1. 启动1号进程
1号进程的创建
kernel的启动开始于start_kernel()函数，在这里完成各种系统初始化后最后进入rest_init函数，这整个过程都可以称之为0进程：
asmlinkage void __init start_kernel(void)
{
...
boot_cpu_init();
page_address_init();
pr_notice("%s", linux_banner);
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();
pr_notice("Kernel command line: %s\n", boot_command_line);
parse_early_param();
...
vfs_caches_init_early();
sort_main_extable();
trap_init();
mm_init();
...
/* Do the rest non-__init'ed, we're now alive */
rest_init();
}
rest_init()创建了两个内核线程kernel_init和kthreadd：
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);
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_startup_entry(CPUHP_ONLINE);
}
top命令可以看到kthreadd的pid为2
top.png
kernel_init后面会尝试装载init进程，并完成内核态到用户态的转换，形成用户态的祖先1号进程。
static int __ref kernel_init(void *unused)
{
...
if (execute_command) {
if (!run_init_process(execute_command))
return 0;
pr_err("Failed to execute %s. Attempting defaults...\n",
execute_command);
}
if (!run_init_process("/etc/preinit") ||
!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.");
}
execute_command为cmdline传递进来的init进程，不存在或运行失败后依次尝试运行：
"/etc/preinit", "/sbin/init", "/etc/init", "/bin/init", "/bin/sh"
