END(__switch_to_asm) 其中 %rdi代表第一个参数， %rsi代表第二个参数，分别代表 prev task， next task 将当前栈顶指针放入prev task_struct 结构 thread_struct的 sp0 中。将next_task栈顶指做为当前栈顶指针，完成切换。

关于 TASK_threadsp，涉及以下几个文件:（当然一开始的时候正是因为找不到才有了有序寻找答案的过程）

编译相关 1 /Kbuild ... ##### # Generate asm-offsets.h offsets-file := include/generated/asm-offsets.h always += $(offsets-file) targets += arch/$(SRCARCH)/kernel/asm-offsets.s arch/$(SRCARCH)/kernel/asm-offsets.s: $(timeconst-file) $(bounds-file) $(offsets-file): arch/$(SRCARCH)/kernel/asm-offsets.s FORCE $(call filechk,offsets,__ASM_OFFSETS_H__) ... 2. /scripts/Makefile.lib ... # ASM offsets # --------------------------------------------------------------------------- # Default sed regexp - multiline due to syntax constraints # # Use [:space:] because LLVM's integrated assembler inserts around # the .ascii directive whereas GCC keeps the as-is. define sed-offsets 's:^[[:space:]]*\.ascii[[:space:]]*"$.*$".*:\1:; \ /^->/{s:->#$.*$:/* \1 */:; \ s:^->$[^ ]*$ [\$$#]*$[^ ]*$ $.*$:#define \1 \2 /* \3 */:; \ s:->::; p;}' endef # Use filechk to avoid rebuilds when a header changes, but the resulting file # does not define filechk_offsets echo "#ifndef $2"; \ echo "#define $2"; \ echo "/*"; \ echo " * DO NOT MODIFY."; \ echo " *"; \ echo " * This file was generated by Kbuild"; \ echo " */"; \ echo ""; \ sed -ne $(sed-offsets) < $" #sym " %0 " #val "\"" : : "i" (val)) #define OFFSET(sym, str, mem) \ DEFINE(sym, offsetof(struct str, mem)) ... /arch/x86/kernel/asm-offsets.c static void __used common(void) { ... OFFSET(TASK_threadsp, task_struct, thread.sp); ... //生成的头文件 /include/generated/asm-offsets.h #ifndef __ASM_OFFSETS_H__ #define __ASM_OFFSETS_H__ /* * DO NOT MODIFY. * * This file was generated by Kbuild */ #define TASK_threadsp 4824 /* offsetof(struct task_struct, thread.sp) */

整体处理流程：

asm-offsets.c 转化为asm-offsets.s通过Makefile.lib 的filechk_offsets ，sed-offsets 将其生成宏定义，这样汇编语言也可以方便的获得相关结构结构体的偏移了。

下面写个demo验证一下

Makefile: offsets-file := asm-offsets.h define sed-yo "/^->/{s:->#$.*$:/* \1 */:; \ s:^->$[^ ]*$ [\$$#]*$[-0-9]*$ $.*$:#define \1 \2 /* \3 */:; \ s:^->$[^ ]*$ [\$$#]*$[^ ]*$ $.*$:#define \1 \2 /* \3 */:; \ s:->::; p;}" endef define sed-y 's:^[[:space:]]*\.ascii[[:space:]]*"$.*$".*:\1:; \ /^->/{s:->#$.*$:/* \1 */:; \ s:^->$[^ ]*$ [\$$#]*$[^ ]*$ $.*$:#define \1 \2 /* \3 */:; \ s:->::; p;}' endef define cmd_offsets (set -e; \ echo "#ifndef __ASM_OFFSETS_H__"; \ echo "#define __ASM_OFFSETS_H__"; \ echo "/*"; \ echo " * DO NOT MODIFY."; \ echo " *"; \ echo " * This file was generated by Kbuild"; \ echo " *"; \ echo " */"; \ echo ""; \ sed -ne $(sed-y) $ $@ endef asm-offsets.s: asm-offsets.c gcc -S $< $(offsets-file): asm-offsets.s @$(cmd_offsets) asm-offsets.c #include "kbuild.h" #include struct thread_struct1 { int sp; }; struct task_struct1 { struct thread_struct1 thread; }; #define __used __attribute__((__used__)) static void __used common(void){ BLANK(); COMMENT("TASK_threadsp1"); OFFSET(TASK_threadsp1, task_struct1, thread.sp); BLANK(); } asm-offsets.h [root@centosgpt generate]# cat asm-offsets.h #ifndef __ASM_OFFSETS_H__ #define __ASM_OFFSETS_H__ /* * DO NOT MODIFY. * * This file was generated by Kbuild * */ /* TASK_threadsp1 */ #define TASK_threadsp1 0 /* offsetof(struct task_struct1, thread.sp) */ #endif

asm-offsets.h 文件生成了。

参考：

What’s purpose of “arm64/kernel/asm-offsets.c”?

lib/asm-offsets.c的作用

linux中的asm-offsets.h和bounds.h

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