基于aarch64分析kernel源码 五：idle进程(0号进程)

一、参考

linux — 0号进程，1号进程，2号进程 - 流水灯 - 博客园 (cnblogs.com)

Linux0号进程，1号进程，2号进程_0号进程和1号进程-CSDN博客

二、idle进程的创建流程

start_kernel --> arch_call_rest_init --> rest_init--> cpu_startup_entry --> while(1) { do_idle(); }

start_kernel 函数在完成系统初始化后会进入死循环调用 do_idle 函数，相当于 start_kernel 函数完成初始化后会退化成 idle 进程。

三、idle 进程控制块

idle 进程控制块是 init_task，init_task 是一个全局静态变量，定义在 init/init_task.c 文件中。

/** Set up the first task table, touch at your own risk!. Base=0,* limit=0x1fffff (=2MB)*/
struct task_struct init_task
#ifdef CONFIG_ARCH_TASK_STRUCT_ON_STACK__init_task_data
#endif__aligned(L1_CACHE_BYTES)
= {
#ifdef CONFIG_THREAD_INFO_IN_TASK.thread_info	= INIT_THREAD_INFO(init_task),.stack_refcount	= REFCOUNT_INIT(1),
#endif.__state	= 0,.stack		= init_stack,		// init_stack 值定义在链接脚本中（init_stack是内核栈的静态的定义）.usage		= REFCOUNT_INIT(2),.flags		= PF_KTHREAD,.prio		= MAX_PRIO - 20,.static_prio	= MAX_PRIO - 20,.normal_prio	= MAX_PRIO - 20,.policy		= SCHED_NORMAL,.cpus_ptr	= &init_task.cpus_mask,.user_cpus_ptr	= NULL,.cpus_mask	= CPU_MASK_ALL,.nr_cpus_allowed= NR_CPUS,.mm		= NULL,.active_mm	= &init_mm,.restart_block	= {.fn = do_no_restart_syscall,},.se		= {.group_node 	= LIST_HEAD_INIT(init_task.se.group_node),},.rt		= {.run_list	= LIST_HEAD_INIT(init_task.rt.run_list),.time_slice	= RR_TIMESLICE,},.tasks		= LIST_HEAD_INIT(init_task.tasks),
#ifdef CONFIG_SMP.pushable_tasks	= PLIST_NODE_INIT(init_task.pushable_tasks, MAX_PRIO),
#endif
#ifdef CONFIG_CGROUP_SCHED.sched_task_group = &root_task_group,
#endif.ptraced	= LIST_HEAD_INIT(init_task.ptraced),.ptrace_entry	= LIST_HEAD_INIT(init_task.ptrace_entry),.real_parent	= &init_task,.parent		= &init_task,.children	= LIST_HEAD_INIT(init_task.children),.sibling	= LIST_HEAD_INIT(init_task.sibling),.group_leader	= &init_task,RCU_POINTER_INITIALIZER(real_cred, &init_cred),RCU_POINTER_INITIALIZER(cred, &init_cred),.comm		= INIT_TASK_COMM,	// #define INIT_TASK_COMM "swapper" - 0号进程的名称.thread		= INIT_THREAD,.fs		= &init_fs,.files		= &init_files,
#ifdef CONFIG_IO_URING.io_uring	= NULL,
#endif.signal		= &init_signals,.sighand	= &init_sighand,.nsproxy	= &init_nsproxy,.pending	= {.list = LIST_HEAD_INIT(init_task.pending.list),.signal = {{0}}},.blocked	= {{0}},.alloc_lock	= __SPIN_LOCK_UNLOCKED(init_task.alloc_lock),.journal_info	= NULL,INIT_CPU_TIMERS(init_task).pi_lock	= __RAW_SPIN_LOCK_UNLOCKED(init_task.pi_lock),.timer_slack_ns = 50000, /* 50 usec default slack */.thread_pid	= &init_struct_pid,.thread_group	= LIST_HEAD_INIT(init_task.thread_group),.thread_node	= LIST_HEAD_INIT(init_signals.thread_head),
#ifdef CONFIG_AUDIT.loginuid	= INVALID_UID,.sessionid	= AUDIT_SID_UNSET,
#endif
#ifdef CONFIG_PERF_EVENTS.perf_event_mutex = __MUTEX_INITIALIZER(init_task.perf_event_mutex),.perf_event_list = LIST_HEAD_INIT(init_task.perf_event_list),
#endif
#ifdef CONFIG_PREEMPT_RCU.rcu_read_lock_nesting = 0,.rcu_read_unlock_special.s = 0,.rcu_node_entry = LIST_HEAD_INIT(init_task.rcu_node_entry),.rcu_blocked_node = NULL,
#endif
#ifdef CONFIG_TASKS_RCU.rcu_tasks_holdout = false,.rcu_tasks_holdout_list = LIST_HEAD_INIT(init_task.rcu_tasks_holdout_list),.rcu_tasks_idle_cpu = -1,
#endif
#ifdef CONFIG_TASKS_TRACE_RCU.trc_reader_nesting = 0,.trc_reader_special.s = 0,.trc_holdout_list = LIST_HEAD_INIT(init_task.trc_holdout_list),.trc_blkd_node = LIST_HEAD_INIT(init_task.trc_blkd_node),
#endif
#ifdef CONFIG_CPUSETS.mems_allowed_seq = SEQCNT_SPINLOCK_ZERO(init_task.mems_allowed_seq,&init_task.alloc_lock),
#endif
#ifdef CONFIG_RT_MUTEXES.pi_waiters	= RB_ROOT_CACHED,.pi_top_task	= NULL,
#endifINIT_PREV_CPUTIME(init_task)
#ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN.vtime.seqcount	= SEQCNT_ZERO(init_task.vtime_seqcount),.vtime.starttime = 0,.vtime.state	= VTIME_SYS,
#endif
#ifdef CONFIG_NUMA_BALANCING.numa_preferred_nid = NUMA_NO_NODE,.numa_group	= NULL,.numa_faults	= NULL,
#endif
#if defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KASAN_SW_TAGS).kasan_depth	= 1,
#endif
#ifdef CONFIG_KCSAN.kcsan_ctx = {.scoped_accesses	= {LIST_POISON1, NULL},},
#endif
#ifdef CONFIG_TRACE_IRQFLAGS.softirqs_enabled = 1,
#endif
#ifdef CONFIG_LOCKDEP.lockdep_depth = 0, /* no locks held yet */.curr_chain_key = INITIAL_CHAIN_KEY,.lockdep_recursion = 0,
#endif
#ifdef CONFIG_FUNCTION_GRAPH_TRACER.ret_stack		= NULL,.tracing_graph_pause	= ATOMIC_INIT(0),
#endif
#if defined(CONFIG_TRACING) && defined(CONFIG_PREEMPTION).trace_recursion = 0,
#endif
#ifdef CONFIG_LIVEPATCH.patch_state	= KLP_UNDEFINED,
#endif
#ifdef CONFIG_SECURITY.security	= NULL,
#endif
#ifdef CONFIG_SECCOMP_FILTER.seccomp	= { .filter_count = ATOMIC_INIT(0) },
#endif
};
EXPORT_SYMBOL(init_task);

四、源码分析

1、设置0号进程PCB

/* 代码位置: /arch/arm64/kernel/head.S *//** Initialize CPU registers with task-specific and cpu-specific context.** Create a final frame record at task_pt_regs(current)->stackframe, so* that the unwinder can identify the final frame record of any task by* its location in the task stack. We reserve the entire pt_regs space* for consistency with user tasks and kthreads.*/.macro	init_cpu_task tsk, tmp1, tmp2msr	sp_el0, \tskldr	\tmp1, [\tsk, #TSK_STACK]add	sp, \tmp1, #THREAD_SIZEsub	sp, sp, #PT_REGS_SIZEstp	xzr, xzr, [sp, #S_STACKFRAME]add	x29, sp, #S_STACKFRAMEscs_load_currentadr_l	\tmp1, __per_cpu_offsetldr	w\tmp2, [\tsk, #TSK_TI_CPU]ldr	\tmp1, [\tmp1, \tmp2, lsl #3]set_this_cpu_offset \tmp1.endm/** The following fragment of code is executed with the MMU enabled.**   x0 = __pa(KERNEL_START)*/
SYM_FUNC_START_LOCAL(__primary_switched)adr_l	x4, init_taskinit_cpu_task x4, x5, x6……bl	start_kernelASM_BUG()
SYM_FUNC_END(__primary_switched)

在 __primary_switched 函数中将 init_task 值保存到 sp_el0 寄存器中，然后调用 start_kernel 函数。至此成功将 init_task 设置成 0 号进程的进程控制块，并成功运行 0 号进程。

2、设置栈底魔数

置栈底魔数，用于栈溢出检查。

/* 函数定义在 /init/main.c 中 */asmlinkage __visible void __init __no_sanitize_address __noreturn start_kernel(void)
{char *command_line;char *after_dashes;set_task_stack_end_magic(&init_task);……
}

3、其他配置

详细配置见 Linux 源码。

五、获取当前PCB

/* 函数定义在 arch/arm64/include/asm/current.h 文件中 *//** We don't use read_sysreg() as we want the compiler to cache the value where* possible.*/
static __always_inline struct task_struct *get_current(void)
{unsigned long sp_el0;asm ("mrs %0, sp_el0" : "=r" (sp_el0));return (struct task_struct *)sp_el0;
}#define current get_current()