参见文章：http://blog.chinaunix.net/uid-22695386-id-272098.html
linux2.4之前的内核有进程最大数的限制，受限制的原因是，每一个进程都有自已的TSS和LDT，而TSS（任务描述符)和LDT(私有描述符)必须放在GDT中，GDT最大只能存放8192个描述符，除掉系统用掉的12描述符之外，最大进程数=(8192-12)/2, 总共4090个进程。从Linux2.4以后，全部进程使用同一个TSS，准确的说是，每个CPU一个TSS，在同一个CPU上的进程使用同一个TSS。TSS的定义在asm-i386/processer.h中，定义如下：
extern struct tss_struct init_tss[NR_CPUS];
在start_kernel()->trap_init()->cpu_init()初始化并加载TSS:
void __init cpu_init (void)
{
int nr = smp_processor_id(); //获取当前cpustruct tss_struct * t = &init_tss[nr]; //当前cpu使用的tss
t->esp0 = current->thread.esp0; //把TSS中esp0更新为当前进程的esp0
set_tss_desc(nr,t);
gdt_table[__TSS(nr)].b &= 0xfffffdff;
load_TR(nr); //加载TSS
load_LDT(&init_mm.context); //加载LDT}
我们知道，任务切换(硬切换)需要用到TSS来保存全部寄存器(2.4以前使用jmp来实现切换)，
中断发生时也需要从TSS中读取ring0的esp0，那么，进程使用相同的TSS，任务切换怎么办？
其实2.4以后不再使用硬切换，而是使用软切换，寄存器不再保存在TSS中了，而是保存在
task->thread中，只用TSS的esp0和IO许可位图，所以，在进程切换过程中，只需要更新TSS中
的esp0、io bitmap，代码在sched.c中：
schedule()->switch_to()->__switch_to()，
void fastcall __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
{
struct thread_struct *prev = &prev_p->thread,
*next = &next_p->thread;
struct tss_struct *tss = init_tss + smp_processor_id(); //当前cpu的TSS/*
* Reload esp0, LDT and the page table pointer:
*/
ttss->esp0 = next->esp0; //用下一个进程的esp0更新tss->esp0//拷贝下一个进程的io_bitmap到tss->io_bitmap
if (prev->ioperm || next->ioperm) {
if (next->ioperm) {
/*
* 4 cachelines copy ... not good, but not that
* bad either. Anyone got something better?
* This only affects processes which use ioperm().
* [Putting the TSSs into 4k-tlb mapped regions
* and playing VM tricks to switch the IO bitmap
* is not really acceptable.]
*/
memcpy(tss->io_bitmap, next->io_bitmap,
IO_BITMAP_BYTES);
tss->bitmap = IO_BITMAP_OFFSET;
} else
/*
* a bitmap offset pointing outside of the TSS limit
* causes a nicely controllable SIGSEGV if a process
* tries to use a port IO instruction. The first
* sys_ioperm() call sets up the bitmap properly.
*/
tss->bitmap = INVALID_IO_BITMAP_OFFSET;
}
}
以及代码：
1: /*
2: * switch_to(x,yn) should switch tasks from x to y.
3: *
4: * We fsave/fwait so that an exception goes off at the right time
5: * (as a call from the fsave or fwait in effect) rather than to
6: * the wrong process. Lazy FP saving no longer makes any sense
7: * with modern CPU's, and this simplifies a lot of things (SMP
8: * and UP become the same).
9: *
10: * NOTE! We used to use the x86 hardware context switching. The
11: * reason for not using it any more becomes apparent when you
12: * try to recover gracefully from saved state that is no longer
13: * valid (stale segment register values in particular). With the
14: * hardware task-switch, there is no way to fix up bad state in
15: * a reasonable manner.
16: *
17: * The fact that Intel documents the hardware task-switching to
18: * be slow is a fairly red herring - this code is not noticeably
19: * faster. However, there _is_ some room for improvement here,
20: * so the performance issues may eventually be a valid point.
21: * More important, however, is the fact that this allows us much
22: * more flexibility.
23: *
24: * The return value (in %ax) will be the "prev" task after
25: * the task-switch, and shows up in ret_from_fork in entry.S,
26: * for example.
27: */
28: __notrace_funcgraph struct task_struct *
29: __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
30: {31: struct thread_struct *prev = &prev_p->thread,
32: *next = &next_p->thread;33: int cpu = smp_processor_id();
34: struct tss_struct *tss = &per_cpu(init_tss, cpu);
35: fpu_switch_t fpu; 36: 37: /* never put a printk in __switch_to... printk() calls wake_up*() indirectly */
38: 39: fpu = switch_fpu_prepare(prev_p, next_p); 40: 41: /*
42: * Reload esp0.
43: */
44: load_sp0(tss, next); 45: 46: /*
47: * Save away %gs. No need to save %fs, as it was saved on the
48: * stack on entry. No need to save %es and %ds, as those are
49: * always kernel segments while inside the kernel. Doing this
50: * before setting the new TLS descriptors avoids the situation
51: * where we temporarily have non-reloadable segments in %fs
52: * and %gs. This could be an issue if the NMI handler ever
53: * used %fs or %gs (it does not today), or if the kernel is
54: * running inside of a hypervisor layer.
55: */
56: lazy_save_gs(prev->gs); 57: 58: /*
59: * Load the per-thread Thread-Local Storage descriptor.
60: */
61: load_TLS(next, cpu); 62: 63: /*
64: * Restore IOPL if needed. In normal use, the flags restore
65: * in the switch assembly will handle this. But if the kernel
66: * is running virtualized at a non-zero CPL, the popf will
67: * not restore flags, so it must be done in a separate step.
68: */
69: if (get_kernel_rpl() && unlikely(prev->iopl != next->iopl))
70: set_iopl_mask(next->iopl); 71: 72: /*
73: * Now maybe handle debug registers and/or IO bitmaps
74: */
75: if (unlikely(task_thread_info(prev_p)->flags & _TIF_WORK_CTXSW_PREV ||
76: task_thread_info(next_p)->flags & _TIF_WORK_CTXSW_NEXT)) 77: __switch_to_xtra(prev_p, next_p, tss); 78: 79: /*
80: * Leave lazy mode, flushing any hypercalls made here.
81: * This must be done before restoring TLS segments so
82: * the GDT and LDT are properly updated, and must be
83: * done before math_state_restore, so the TS bit is up
84: * to date.
85: */
86: arch_end_context_switch(next_p); 87: 88: /*
89: * Restore %gs if needed (which is common)
90: */
91: if (prev->gs | next->gs)
92: lazy_load_gs(next->gs); 93: 94: switch_fpu_finish(next_p, fpu); 95: 96: percpu_write(current_task, next_p); 97: 98: return prev_p;
99: }先分析一下comments
/*
* switch_to(x,yn) should switch tasks from x to y.
*
* We fsave/fwait so that an exception goes off at the right time
* (as a call from the fsave or fwait in effect) rather than to
* the wrong process. Lazy FP saving no longer makes any sense
* with modern CPU's, and this simplifies a lot of things (SMP
* and UP become the same).
*
* NOTE! We used to use the x86 hardware context switching. The
* reason for not using it any more becomes apparent when you
* try to recover gracefully from saved state that is no longer
* valid (stale 【变味的，失效的】segment register values in particular). With the
* hardware task-switch, there is no way to fix up bad state in
* a reasonable manner.
*
* The fact that Intel documents the hardware task-switching to
* be slow is a fairly red herring【题外话】 - this code is not noticeably
* faster. However, there _is_ some room for improvement here,
* so the performance issues may eventually be a valid point.
* More important, however, is the fact that this allows us much
* more flexibility.
*
* The return value (in %ax) will be the "prev" task after
* the task-switch, and shows up in ret_from_fork in entry.S,
* for example.
*/
大致意思是，为了灵活起见，我们将Intel硬件任务切换变为软任务切换。
根据开关引用的文章，每个CPU上执行的进程都使用同一个TSS段，
int cpu = smp_processor_id();
struct tss_struct *tss = &per_cpu(init_tss, cpu);
而且里面有效的信息只有esp0和io_map成员。
/*
* Reload esp0.
*/
load_sp0(tss, next);
原来保存在TSS段中，属于每个进程的上下文信息都保存在下面的结构体（thread_struct）中：
struct thread_struct *prev = &prev_p->thread,
*next = &next_p->thread;
1: struct thread_struct {
2: /* Cached TLS descriptors: */
3: struct desc_struct tls_array[GDT_ENTRY_TLS_ENTRIES];
4: unsigned long sp0;
5: unsigned long sp;
6: #ifdef CONFIG_X86_32
7: unsigned long sysenter_cs;
8: #else
9: unsigned long usersp; /* Copy from PDA */
10: unsigned short es;
11: unsigned short ds;
12: unsigned short fsindex;
13: unsigned short gsindex;
14: #endif
15: #ifdef CONFIG_X86_32
16: unsigned long ip;
17: #endif
18: #ifdef CONFIG_X86_64
19: unsigned long fs;
20: #endif
21: unsigned long gs;
22: /* Save middle states of ptrace breakpoints */
23: struct perf_event *ptrace_bps[HBP_NUM];
24: /* Debug status used for traps, single steps, etc... */
25: unsigned long debugreg6;
26: /* Keep track of the exact dr7 value set by the user */
27: unsigned long ptrace_dr7;
28: /* Fault info: */
29: unsigned long cr2;
30: unsigned long trap_no;
31: unsigned long error_code;
32: /* floating point and extended processor state */
33: unsigned long has_fpu;
34: struct fpu fpu;
35: #ifdef CONFIG_X86_32
36: /* Virtual 86 mode info */
37: struct vm86_struct __user *vm86_info;
38: unsigned long screen_bitmap;
39: unsigned long v86flags;
40: unsigned long v86mask;
41: unsigned long saved_sp0;
42: unsigned int saved_fs;
43: unsigned int saved_gs;
44: #endif
45: /* IO permissions: */
46: unsigned long *io_bitmap_ptr;
47: unsigned long iopl;
48: /* Max allowed port in the bitmap, in bytes: */
49: unsigned io_bitmap_max;
50: };在Linux操作系统中，gs寄存器用于存储存放TLS的地址。（在Windows中，使用fs寄存器来存放TEB结构体的地址）。
参见：http://www.linuxidc.com/Linux/2012-06/64079p2.htm
Linux的glibc使用GS寄存器来访问TLS，也就是说，GS寄存器指示的段指向本线程的TEB(Windows的术语)，也就是TLS，这么做有个好处，那就是可以高效的访问TLS里面存储的信息而不用一次次的调用系统调用，当然使用系统调用的方式也是可以的。之所以可以这么做，是因为Intel对各个寄存器的作用的规范规定的比较松散，因此你可以拿GS，FS等段寄存器来做几乎任何事，当然也就可以做TLS直接访问了，最终glibc在线程启动的时候首先将GS寄存器指向GDT的第6个段，完全使用段机制来支持针对TLS的寻址访问，后续的访问TLS信息就和访问用户态的信息一样高效了。
下面代码，将当前的CPU中的gs寄存器的内容写回到prev结构体中。
/*
* Save away %gs. No need to save %fs, as it was saved on the
* stack on entry. No need to save %es and %ds, as those are
* always kernel segments while inside the kernel. Doing this
* before setting the new TLS descriptors avoids the situation
* where we temporarily have non-reloadable segments in %fs
* and %gs. This could be an issue if the NMI handler ever
* used %fs or %gs (it does not today), or if the kernel is
* running inside of a hypervisor layer.
*/
lazy_save_gs(prev->gs);
接下来
/*
* Load the per-thread Thread-Local Storage descriptor.
*/
load_TLS(next, cpu);
更新GDT表中表示TLS的相关表项。
1: #define load_TLS(t, cpu) native_load_tls(t, cpu)
2: 3: static inline void native_load_tls(struct thread_struct *t, unsigned int cpu)
4: {5: struct desc_struct *gdt = get_cpu_gdt_table(cpu);
6: unsigned int i;
7: 8: for (i = 0; i < GDT_ENTRY_TLS_ENTRIES; i++)
9: gdt[GDT_ENTRY_TLS_MIN + i] = t->tls_array[i]; 10: }首先获取到当前CPU的GDT表
struct gdt_page {
struct desc_struct gdt[GDT_ENTRIES];
} __attribute__((aligned(PAGE_SIZE)));DECLARE_PER_CPU_PAGE_ALIGNED(struct gdt_page, gdt_page);
static inline struct desc_struct *get_cpu_gdt_table(unsigned int cpu)
{
return per_cpu(gdt_page, cpu).gdt;
}
per_cpu机制，是保证每个CPU都有一份自己的关键数据结构
参见：http://www.unixresources.net/linux/clf/linuxK/archive/00/00/47/91/479165.html
在该函数中，为每个CPU分配一段专有数据区，并将.data.percpu中的数据拷贝到其中，
每个CPU各有一份。由于数据从__per_cpu_start处转移到各CPU自己的专有数据区中了，
因此存取其中的变量就不能再用原先的值了，比如存取per_cpu__runqueues
就不能再用per_cpu__runqueues了，需要做一个偏移量的调整，
即需要加上各CPU自己的专有数据区首地址相对于__per_cpu_start的偏移量。
在这里也就是__per_cpu_offset[i]，其中CPU i的专有数据区相对于
__per_cpu_start的偏移量为__per_cpu_offset[i]。
这样，就可以方便地计算专有数据区中各变量的新地址，比如对于per_cpu_runqueues，
其新地址即变成per_cpu_runqueues+__per_cpu_offset[i]。
load_TLS的深入到此结束，言归正传。
/*
* Now maybe handle debug registers and/or IO bitmaps
*/
if (unlikely(task_thread_info(prev_p)->flags & _TIF_WORK_CTXSW_PREV ||
task_thread_info(next_p)->flags & _TIF_WORK_CTXSW_NEXT))
__switch_to_xtra(prev_p, next_p, tss);
接下来处理调试寄存器，以及io位图。
1: void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p,
2: struct tss_struct *tss)
3: {4: struct thread_struct *prev, *next;
5: 6: prev = &prev_p->thread; 7: next = &next_p->thread; 8: 9: if (test_tsk_thread_flag(prev_p, TIF_BLOCKSTEP) ^
10: test_tsk_thread_flag(next_p, TIF_BLOCKSTEP)) {11: unsigned long debugctl = get_debugctlmsr();
12: 13: debugctl &= ~DEBUGCTLMSR_BTF;14: if (test_tsk_thread_flag(next_p, TIF_BLOCKSTEP))
15: debugctl |= DEBUGCTLMSR_BTF; 16: 17: update_debugctlmsr(debugctl); 18: } 19: 20: if (test_tsk_thread_flag(prev_p, TIF_NOTSC) ^
21: test_tsk_thread_flag(next_p, TIF_NOTSC)) {22: /* prev and next are different */
23: if (test_tsk_thread_flag(next_p, TIF_NOTSC))
24: hard_disable_TSC();25: else
26: hard_enable_TSC(); 27: } 28: 29: if (test_tsk_thread_flag(next_p, TIF_IO_BITMAP)) {
30: /*
31: * Copy the relevant range of the IO bitmap.
32: * Normally this is 128 bytes or less:
33: */
34: memcpy(tss->io_bitmap, next->io_bitmap_ptr, 35: max(prev->io_bitmap_max, next->io_bitmap_max));36: } else if (test_tsk_thread_flag(prev_p, TIF_IO_BITMAP)) {
37: /*
38: * Clear any possible leftover bits:
39: */
40: memset(tss->io_bitmap, 0xff, prev->io_bitmap_max); 41: } 42: propagate_user_return_notify(prev_p, next_p); 43: }