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(1)进程状态转换的时刻:进程终止、进程睡眠,这些过程会主动调用调度程序进行进程调度。 (2)当前进程时间片用完时 (3)进程从中断、异常及系统调用返回到用户态时
(1)正在运行的用户态进程X (2)发生中断——save cs:eip/esp/eflags(current) to kernel stack,then load cs:eip(entry of a specific ISR) and ss:esp(point to kernel stack). (3)SAVE_ALL //保存现场 (4)中断处理过程中或中断返回前调用了schedule(),其中的switch_to做了关键的进程上下文切换 (5) 标号1之后开始运行用户态进程Y(这里Y曾经通过以上步骤被切换出去过因此可以从标号1继续执行) (6) restore_all //恢复现场 (7)iret - pop cs:eip/ss:esp/eflags from kernel stack (8)继续运行用户态进程Y
switch_to代码
switch_to是通过内联汇编操作的,在调试过程中无法进入,只能进入__switch_to函数。
#define switch_to(prev, next, last) \
do { \
/* \
* Context-switching clobbers all registers, so we clobber \
* them explicitly, via unused output variables. \
* (EAX and EBP is not listed because EBP is saved/restored \
* explicitly for wchan access and EAX is the return value of \
* __switch_to()) \
*/ \
unsigned long ebx, ecx, edx, esi, edi; \
\
asm volatile("pushfl\n\t" /* save flags */ \
"pushl %%ebp\n\t" /* save EBP */ \//保存当前进程的栈基址
"movl %%esp,%[prev_sp]\n\t" /* save ESP */ \//保存当前的栈顶
"movl %[next_sp],%%esp\n\t" /* restore ESP */ \//这里实现内核堆栈的切换
"movl $1f,%[prev_ip]\n\t" /* save EIP */ \
//保存当前进程的EIP,next_ip一般是$1f,对于新创建的子进程是ret_from_fork
"pushl %[next_ip]\n\t" /* restore EIP */ \//将下一个进程的起始位置压栈
__switch_canary \
"jmp __switch_to\n" /* regparm call */ \//通过寄存器传参数,返回1f位置
"1:\t" \
"popl %%ebp\n\t" /* restore EBP */ \//弹出之前被调度时的ebp
"popfl\n" /* restore flags */ \
\
/* output parameters */ \
: [prev_sp] "=m" (prev->thread.sp), \//保存当前进程的esp
[prev_ip] "=m" (prev->thread.ip), \//保存当前进程的eip
"=a" (last), \
\
/* clobbered output registers: */ \
"=b" (ebx), "=c" (ecx), "=d" (edx), \
"=S" (esi), "=D" (edi) \
\
__switch_canary_oparam \
\
/* input parameters: */ \
: [next_sp] "m" (next->thread.sp), \
[next_ip] "m" (next->thread.ip), \
\
/* regparm parameters for __switch_to(): */ \
[prev] "a" (prev), \
[next] "d" (next) \
\
__switch_canary_iparam \
\
: /* reloaded segment registers */ \
"memory"); \
} while (0)
在以下几个地方下断点: schedule、__schedule、context_switch、switch_to(断点无法断下)、__switch_to 运行后单步跟踪到schedule函数,发现实际调用的是__schedule函数,接下来完成进程切换的操作:
context_switch函数:
switch_to函数:
__switch_to函数: