Linux既支持普通的分时进程，也支持实时进程，即基于优先级。Linux中调度是多种调度策略和调度算法的混合，因为不同类型的进程有不同的调度需求，所以其调度时机也是不同的。这些算法让系统运行的更高效，让CPU资源得到最大限度使用。而所谓的策略就是一组规则，决定了什么时候以怎样的方式选择一个新的进程运行。

操作系统原理中介绍了大量进程调度算法，这些算法从实现的角度看仅仅是从运行队列中选择一个新进程，选择的过程中运用了不同的策略而已。对于理解操作系统的工作机制，反而是进程的调度时机与进程的切换机制更为关键。

#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   */ \
45             "movl %[next_sp],%%esp\n\t"    /* restore ESP   */ \
46             "movl $1f,%[prev_ip]\n\t"    /* save    EIP   */    \
47             "pushl %[next_ip]\n\t"    /* restore EIP   */    \
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)

asm volatile("pushfl\n\t"        /* save    flags */    \
43             "pushl %%ebp\n\t"        /* save    EBP   */    \
44             "movl %%esp,%[prev_sp]\n\t"    /* save    ESP   */ \
45             "movl %[next_sp],%%esp\n\t"    /* restore ESP   */ \
46             "movl $1f,%[prev_ip]\n\t"    /* save    EIP   */    \
47             "pushl %[next_ip]\n\t"    /* restore EIP   */    \

"movl %[next_sp],%%esp\n\t" /* restore ESP   */ "movl $1f,%[prev_ip]\n\t"   /* save    EIP   */这两句将堆栈的sp作了保存和切换
然后将下一个进程的ip压栈

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 */    \