1. 内核栈
在每一个进程的生命周期中，必然会通过到系统调用陷入内核。在执行系统调用陷入内核之后，这些内核代码所使用的栈并不是原先进程用户空间中的栈，而是一个单独内核空间的栈，这个称作进程内核栈。
1.1. init_task
init_task，也就是bootcpu的swapper进程对应的task_struct，其内核栈是静态初始化的。
1/// init/init_task.c
2/*
3 * Set up the first task table, touch at your own risk!. Base=0,
4 * limit=0x1fffff (=2MB)
5 */
6struct task_struct init_task
7#ifdef CONFIG_ARCH_TASK_STRUCT_ON_STACK
8 __init_task_data
9#endif
10 __aligned(L1_CACHE_BYTES)
11= {
12#ifdef CONFIG_THREAD_INFO_IN_TASK
13 .thread_info = INIT_THREAD_INFO(init_task),
14 .stack_refcount = REFCOUNT_INIT(1),
15#endif
16 .__state = 0,
17 .stack = init_stack,
18 .usage = REFCOUNT_INIT(2),
19 .flags = PF_KTHREAD,
20 .prio = MAX_PRIO - 20,
21 .static_prio = MAX_PRIO - 20,
22 .normal_prio = MAX_PRIO - 20,
23 .policy = SCHED_NORMAL,
24 /// ... ...
1/// include/linux/sched.h
2extern unsigned long init_stack[THREAD_SIZE / sizeof(unsigned long)];
从include/asm-generic/vmlinux.lds.h可以看到，init_stack是通过链接时来分配的，大小为THREAD_SIZE，且与init_thread_union共用同一段内存。init_thread_union在内核栈空间的低地址。
1/// include/asm-generic/vmlinux.lds.h
2#define INIT_TASK_DATA(align) \
3 . = ALIGN(align); \
4 __start_init_task = .; \
5 init_thread_union = .; \
6 init_stack = .; \
7 KEEP(*(.data..init_task)) \
8 KEEP(*(.data..init_thread_info)) \
9 . = __start_init_task + THREAD_SIZE; \
10 __end_init_task = .;
1.2. 非init_task的内核栈创建
除了init_task的内核栈，包括非bootcpu的swapper进程的所有内核线程和进程，都是通过fork_idle调用copy_process来动态申请的。
调用路径，copy_process -> dup_task_struct -> alloc_thread_stack_node。
以ARM64为例：不论是用户进程还是内核线程，都会在copy_thread中，将cpu_context.sp指向内核栈的栈顶。
1/// arch/arm64/kernel/process.c
2int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
3{
4 unsigned long clone_flags = args->flags;
5 unsigned long stack_start = args->stack;
6 unsigned long tls = args->tls;
7 struct pt_regs *childregs = task_pt_regs(p);
8
9 memset(&p->thread.cpu_context, 0, sizeof(struct cpu_context));
10
11 /// ... ...
12 p->thread.cpu_context.pc = (unsigned long)ret_from_fork;
13 p->thread.cpu_context.sp = (unsigned long)childregs;
14 /*
15 * For the benefit of the unwinder, set up childregs->stackframe
16 * as the final frame for the new task.
17 */
18 p->thread.cpu_context.fp = (unsigned long)childregs->stackframe;
19
20 ptrace_hw_copy_thread(p);
21
22 return 0;
23}
2. 中断栈
ARM64默认不使用中断栈，而是直接复用当前进程的内核栈。
在特殊情况下，会用到call_on_irq_stack，如do_softirq_own_stack和do_interrupt_handler中on_thread_stack为true的情况，会用到专门的中断栈。
2.1. 中断栈的初始化
中断栈空间的低地址保存在per_cpu(irq_stack_ptr, cpu)中。中断栈的大小IRQ_STACK_SIZE与THREAD_SIZE相同。
1/// arch/arm64/kernel/irq.c
2#ifdef CONFIG_VMAP_STACK
3static void init_irq_stacks(void)
4{
5 int cpu;
6 unsigned long *p;
7
8 for_each_possible_cpu(cpu) {
9 p = arch_alloc_vmap_stack(IRQ_STACK_SIZE, cpu_to_node(cpu));
10 per_cpu(irq_stack_ptr, cpu) = p;
11 }
12}
13#else
14/* irq stack only needs to be 16 byte aligned - not IRQ_STACK_SIZE aligned. */
15DEFINE_PER_CPU_ALIGNED(unsigned long [IRQ_STACK_SIZE/sizeof(long)], irq_stack);
16
17static void init_irq_stacks(void)
18{
19 int cpu;
20
21 for_each_possible_cpu(cpu)
22 per_cpu(irq_stack_ptr, cpu) = per_cpu(irq_stack, cpu);
23}
24#endif
25
26#ifndef CONFIG_PREEMPT_RT
27static void ____do_softirq(struct pt_regs *regs)
28{
29 __do_softirq();
30}
31
32void do_softirq_own_stack(void)
33{
34 call_on_irq_stack(NULL, ____do_softirq);
35}
36#endif
2.2. 中断栈的切换
在call_on_irq_stack中，读取当前cpu的irq_stack_ptr，然后加上IRQ_STACK_SIZE，作为中断栈顶地址。
1/// arch/arm64/kernel/entry.S
2/*
3 * void call_on_irq_stack(struct pt_regs *regs,
4 * void (*func)(struct pt_regs *));
5 *
6 * Calls func(regs) using this CPU's irq stack and shadow irq stack.
7 */
8SYM_FUNC_START(call_on_irq_stack)
9#ifdef CONFIG_SHADOW_CALL_STACK
10 get_current_task x16
11 scs_save x16
12 ldr_this_cpu scs_sp, irq_shadow_call_stack_ptr, x17
13#endif
14
15 /* Create a frame record to save our LR and SP (implicit in FP) */
16 stp x29, x30, [sp, #-16]!
17 mov x29, sp
18
19 /// 读取irq_stack_ptr，获取当前cpu的中断栈地址
20 ldr_this_cpu x16, irq_stack_ptr, x17
21
22 /* Move to the new stack and call the function there */
23 add sp, x16, #IRQ_STACK_SIZE
24 blr x1
25
26 /*
27 * Restore the SP from the FP, and restore the FP and LR from the frame
28 * record.
29 */
30 mov sp, x29
31 ldp x29, x30, [sp], #16
32 scs_load_current
33 ret
34SYM_FUNC_END(call_on_irq_stack)
35NOKPROBE(call_on_irq_stack)
