Linux内核启动流程

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movsr10, r5@ invalid processor (r5=0)? THUMB( iteq )@ force fixup-able long branch encodingbeq__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. */ldrr13, =__mmap_switched@ address to jump to after                 //保存MMU使能后跳转地址@ mmu has been enabledadrlr, BSYM(1f)@ return (PIC) addressmovr8, r4@ set TTBR1 to swapper_pg_dir ARM(addpc, r10, #PROCINFO_INITFUNC) THUMB(addr12, r10, #PROCINFO_INITFUNC) THUMB(movpc, 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。。。bstart_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_INITRDif (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;}#endifpage_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_X86if (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();#endifthread_info_cache_init();cred_init();fork_init(totalram_pages);                                                             //初始化forkproc_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_FSproc_root_init();#endifcgroup_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.");}

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：