linux 启动过程一

Linux Kernel

此文档基于linux3.0.9内核。

一、rest_init 分析

在kernel启动最后会调用rest_init,内核启动即将完成，下面分析其流程：该函数主要功能是启动内核线程kernel_init创建init任务1.static noinline void__init_refok rest_init(void)2.{3.int pid;内核RCU锁机制调度启动(新版本取消大内核锁)，此机制以后分析；4.rcu_scheduler_starting();5./*6.We need to spawn init first sothat it obtains pid 1, however7.the init task will end upwanting to create kthreads, which, if8.we schedule it before we createkthreadd, will OOPS.9.*/10.创建init线程，见下面分析；11.kernel_thread(kernel_init, NULL, CLONE_FS |CLONE_SIGHAND);12.numa_default_policy();13.pid = kernel_thread(kthreadd,NULL, CLONE_FS | CLONE_FILES);14.rcu_read_lock();15.kthreadd_task =find_task_by_pid_ns(pid, &init_pid_ns);16.rcu_read_unlock();17.complete(&kthreadd_done);
18./*19.The boot idle thread mustexecute schedule()20.at least once to get thingsmoving:21.*/22.init_idle_bootup_task(current);23.schedule_preempt_disabled();24./* Call into cpu_idle withpreempt disabled */25.cpu_idle();26.}kernel_init线程流程：在以前内核采用大内核锁，锁住线程，这里采用completion进行线程通信static int __refkernel_init(void *unused){kernel_init_freeable();/* need to finish all async __init codebefore freeing the memory */async_synchronize_full();释放所有init.* 段中的内存。free_initmem();mark_rodata_ro();设置系统状态为运行状态system_state = SYSTEM_RUNNING;设定NUMA系统的内存访问策略为默认numa_default_policy();
flush_delayed_fput();
如果ramdisk_execute_command有指定的init程序，就执行它。if (ramdisk_execute_command) {if(!run_init_process(ramdisk_execute_command))return 0;printk(KERN_WARNING "Failedto execute %s\n",ramdisk_execute_command);}
/**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.*/如果execute_command有指定的init程序，就执行它if (execute_command) {if(!run_init_process(execute_command))return 0;printk(KERN_WARNING "Failedto execute %s.Attempting ""defaults...\n",execute_command);}if(!run_init_process("/sbin/init") ||!run_init_process("/etc/init") ||!run_init_process("/bin/init") ||!run_init_process("/bin/sh"))return 0;
panic("No init found.Try passing init= option to kernel. ""See Linux Documentation/init.txt for guidance.");}
static noinline void__init kernel_init_freeable(void){/**Wait until kthreadd is all set-up.*/等待kthreadd线程执行完成，阻塞wait_for_completion(&kthreadd_done);
/* Now the scheduler is fully set up andcan do blocking allocations */gfp_allowed_mask = __GFP_BITS_MASK;
/**init can allocate pages on any node*/set_mems_allowed(node_states[N_MEMORY]);/**init can run on any cpu.*/set_cpus_allowed_ptr(current,cpu_all_mask);
cad_pid = task_pid(current);
smp_prepare_cpus(setup_max_cpus);
do_pre_smp_initcalls();lockup_detector_init();这里为SMP初始化，并启动SMP调度smp_init();sched_init_smp();到此，与构架相关的部分已经初始化完成了，do_basic_setup函数主要是初始化设备驱动，完成其他驱动程序（直接编译进内核的模块）的初始化。内核中大部分的启动数据输出（都是各设备的驱动模块输出）都是这里产生的。这个函数非常重要，下面分析do_basic_setup();
复制两次标准输入（0）的文件描述符（它是上面打开的/dev/console，也就是系统控制台）：一个作为标准输出（1）一个作为标准出错（2）现在标准输入、标准输出、标准出错都是/dev/console了。这个console在内核启动参数中可以配置为某个串口（ttySn、ttyOn等等），也可以是虚拟控制台（tty0）。所以我们就在串口或者显示器上看到了之后的系统登录提示。
/* Open the /dev/console on the rootfs,this should never fail */if (sys_open((const char __user *)"/dev/console", O_RDWR, 0) < 0)printk(KERN_WARNING "Warning:unable to open an initial console.\n");
(void) sys_dup(0);(void) sys_dup(0);/**check if there is an early userspace init.If yes, let it do all*the work*/
if (!ramdisk_execute_command)ramdisk_execute_command ="/init";
if (sys_access((const char __user *)ramdisk_execute_command, 0) != 0) {ramdisk_execute_command = NULL;prepare_namespace();}
/**Ok, we have completed the initial bootup, and*we're essentially up and running. Get rid of the*initmem segments and start the user-mode stuff..*/
/* rootfs is available now, try loadingdefault modules */load_default_modules();}
do_basic_setup()函数分析：static void__init do_basic_setup(void){cpuset_init_smp();usermodehelper_init();shmem_init();driver_init();init_irq_proc();do_ctors();usermodehelper_enable();do_initcalls();}

二、驱动模型子系统：

void __initdriver_init(void){/* These are the core pieces */devtmpfs_init();创建节点devicesdev dev/char /dev/block;devices_init();创建节点bus 并常见/devices/systembuses_init();创建节点classclasses_init();创建节点firmwarefirmware_init();创建节点hypervisorhypervisor_init();
/* These are also core pieces, but mustcome after the*core core pieces.*/初始化平台设备platform_bus_init();/sys/devices/system/cpucpu_dev_init();/sys/devices/system/memorymemory_dev_init();}

三、子系统初始化：

这个函数调用个子系统初始化函数，按照优先级。staticinitcall_t *initcall_levels[] __initdata = {__initcall0_start,__initcall1_start,__initcall2_start,__initcall3_start,__initcall4_start,__initcall5_start,__initcall6_start,__initcall7_start,__initcall_end,};
/* Keep these insync with initcalls in include/linux/init.h */static char*initcall_level_names[] __initdata = {"early","core","postcore","arch","subsys","fs","device","late",};static void__init do_initcalls(void){int level;
for (level = 0; level <ARRAY_SIZE(initcall_levels) - 1; level++)do_initcall_level(level);}static void__init do_initcall_level(int level){extern const struct kernel_param__start___param[], __stop___param[];initcall_t *fn;
strcpy(static_command_line,saved_command_line);parse_args(initcall_level_names[level],static_command_line, __start___param,__stop___param - __start___param,level, level,&repair_env_string);
for (fn = initcall_levels[level]; fn <initcall_levels[level+1]; fn++)do_one_initcall(*fn);}int__init_or_module do_one_initcall(initcall_t fn){int count = preempt_count();int ret;
if (initcall_debug)ret = do_one_initcall_debug(fn);elseret =fn();
msgbuf[0] = 0;
if (ret && ret != -ENODEV&& initcall_debug)sprintf(msgbuf, "error code%d ", ret);
if (preempt_count() != count) {strlcat(msgbuf, "preemptionimbalance ", sizeof(msgbuf));preempt_count() = count;}if (irqs_disabled()) {strlcat(msgbuf, "disabledinterrupts ", sizeof(msgbuf));local_irq_enable();}if (msgbuf[0]) {printk("initcall %pF returnedwith %s\n", fn, msgbuf);}
return ret;}

本文出自 “陈立峰嵌入式之路” 博客，请务必保留此出处http://linuxkernel.blog.51cto.com/1308410/1293754