理解进程调度时机跟踪分析进程调度与进程切换的过程

陈良 + 原创作品转载请注明出处 + 《Linux内核分析》MOOC课程http://www.xuetangx.com/courses/course-v1:ustcX+USTC001+_/about

1. 使用gdb跟踪schedule()函数



2. 跟踪schedule()函数



3. 跟踪__schedule()内部的context_switch()函数



4. 由于switch_to不是函数，无法设置断点，我们进入内核源码查看context_switch（）函数内部

2335    static inline void
2336    context_switch(struct rq *rq, struct task_struct *prev,
2337	       struct task_struct *next)

2371	context_tracking_task_switch(prev, next);
2372	/* Here we just switch the register state and the stack. */
2373	switch_to(prev, next, prev);
//分析switch_to宏
31#define switch_to(prev, next, last) \

32do {									\
33	/*								\
34	 * Context-switching clobbers all registers, so we clobber	\
35	 * them explicitly, via unused output variables.		\
36	 * (EAX and EBP is not listed because EBP is saved/restored	\
37	 * explicitly for wchan access and EAX is the return value of	\
38	 * __switch_to())						\
39	 */								\
40	unsigned long ebx, ecx, edx, esi, edi;				\
41									\
42	asm volatile("pushfl\n\t"		/* save    flags */	\
43		     "pushl %%ebp\n\t"		/* save    EBP   */	\
44		     "movl %%esp,%[prev_sp]\n\t"	/* save    ESP   */ \ //将当前栈顶存储到prev_sp寄存器
45		     "movl %[next_sp],%%esp\n\t"	/* restore ESP   */ \ //将下一个进程的esp,next_esp赋予esp寄存器,用新进程的esp替换了当前进程的esp寄存器
46		     "movl $1f,%[prev_ip]\n\t"	/* save    EIP   */	\// 保存当前进程的eip
47		     "pushl %[next_ip]\n\t"	/* restore EIP   */	\ //将下一个进程的eip压入栈顶，使得pop出栈顶的eip为新进程的eip，完成进程切换
48		     __switch_canary					\
49		     "jmp __switch_to\n"	/* regparm call  */	\
50		     "1:\t"						\
51		     "popl %%ebp\n\t"		/* restore EBP   */	\
52		     "popfl\n"			/* restore flags */	\
53									\
54		     /* output parameters */				\
55		     : [prev_sp] "=m" (prev->thread.sp),		\
56		       [prev_ip] "=m" (prev->thread.ip),		\
57		       "=a" (last),					\
58									\
59		       /* clobbered output registers: */		\
60		       "=b" (ebx), "=c" (ecx), "=d" (edx),		\
61		       "=S" (esi), "=D" (edi)				\
62		       							\
63		       __switch_canary_oparam				\
64									\
65		       /* input parameters: */				\
66		     : [next_sp]  "m" (next->thread.sp),		\
67		       [next_ip]  "m" (next->thread.ip),		\
68		       							\
69		       /* regparm parameters for __switch_to(): */	\
70		       [prev]     "a" (prev),				\
71		       [next]     "d" (next)				\
72									\
73		       __switch_canary_iparam				\
74									\
75		     : /* reloaded segment registers */			\
76			"memory");					\
77} while (0)
//以下4行为核心代码
44		     "movl %%esp,%[prev_sp]\n\t"	/* save    ESP   */ \ //将当前栈顶存储到prev_sp寄存器
45		     "movl %[next_sp],%%esp\n\t"	/* restore ESP   */ \ //将下一个进程的esp,next_esp赋予esp寄存器,用新进程的esp替换了当前进程的esp寄存器
46		     "movl $1f,%[prev_ip]\n\t"	/* save    EIP   */	\ //保存当前进程的eip
47		     "pushl %[next_ip]\n\t"	/* restore EIP   */	\ //将下一个进程的eip压入栈顶，使得pop出栈顶的eip为新进程的eip，完成进程切换

//以上4行核心代码完成了进程的切换

5. 总结

一般执行过程（用户进程x切换到用户进程y）

（1）中断，保存x用户堆栈，载入x内核堆栈，进入x内核态

（2）内核态下运行schedule，使用switch_to进行进程上下文切换

（3）进入y内核态

（4）恢复现场，返回y用户态

（5）结束