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电梯直达

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发表于 2012-12-26 13:44 |只看该作者 |倒序浏览

内核中两大重要的线程，migration_thread负责cpu的负载均衡(将进程从本地队列移动到目标cpu的队列)，kthreadd负责为kthread_create_list链表中的成员创建内核线程。

内核版本2.6.24中的引导部分，start_kernel()->rest_init():
static void noinline __init_refok rest_init(void)
__releases(kernel_lock)
{
int pid;

kernel_thread(kernel_init, NULL, CLONE_FS | CLONE_SIGHAND);
numa_default_policy();
pid = kernel_thread(kthreadd, NULL, CLONE_FS | CLONE_FILES);
kthreadd_task = find_task_by_pid(pid);
……
}
以上依次创建了kernel_init线程和kthreadd线程，rest_init()是在禁用抢占(preempt_disable)的情况下运行，因此保证了kernel_init()运行时kthreadd_task 已经指向ktheadd线程。

kernel_init（）调用do_pre_smp_initcalls()->migration_init();创建了负责进程在cpu间移动(cpu负载均衡)的内核线程migration_thread，创建线程是通过将包含待运行函数及参数的kthread_create_info结构挂入kthread_create_list链表，然后唤醒kthreadd_task(即ktheadd线程)，而ktheadd线程负责为链表上的每个结构创建相应的线程。
void __init migration_init(void)
{
void *cpu = (void *)(long)smp_processor_id();
int err;

/* Start one for the boot CPU: */
err = migration_call(&migration_notifier, CPU_UP_PREPARE, cpu);
……
}

static int __cpuinit
migration_call(struct notifier_block *nfb, unsigned long action, void *hcpu)
{
struct task_struct *p;
int cpu = (long)hcpu;
unsigned long flags;
struct rq *rq;

switch (action) {
case CPU_LOCK_ACQUIRE:
mutex_lock(&sched_hotcpu_mutex);
break;

case CPU_UP_PREPARE:
case CPU_UP_PREPARE_FROZEN:
p = kthread_create(migration_thread, hcpu, "migration/%d", cpu);
……
}

struct task_struct *kthread_create(int (*threadfn)(void *data),
   void *data,
   const char namefmt[],
   ...)
{
struct kthread_create_info create;

create.threadfn = threadfn;
create.data = data;
init_completion(&create.started);
init_completion(&create.done);

spin_lock(&kthread_create_lock);
list_add_tail(&create.list, &kthread_create_list);
wake_up_process(kthreadd_task);
……
}