linux內核啓動順序

參見linux源碼樹結構

linux內核的中心是文件init/main.c。

在底層架構相關的設置完成之後立刻調用了start_kernel函數。start_kernel函數就位於init/main.c。

kernel代碼執行順序大致如下：

架構相關  ( arch / * /） ---> 函數start_kernel() (init/main.c)  ---> 函數init( init/main.c ) --->  啓動用戶進程init

架構相關：

         -.解壓和移動內核代碼本身

         -.初始化硬件（包括設置底層內存管理）

         -.把控制權交給start_kernel

start_kernel():

         -.打印內核版本信息，初始化命令行

          -.初始化終端輸出設備

          -.開啓中斷

  -.初始化內存，調度，時鐘等

         -.調用rest_init()    // 啓動一個內核線程運行 init(),  進入idle循環

init():

          -.啓動其他處理器(SMP機器)

          -.啓動設備子系統

         -.掛載root文件系統

          -.釋放不用的內核內存

          -.運行 /sbin/init 程序   也就是1號進程（用戶進程），到此內核啓動完成。後續各種程序有1號進程啓動，像 網絡服務，getty。

你可以在start_kernel() , init() 和各種模塊中 添加printk()函數打印信息，幫助理解。

start_kernel()代碼

asmlinkage __visible void __init start_kernel(void)
{
	char *command_line;
	char *after_dashes;

	/*
	 * Need to run as early as possible, to initialize the
	 * lockdep hash:
	 */
	lockdep_init();
	set_task_stack_end_magic(&init_task);
	smp_setup_processor_id();
	debug_objects_early_init();

	/*
	 * Set up the the initial canary ASAP:
	 */
	boot_init_stack_canary();

	cgroup_init_early();

	local_irq_disable();
	early_boot_irqs_disabled = true;

/*
 * Interrupts are still disabled. Do necessary setups, then
 * enable them
 */
	boot_cpu_init();
	page_address_init();
	pr_notice("%s", linux_banner);
	setup_arch(&command_line);
	mm_init_cpumask(&init_mm);
	setup_command_line(command_line);
	setup_nr_cpu_ids();
	setup_per_cpu_areas();
	smp_prepare_boot_cpu();	/* arch-specific boot-cpu hooks */

	build_all_zonelists(NULL, NULL);
	page_alloc_init();

	pr_notice("Kernel command line: %s\n", boot_command_line);
	parse_early_param();
	after_dashes = parse_args("Booting kernel",
				  static_command_line, __start___param,
				  __stop___param - __start___param,
				  -1, -1, &unknown_bootoption);
	if (!IS_ERR_OR_NULL(after_dashes))
		parse_args("Setting init args", after_dashes, NULL, 0, -1, -1,
			   set_init_arg);

	jump_label_init();

	/*
	 * These use large bootmem allocations and must precede
	 * kmem_cache_init()
	 */
	setup_log_buf(0);
	pidhash_init();
	vfs_caches_init_early();
	sort_main_extable();
	trap_init();
	mm_init();

	/*
	 * Set up the scheduler prior starting any interrupts (such as the
	 * timer interrupt). Full topology setup happens at smp_init()
	 * time - but meanwhile we still have a functioning scheduler.
	 */
	sched_init();
	/*
	 * Disable preemption - early bootup scheduling is extremely
	 * fragile until we cpu_idle() for the first time.
	 */
	preempt_disable();
	if (WARN(!irqs_disabled(),
		 "Interrupts were enabled *very* early, fixing it\n"))
		local_irq_disable();
	idr_init_cache();
	rcu_init();

	/* trace_printk() and trace points may be used after this */
	trace_init();

	context_tracking_init();
	radix_tree_init();
	/* init some links before init_ISA_irqs() */
	early_irq_init();
	init_IRQ();
	tick_init();
	rcu_init_nohz();
	init_timers();
	hrtimers_init();
	softirq_init();
	timekeeping_init();
	time_init();
	sched_clock_postinit();
	perf_event_init();
	profile_init();
	call_function_init();
	WARN(!irqs_disabled(), "Interrupts were enabled early\n");
	early_boot_irqs_disabled = false;
	local_irq_enable();

	kmem_cache_init_late();

	/*
	 * HACK ALERT! This is early. We're enabling the console before
	 * we've done PCI setups etc, and console_init() must be aware of
	 * this. But we do want output early, in case something goes wrong.
	 */
	console_init();
	if (panic_later)
		panic("Too many boot %s vars at `%s'", panic_later,
		      panic_param);

	lockdep_info();

	/*
	 * Need to run this when irqs are enabled, because it wants
	 * to self-test [hard/soft]-irqs on/off lock inversion bugs
	 * too:
	 */
	locking_selftest();

#ifdef CONFIG_BLK_DEV_INITRD
	if (initrd_start && !initrd_below_start_ok &&
	    page_to_pfn(virt_to_page((void *)initrd_start)) < min_low_pfn) {
		pr_crit("initrd overwritten (0x%08lx < 0x%08lx) - disabling it.\n",
		    page_to_pfn(virt_to_page((void *)initrd_start)),
		    min_low_pfn);
		initrd_start = 0;
	}
#endif
	page_ext_init();
	debug_objects_mem_init();
	kmemleak_init();
	setup_per_cpu_pageset();
	numa_policy_init();
	if (late_time_init)
		late_time_init();
	sched_clock_init();
	calibrate_delay();
	pidmap_init();
	anon_vma_init();
	acpi_early_init();
#ifdef CONFIG_X86
	if (efi_enabled(EFI_RUNTIME_SERVICES))
		efi_enter_virtual_mode();
#endif
#ifdef CONFIG_X86_ESPFIX64
	/* Should be run before the first non-init thread is created */
	init_espfix_bsp();
#endif
	thread_info_cache_init();
	cred_init();
	fork_init(totalram_pages);
	proc_caches_init();
	buffer_init();
	key_init();
	security_init();
	dbg_late_init();
	vfs_caches_init(totalram_pages);
	signals_init();
	/* rootfs populating might need page-writeback */
	page_writeback_init();
	proc_root_init();
	nsfs_init();
	cgroup_init();
	cpuset_init();
	taskstats_init_early();
	delayacct_init();

	check_bugs();

	sfi_init_late();

	if (efi_enabled(EFI_RUNTIME_SERVICES)) {
		efi_late_init();
		efi_free_boot_services();
	}

	ftrace_init();

	/* Do the rest non-__init'ed, we're now alive */
	rest_init();
}