Linux内核分析3
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gdb跟踪start_kernel()函数
start_kernel()为系统内核的入口函数，该函数主要是对环境和组件等进行初始化操作。
课程中通过运行跟踪MenuOS来了解从start_kernel()做了哪些事情，以下为我跟踪调试的截图。后面将分析函数的执行过程。
分析start_kernel()函数
start_kernel(): /linux-3.18.6/init/main.c
asmlinkage __visible void __init start_kernel(void)
{
// ...
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();
// ...
/* Do the rest non-__init'ed, we're now alive */
rest_init();
}
这个函数的大部分代码被省略，留下关注的4个初始化函数，其实是3个，mm_init()是内存管理初始化，暂时不分析。
trap_init()
中断向量表的初始化函数，设置了很多中断门(Interrupt Gate)，其中设置了后面会关注到的system_call
trap_init(): /linux-3.18.6/arch/x86/kernel/traps.c
void __init trap_init(void)
{
// ...
#ifdef CONFIG_X86_32
set_system_trap_gate(SYSCALL_VECTOR, &system_call);
set_bit(SYSCALL_VECTOR, used_vectors);
#endif
// ...
}sched_init()
进程调度初始化函数，函数内做了很关键的一步初始化——对0号进程，即idle进程进行初始化
rest_init()
其他初始化函数，函数内将创建1号进程，即init进程。下面主要来分析该函数。
分析rest_init()函数
rest_init(): /linux-3.18.6/init/main.c
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);
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);
}其中，创建了一个线程kernel_init，不过内核中并没有线程的概念，这里创建的是一个任务(进程)。如果把系统当成是一个大进程，那么init进程就相当于大进程中的一个线程，因为内核中的进程调度会为每一个像init进程分配时间片来执行。
kernel_thread(kernel_init, NULL, CLONE_FS);
kernel_init(): /linux-3.18.6/init/main.c
static int __ref kernel_init(void *unused)
{
int ret;
kernel_init_freeable();
/* need to finish all async __init code before freeing the memory */
async_synchronize_full();
free_initmem();
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.
*/
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.");
}
代码中有3个if，这三个if分别以三种不同方式来启动init，但只会有1个init会被启动
- 如果ramdisk_execute_command被设置，执行-initrd指定的rootfs中的init
- 如果execute_command有值，执行命令行传入的init（猜测）
- 最后再在系统文件中的/sbin/init; /etc/init; /bin/init; /bin/sh查找文件是否存在，存在即作为1号进程启动
到此1号进程的启动分析完成
下面分析0号idle进程从哪里启动
static noinline void __init_refok rest_init(void)
{
// ...
/* Call into cpu_idle with preempt disabled */
cpu_startup_entry(CPUHP_ONLINE);
}
在rest_init()函数的末尾，0号进程idle就是在这里启动的。
cpu_startup_entry(): /linux-3.18.6/kernel/sched/idle.c
void cpu_startup_entry(enum cpuhp_state state)
{
/*
* This #ifdef needs to die, but it's too late in the cycle to
* make this generic (arm and sh have never invoked the canary
* init for the non boot cpus!). Will be fixed in 3.11
*/
#ifdef CONFIG_X86
/*
* If we're the non-boot CPU, nothing set the stack canary up
* for us. The boot CPU already has it initialized but no harm
* in doing it again. This is a good place for updating it, as
* we wont ever return from this function (so the invalid
* canaries already on the stack wont ever trigger).
*/
boot_init_stack_canary();
#endif
arch_cpu_idle_prepare();
cpu_idle_loop();
}
static void cpu_idle_loop(void)
{
while (1) {
/*
* If the arch has a polling bit, we maintain an invariant:
*
* Our polling bit is clear if we're not scheduled (i.e. if
* rq->curr != rq->idle). This means that, if rq->idle has
* the polling bit set, then setting need_resched is
* guaranteed to cause the cpu to reschedule.
*/
__current_set_polling();
tick_nohz_idle_enter();
while (!need_resched()) {
check_pgt_cache();
rmb();
if (cpu_is_offline(smp_processor_id()))
arch_cpu_idle_dead();
local_irq_disable();
arch_cpu_idle_enter();
/*
* In poll mode we reenable interrupts and spin.
*
* Also if we detected in the wakeup from idle
* path that the tick broadcast device expired
* for us, we don't want to go deep idle as we
* know that the IPI is going to arrive right
* away
*/
if (cpu_idle_force_poll || tick_check_broadcast_expired())
cpu_idle_poll();
else
cpuidle_idle_call();
arch_cpu_idle_exit();
}
/*
* Since we fell out of the loop above, we know
* TIF_NEED_RESCHED must be set, propagate it into
* PREEMPT_NEED_RESCHED.
*
* This is required because for polling idle loops we will
* not have had an IPI to fold the state for us.
*/
preempt_set_need_resched();
tick_nohz_idle_exit();
__current_clr_polling();
/*
* We promise to call sched_ttwu_pending and reschedule
* if need_resched is set while polling is set. That
* means that clearing polling needs to be visible
* before doing these things.
*/
smp_mb__after_atomic();
sched_ttwu_pending();
schedule_preempt_disabled();
}
}
