linux内核启动流程微信可以设置雪花昵称了，真漂亮！！！本文以Linux3.14版本源码为例分析其启动流程。各版本启动

本文以Linux3.14版本源码为例分析其启动流程。各版本启动代码略有不同，但核心流程与思想万变不离其宗。

内核映像被加载到内存并获得控制权之后，内核启动流程开始。通常，内核映像以压缩形式存储，并不是一个可以执行的内核。因此，内核阶段的首要工作是自解压内核映像。

内核编译生成vmliunx后，通常会对其进行压缩，得到zImage（小内核，小于512KB）或bzImage（大内核，大于512KB）。在它们的头部嵌有解压缩程序。

通过linux/arch/arm/boot/compressed目录下的Makefile寻找到vmlinux文件的链接脚本（vmlinux.lds），从中查找系统启动入口函数。

$(obj)/vmlinux:$ (obj)/vmlinux.lds $(obj)/$ (HEAD) $(obj)/piggy.$ (suffix_y).o
$(addprefix$ (obj)/, $(OBJS))$ (lib1funcs) $(ashldi3) \$ (bswapsdi2) FORCE @ $(check_for_multiple_zreladdr)$ (call if_changed,ld) @$(check_for_bad_syms) vmlinux.lds(linux/arch/arm/kernel/vmlinux.lds)链接脚本开头内容

OUTPUT_ARCH(arm) ENTRY(stext) jiffies = jiffies_64; SECTIONS { 。。。得到内核入口函数为 stext（linux/arch/arm/kernel/head.S）

内核引导阶段 ENTRY(stext) 。。。 bl __lookup_processor_type @ r5=procinfo r9=cpuid //处理器是否支持 movs r10, r5 @ invalid processor (r5=0)? THUMB( it eq ) @ force fixup-able long branch encoding beq __error_p @ yes, error 'p' //不支持则打印错误信息

      。
。
。
bl	__create_page_tables                                                       //创建页表
 
/*
 * The following calls CPU specific code in a position independent
 * manner.  See arch/arm/mm/proc-*.S for details.  r10 = base of
 * xxx_proc_info structure selected by __lookup_processor_type
 * above.  On return, the CPU will be ready for the MMU to be
 * turned on, and r0 will hold the CPU control register value.
 */
ldr	r13, =__mmap_switched		@ address to jump to after                 //保存MMU使能后跳转地址
					@ mmu has been enabled
adr	lr, BSYM(1f)			@ return (PIC) address
mov	r8, r4				@ set TTBR1 to swapper_pg_dir

ARM( add pc, r10, #PROCINFO_INITFUNC ) THUMB( add r12, r10, #PROCINFO_INITFUNC ) THUMB( mov pc, r12 ) 1: b __enable_mmu //使能MMU后跳转到__mmap_switched 查找标签__mmap_switched所在位置：/linux/arch/arm/kernel/head-common.S __mmap_switched: /* * The following fragment of code is executed with the MMU on in MMU mode, * and uses absolute addresses; this is not position independent. * * r0 = cp#15 control register * r1 = machine ID * r2 = atags/dtb pointer * r9 = processor ID */ //保存设备信息、设备树及启动参数存储地址。。。 b start_kernel

内核初始化阶段从start_kernel函数开始，内核进入C语言部分，完成内核的大部分初始化工作。

函数所在位置：/linux/init/Main.c

start_kernel涉及大量初始化工作，只例举重要的初始化工作。

asmlinkage void __init start_kernel(void) { …… //类型判断 smp_setup_processor_id(); //smp相关，返回启动CPU号 …… local_irq_disable(); //关闭当前CPU中断 early_boot_irqs_disabled = true; /*

Interrupts are still disabled. Do necessary setups, then
enable them / boot_cpu_init(); page_address_init(); //初始化页地址 pr_notice("%s", linux_banner); //显示内核版本信息 setup_arch(&command_line); mm_init_owner(&init_mm, &init_task); 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(); //页内存申请初始化

pr_notice("Kernel command line: %s\n", boot_command_line); //打印内核启动命令行参数 parse_early_param(); parse_args("Booting kernel", static_command_line, __start___param, __stop___param - __start___param, -1, -1, &unknown_bootoption);

…… /*
- 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(); //进程调度器初始化 /
- Disable preemption - early bootup scheduling is extremely
- fragile until we cpu_idle() for the first time. */ preempt_disable(); //禁止内核抢占 if (WARN(!irqs_disabled(), "Interrupts were enabled very early, fixing it\n")) local_irq_disable(); //检查关闭CPU中断
  
  /大量初始化内容见名知意/ idr_init_cache(); rcu_init(); tick_nohz_init(); context_tracking_init(); radix_tree_init(); /* init some links before init_ISA_irqs() */ early_irq_init(); init_IRQ(); tick_init(); 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(); //本地中断可以使用了
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(); //初始化控制台，可以使用printk了 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

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page_cgroup_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

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thread_info_cache_init();
cred_init();
fork_init(totalram_pages);                                                             //初始化fork
proc_caches_init();
buffer_init();
key_init();
security_init();
dbg_late_init();
vfs_caches_init(totalram_pages);                                                      //虚拟文件系统初始化
signals_init();
/* rootfs populating might need page-writeback */
page_writeback_init();

#ifdef CONFIG_PROC_FS proc_root_init(); #endif 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();

} 函数最后调用rest_init()函数

/最重要使命：创建kernel_init进程，并进行后续初始化/ static noinline void __init_refok rest_init(void) { int pid;

rcu_scheduler_starting();
/*
 * We need to spawn init first so that it obtains pid 1, however
 * the init task will end up wanting to create kthreads, which, if
 * we schedule it before we create kthreadd, will OOPS.
 */

kernel_thread(kernel_init, NULL, CLONE_FS | CLONE_SIGHAND);                             //创建kernel_init进程

numa_default_policy();
pid = kernel_thread(kthreadd, NULL, CLONE_FS | CLONE_FILES);
rcu_read_lock();
kthreadd_task = find_task_by_pid_ns(pid, &init_pid_ns);
rcu_read_unlock();
complete(&kthreadd_done);
 
/*
 * The boot idle thread must execute schedule()
 * at least once to get things moving:
 */
init_idle_bootup_task(current);
schedule_preempt_disabled();
/* Call into cpu_idle with preempt disabled */
//cpu_idle就是在系统闲置时用来降低电力的使用和减少热的产生的空转函数，函数至此不再返回，其余工作从kernel_init进程处发起
cpu_startup_entry(CPUHP_ONLINE);

} kernel_init函数将完成设备驱动程序的初始化，并调用init_post函数启动用户进程

部分书籍介绍的内核启动流程基于经典的2.6版本，kernel_init函数还会调用init_post函数专门负责_init进程的启动，现版本已经被整合到了一起。

static int __ref kernel_init(void *unused) { int ret;

kernel_init_freeable();                 //该函数中完成smp开启  驱动初始化 共享内存初始化等工作
/* need to finish all async __init code before freeing the memory */
async_synchronize_full();
free_initmem();                         //初始化尾声，清除内存无用数据
mark_rodata_ro();
system_state = SYSTEM_RUNNING;
numa_default_policy();
 
flush_delayed_fput();
 
if (ramdisk_execute_command) {
	ret = run_init_process(ramdisk_execute_command);
	if (!ret)
		return 0;
	pr_err("Failed to execute %s (error %d)\n",
	       ramdisk_execute_command, ret);
}
 
/*
 * We try each of these until one succeeds.
 *
 * The Bourne shell can be used instead of init if we are
 * trying to recover a really broken machine.
                                                      *寻找init函数，创建一号进程_init (第一个用户空间进程)*/
if (execute_command) {
	ret = run_init_process(execute_command);
	if (!ret)
		return 0;
	pr_err("Failed to execute %s (error %d).  Attempting defaults...\n",
		execute_command, ret);
}
if (!try_to_run_init_process("/sbin/init") ||
    !try_to_run_init_process("/etc/init") ||
    !try_to_run_init_process("/bin/init") ||
    !try_to_run_init_process("/bin/sh"))
	return 0;
 
panic("No working init found.  Try passing init= option to kernel. "
      "See Linux Documentation/init.txt for guidance.");

} static int __ref kernel_init(void *unused) { int ret;

kernel_init_freeable();                 //该函数中完成smp开启  驱动初始化 共享内存初始化等工作
/* need to finish all async __init code before freeing the memory */
async_synchronize_full();
free_initmem();                         //初始化尾声，清除内存无用数据
mark_rodata_ro();
system_state = SYSTEM_RUNNING;
numa_default_policy();
 
flush_delayed_fput();
 
if (ramdisk_execute_command) {
	ret = run_init_process(ramdisk_execute_command);
	if (!ret)
		return 0;
	pr_err("Failed to execute %s (error %d)\n",
	       ramdisk_execute_command, ret);
}
 
/*
 * We try each of these until one succeeds.
 *
 * The Bourne shell can be used instead of init if we are
 * trying to recover a really broken machine.
                                                      *寻找init函数，创建一号进程_init (第一个用户空间进程)*/
if (execute_command) {
	ret = run_init_process(execute_command);
	if (!ret)
		return 0;
	pr_err("Failed to execute %s (error %d).  Attempting defaults...\n",
		execute_command, ret);
}
if (!try_to_run_init_process("/sbin/init") ||
    !try_to_run_init_process("/etc/init") ||
    !try_to_run_init_process("/bin/init") ||
    !try_to_run_init_process("/bin/sh"))
	return 0;
 
panic("No working init found.  Try passing init= option to kernel. "
      "See Linux Documentation/init.txt for guidance.");

}

到此，内核初始化已经接近尾声，所有的初始化函数都已经调用，因此free_initmem函数可以舍弃内存的__init_begin至__init_end之间的数据。

当内核被引导并进行初始化后，内核启动了自己的第一个用户空间应用程序_init，这是调用的第一个使用标准C库编译的程序，其进程编号时钟为1.

_init负责出发其他必须的进程，以使系统进入整体可用的状态。

以下为内核启动流程图：

以上就是良许教程网为各位朋友分享的Linux相关知识。