内核随记(三)--同步(2)
2.2、睡眠与唤醒
在操作系统中，睡眠和唤醒原语实际上是操作系统的基本原语，也是实现同步的一种方式，而且它还是实现信号量的基础。当进程请求的资源(如内存、文件等)不能得到满足时，就会主动放弃CPU，进入等待状态(可中断等待或者不可中断等待)。当资源满足时，就会由别的进程唤醒，从而投入运行。
2.2.1、等待队列
等待队列表示一组睡眠的进程，这些进程正在等待特定的事件发生(或者说条件为真)，比如，等待足够的内存。等待队列是一个双链表，每个队列都有一个队列头，其定义如下：
//include/linux/wait.h
//等待队列头
struct __wait_queue_head {
// 自旋锁
spinlock_t lock;
struct list_head task_list;
};
typedef struct __wait_queue_head wait_queue_head_t;
等待队列链表中的元素类型为：
typedef struct __wait_queue wait_queue_t;
//唤醒函数指针
typedef int (*wait_queue_func_t)(wait_queue_t *wait, unsigned mode, int sync, void *key);
//默认的唤醒函数
int default_wake_function(wait_queue_t *wait, unsigned mode, int sync, void *key);
struct __wait_queue {
/*取值为WQ_FLAG_EXCLUSIVE(=1)表示互斥进程,由内核有选择的唤醒.为0时表示非互斥进程,由内核在
**事件发生时唤醒所有等待进程.
**/
unsigned int flags;
#define WQ_FLAG_EXCLUSIVE 0x01
//等待的任务描述符
struct task_struct * task;
//唤醒函数,默认为default_wake_function
wait_queue_func_t func;
struct list_head task_list;
};
其典型的结构如下：
等待队列头的初始化：
DECLARE_WAIT_QUEUE_HEAD(name)；
其定义如下：
//incude/linux/wait.h
#define __WAIT_QUEUE_HEAD_INITIALIZER(name) { \
.lock = SPIN_LOCK_UNLOCKED, \
.task_list = { &(name).task_list, &(name).task_list } }
//初始化等待队列头
#define DECLARE_WAIT_QUEUE_HEAD(name) \
wait_queue_head_t name = __WAIT_QUEUE_HEAD_INITIALIZER(name)
或者如下：
wait_queue_head_t my_queue;
init_waitqueue_head(&my_queue);
等待队列元素初始化：
//linux/wait.h
//wait_queue_t初始化
static inline void init_waitqueue_entry(wait_queue_t *q, struct task_struct *p)
{
q->flags = 0;
q->task = p;
q->func = default_wake_function;
}
2.2.2、等待事件(Waiting on the Event)
内核提供的等待接口包括wait_event(), wait_event_ interruptible(), 和wait_event_interruptible_timeout()。此外sleep_on(), sleep_on_timeout(), 和interruptible_sleep_on()在2.6中仍然支持，但已经过时。这些接口的基本实现如下：
具体代码如下：
//linux/wait.h
#define wait_event(wq, condition) \
do { \
if (condition) //条件发生 \
break; \
__wait_event(wq, condition); \
} while (0)
#define __wait_event(wq, condition) \
do { \
DEFINE_WAIT(__wait); \
\
for (;;) { \
prepare_to_wait(&wq, &__wait, TASK_UNINTERRUPTIBLE); \
if (condition) \
break; \
schedule();//调度 \
} \
finish_wait(&wq, &__wait); \
} while (0)
//kernel/wait.c
void fastcall
prepare_to_wait(wait_queue_head_t *q, wait_queue_t *wait, int state)
{
unsigned long flags;
//非互斥进程
wait->flags &= ~WQ_FLAG_EXCLUSIVE;
//关中断,并请求自旋锁
spin_lock_irqsave(&q->lock, flags);
if (list_empty(&wait->task_list))
__add_wait_queue(q, wait); //将等待任务加入等待队列
/*
* don't alter the task state if this is just going to
* queue an async wait queue callback
*/
if (is_sync_wait(wait))
set_current_state(state); //设置任务当前的状态
//释放自旋锁,并恢复处理器状态
spin_unlock_irqrestore(&q->lock, flags);
}
//等待完成之后,应该设置任务的状态为运行状态,并从等待队列中删除
void fastcall finish_wait(wait_queue_head_t *q, wait_queue_t *wait)
{
unsigned long flags;
__set_current_state(TASK_RUNNING); //设置为运行状态
if (!list_empty_careful(&wait->task_list)) {
spin_lock_irqsave(&q->lock, flags);
list_del_init(&wait->task_list); //从等待队列中删除
spin_unlock_irqrestore(&q->lock, flags);
}
}
2.2.3、唤醒(Waking Up)
接口如下：
//include/inux/wait.h
#define wake_up(x) __wake_up(x, TASK_UNINTERRUPTIBLE | TASK_INTERRUPTIBLE, 1, NULL)
#define wake_up_nr(x, nr) __wake_up(x, TASK_UNINTERRUPTIBLE | TASK_INTERRUPTIBLE, nr, NULL)
#define wake_up_all(x) __wake_up(x, TASK_UNINTERRUPTIBLE | TASK_INTERRUPTIBLE, 0, NULL)
#define wake_up_interruptible(x) __wake_up(x, TASK_INTERRUPTIBLE, 1, NULL)
#define wake_up_interruptible_nr(x, nr) __wake_up(x, TASK_INTERRUPTIBLE, nr, NULL)
#define wake_up_interruptible_all(x) __wake_up(x, TASK_INTERRUPTIBLE, 0, NULL)
#define wake_up_locked(x) __wake_up_locked((x), TASK_UNINTERRUPTIBLE | TASK_INTERRUPTIBLE)
#define wake_up_interruptible_sync(x) __wake_up_sync((x),TASK_INTERRUPTIBLE, 1)
具体实现：
//kernel/sched.c
void fastcall __wake_up(wait_queue_head_t *q, unsigned int mode,
int nr_exclusive, void *key)
{
unsigned long flags;
//请求自旋锁,并关中断
spin_lock_irqsave(&q->lock, flags);
__wake_up_common(q, mode, nr_exclusive, 0, key);
spin_unlock_irqrestore(&q->lock, flags);
}
static void __wake_up_common(wait_queue_head_t *q, unsigned int mode,
int nr_exclusive, int sync, void *key)
{
struct list_head *tmp, *next;
list_for_each_safe(tmp, next, &q->task_list) {
wait_queue_t *curr;
unsigned flags;
curr = list_entry(tmp, wait_queue_t, task_list);
flags = curr->flags;
//调用相应的唤醒函数, 唤醒第1个有WQ_FLAG_EXCLUSIVE标志的进程后停止
if (curr->func(curr, mode, sync, key) &&
(flags & WQ_FLAG_EXCLUSIVE) &&
!--nr_exclusive)
break;
}
}
//默认的唤醒函数
int default_wake_function(wait_queue_t *curr, unsigned mode, int sync, void *key)
{
task_t *p = curr->task;
return try_to_wake_up(p, mode, sync);
}
try_to_wake_up是唤醒原语中核心部分，其具体代码如下：
/*p被唤醒的进程描述符.
**state被唤醒的进程的状态掩码
**sync禁止被唤醒的进程抢占本地CPU正在运行的进程
*/
static int try_to_wake_up(task_t * p, unsigned int state, int sync)
{
int cpu, this_cpu, success = 0;
unsigned long flags;
long old_state;
runqueue_t *rq;
#ifdef CONFIG_SMP
unsigned long load, this_load;
struct sched_domain *sd;
int new_cpu;
#endif
//关闭中断,并获取最后执行该进程的CPU(可能不同于本地CPU)的运行队列的锁
rq = task_rq_lock(p, &flags);
schedstat_inc(rq, ttwu_cnt);
old_state = p->state;
if (!(old_state & state))
goto out;
if (p->array)
goto out_running;
//最后执行该任务的CPU
cpu = task_cpu(p);
//本地CPU
this_cpu = smp_processor_id();
/*对于多CPU系统,检查要被唤醒的进程是否应该从最近执行该进程的CPU的运行队列,
**转移到另外一个CPU的运行队列.
*/
#ifdef CONFIG_SMP
if (unlikely(task_running(rq, p)))
goto out_activate;
new_cpu = cpu;
if (cpu == this_cpu || unlikely(!cpu_isset(this_cpu, p->cpus_allowed)))
goto out_set_cpu;
load = source_load(cpu);
this_load = target_load(this_cpu);
/*
* If sync wakeup then subtract the (maximum possible) effect of
* the currently running task from the load of the current CPU:
*/
if (sync)
this_load -= SCHED_LOAD_SCALE;
/* Don't pull the task off an idle CPU to a busy one */
if (load < SCHED_LOAD_SCALE/2 && this_load > SCHED_LOAD_SCALE/2)
goto out_set_cpu;
new_cpu = this_cpu; /* Wake to this CPU if we can */
/*
* Scan domains for affine wakeup and passive balancing
* possibilities.
*/
for_each_domain(this_cpu, sd) {
unsigned int imbalance;
/*
* Start passive balancing when half the imbalance_pct
* limit is reached.
*/
imbalance = sd->imbalance_pct + (sd->imbalance_pct - 100) / 2;
if ((sd->flags & SD_WAKE_AFFINE) &&
!task_hot(p, rq->timestamp_last_tick, sd)) {
/*
* This domain has SD_WAKE_AFFINE and p is cache cold
* in this domain.
*/
if (cpu_isset(cpu, sd->span)) {
schedstat_inc(sd, ttwu_wake_affine);
goto out_set_cpu;
}
} else if ((sd->flags & SD_WAKE_BALANCE) &&
imbalance*this_load <= 100*load) {
/*
* This domain has SD_WAKE_BALANCE and there is
* an imbalance.
*/
if (cpu_isset(cpu, sd->span)) {
schedstat_inc(sd, ttwu_wake_balance);
goto out_set_cpu;
}
}
}
new_cpu = cpu; /* Could not wake to this_cpu. Wake to cpu instead */
out_set_cpu:
schedstat_inc(rq, ttwu_attempts);
new_cpu = wake_idle(new_cpu, p);
if (new_cpu != cpu && cpu_isset(new_cpu, p->cpus_allowed)) {
schedstat_inc(rq, ttwu_moved);
set_task_cpu(p, new_cpu);
task_rq_unlock(rq, &flags);
/* might preempt at this point */
rq = task_rq_lock(p, &flags);
old_state = p->state;
if (!(old_state & state))
goto out;
if (p->array)
goto out_running;
this_cpu = smp_processor_id();
cpu = task_cpu(p);
}
out_activate:
#endif /* CONFIG_SMP */
if (old_state == TASK_UNINTERRUPTIBLE) {
rq->nr_uninterruptible--;
/*
* Tasks on involuntary sleep don't earn
* sleep_avg beyond just interactive state.
*/
p->activated = -1;
}
/*
* Sync wakeups (i.e. those types of wakeups where the waker
* has indicated that it will leave the CPU in short order)
* don't trigger a preemption, if the woken up task will run on
* this cpu. (in this case the 'I will reschedule' promise of
* the waker guarantees that the freshly woken up task is going
* to be considered on this CPU.)
*/
//将进程p加入目标CPU的可运行队列
activate_task(p, rq, cpu == this_cpu);
/*如果没有设置sync标志(表示允许抢占),且目标CPU不是本地CPU,则检查p是否比rq运行队列中当前进程的动态优先级高.
**即(p)->prio < (rq)->curr->prio,如果是,则调用resched_task()抢占rq->curr。
*/
if (!sync || cpu != this_cpu) {
if (TASK_PREEMPTS_CURR(p, rq))
/*在单CPU中,仅仅设置TIF_NEED_RESCHED标志.多CPU系统中,则检查相应标志,并使目标CPU重新调度
*/
resched_task(rq->curr);
}
success = 1;
out_running:
//设置进程的状态
p->state = TASK_RUNNING;
out:
//释放rq的锁,并打开本地中断
task_rq_unlock(rq, &flags);
return success;
}
#ifdef CONFIG_SMP
//多CPU系统
static void resched_task(task_t *p)
{
int need_resched, nrpolling;
BUG_ON(!spin_is_locked(&task_rq(p)->lock));
/* minimise the chance of sending an interrupt to poll_idle() */
nrpolling = test_tsk_thread_flag(p,TIF_POLLING_NRFLAG);
need_resched = test_and_set_tsk_thread_flag(p,TIF_NEED_RESCHED);
nrpolling |= test_tsk_thread_flag(p,TIF_POLLING_NRFLAG);
if (!need_resched && !nrpolling && (task_cpu(p) != smp_processor_id()))
//产生IPI,强制目标CPU重新调度
smp_send_reschedule(task_cpu(p));
}
#else
//单CPU系统
static inline void resched_task(task_t *p)
{
set_tsk_need_resched(p);
}
#endif
2.2.4、互斥等待
当调用wake_up唤醒等待队列时，等待队列上的所有进程都转置为可运行。在一些情况下，这种做法是正确的，比如等待某个特定的事件。但是在另外一些情况，可以提前知道只有一个被唤醒的进程能够成功的获取资源，比如等待临界区资源，其它的进程将再次睡眠。如果等待队列中的进程数量太大，将会严重影响系统性能，这就是所谓的thundering herd行为。为此，内核引入互斥等待，它与非互斥等待的区别如下：
(1) 当一个等待队列入口有 WQ_FLAG_EXCLUSEVE 标志置位, 它被添加到等待队列的尾部. 没有这个标志的入口项, 相反, 添加到开始。
(2) 当 wake_up 被在一个等待队列上调用, 它在唤醒第一个有 WQ_FLAG_EXCLUSIVE 标志的进程后停止。
这样，进行互斥等待的进程一次只唤醒一个。使一个进程进入互斥等待是调用prepare_to_wait_exclusive完成的。
//kernel/wait.c
void fastcall
prepare_to_wait_exclusive(wait_queue_head_t *q, wait_queue_t *wait, int state)
{
unsigned long flags;
//互斥标志
wait->flags |= WQ_FLAG_EXCLUSIVE;
spin_lock_irqsave(&q->lock, flags);
if (list_empty(&wait->task_list))
__add_wait_queue_tail(q, wait);
/*
* don't alter the task state if this is just going to
* queue an async wait queue callback
*/
if (is_sync_wait(wait))
set_current_state(state);
spin_unlock_irqrestore(&q->lock, flags);
}
