AndroidO bluedroid alarm 机制分析
bluedroid的alarm 机制实现在osi/osi/src/alarm.cc 中：
这里面实现了很多的接口：
alarm_t* alarm_new(const char* name)； alarm_t* alarm_new_periodic(const char* name) ； static alarm_t* alarm_new_internal(const char* name, bool is_periodic) ； void alarm_free(alarm_t* alarm)； void alarm_set(alarm_t* alarm, period_ms_t interval_ms, alarm_callback_t cb, void* data) ； void alarm_cancel(alarm_t* alarm)； void alarm_cleanup(void)； static bool lazy_initialize(void) ；
我们先看一个使用alarm 的事例：
在hci_layer.cc 文件中关于hci_module_start_up 的实现：
startup_timer = alarm_new("hci.startup_timer"); ... alarm_set(startup_timer, startup_timeout_ms, startup_timer_expired, NULL);
当startup_timeout_ms 时间到达的时候，如果startup_timer还没有被取消的话，那么startup_timer_expired函数将会被执行。
我们具体看看 其实现的过程：
我们先看看新建定时器的代码：
alarm_t* alarm_new(const char* name) { return alarm_new_internal(name, false); }
static alarm_t* alarm_new_internal(const char* name, bool is_periodic) { // Make sure we have a list we can insert alarms into. if (!alarms && !lazy_initialize()) { //最初alarm 还没有初始化，需要执行初始化流程 CHECK(false); // if initialization failed, we should not continue return NULL; } alarm_t* ret = static_cast<alarm_t*>(osi_calloc(sizeof(alarm_t))); ret->callback_mutex = new std::recursive_mutex; ret->is_periodic = is_periodic; ret->stats.name = osi_strdup(name); // NOTE: The stats were reset by osi_calloc() above return ret; }
我们继续看看 lazy_initialize 是如何 初始化的：
static bool lazy_initialize(void) { bool timer_initialized = false; bool wakeup_timer_initialized = false; std::lock_guard<std::mutex> lock(alarms_mutex); alarms = list_new(NULL); if (!timer_create_internal(CLOCK_ID, &timer)) goto error;//timer_create timer_initialized = true; if (!timer_create_internal(CLOCK_ID_ALARM, &wakeup_timer)) goto error; wakeup_timer_initialized = true; alarm_expired = semaphore_new(0);//新建信号量 default_callback_thread = thread_new_sized("alarm_default_callbacks", SIZE_MAX);//新建线程 thread_set_rt_priority(default_callback_thread, THREAD_RT_PRIORITY);//提高线程优先级 default_callback_queue = fixed_queue_new(SIZE_MAX);//新建队列 alarm_register_processing_queue(default_callback_queue, default_callback_thread);//将线程和队列绑定 dispatcher_thread_active = true; dispatcher_thread = thread_new("alarm_dispatcher");//新建定时器分发线程,该线程不停运行while(true) thread_set_rt_priority(dispatcher_thread, THREAD_RT_PRIORITY); thread_post(dispatcher_thread, callback_dispatch, NULL);//运行callback_dispatch return true; error: ... return false; }
这里有两个线程,一个是dispatcher_thread,它一直轮询,如果有定时器到期那么他就将这个定时器放到一个(当初创建啊定时器的时候关联的队列)特定的队列里面,默认的就是default_callback_queue ,然后由另外一个线程default_callback_thread 来处理该队列里面的已经expire的定时器.
我们看看timer_create_internal的实现:
static bool timer_create_internal(const clockid_t clock_id, timer_t* timer) { CHECK(timer != NULL); struct sigevent sigevent; // create timer with RT priority thread pthread_attr_t thread_attr; pthread_attr_init(&thread_attr); pthread_attr_setschedpolicy(&thread_attr, SCHED_FIFO); struct sched_param param; param.sched_priority = THREAD_RT_PRIORITY; pthread_attr_setschedparam(&thread_attr, ¶m); memset(&sigevent, 0, sizeof(sigevent)); sigevent.sigev_notify = SIGEV_THREAD; sigevent.sigev_notify_function = (void (*)(union sigval))timer_callback;//发送信号量 sigevent.sigev_notify_attributes = &thread_attr; if (timer_create(clock_id, &sigevent, timer) == -1) { //创建定时器 /*错误处理*/ } return false; } return true; }
这里我们注意到,当定时器到期的时候,会执行timer_callback,其就是发送了一个alarm_expired的信号量:
static void timer_callback(UNUSED_ATTR void* ptr) { semaphore_post(alarm_expired); }
这里有发送信号量,那么一定有一个地方会等待这个信号量,就是在定时器的不断等待的线程里面:
static void callback_dispatch(UNUSED_ATTR void* context) { while (true) { semaphore_wait(alarm_expired);//一直循环等待信号量 ... } }
lazy_initialize 初始化完成之后,定时器并没有启动,只是创建了定时器.那是在哪里启动的呢?根据我们上面展示出的使用示例,在alarm_set 肯定是有启动定时器的操作的:
void alarm_set(alarm_t* alarm, period_ms_t interval_ms, alarm_callback_t cb, void* data) { alarm_set_on_queue(alarm, interval_ms, cb, data, default_callback_queue); }
这里发现 调用alarm_set 传入的 队列都是default_callback_queue,
void alarm_set_on_queue(alarm_t* alarm, period_ms_t interval_ms, alarm_callback_t cb, void* data, fixed_queue_t* queue) { CHECK(queue != NULL); alarm_set_internal(alarm, interval_ms, cb, data, queue); }
// Runs in exclusion with alarm_cancel and timer_callback. static void alarm_set_internal(alarm_t* alarm, period_ms_t period, alarm_callback_t cb, void* data, fixed_queue_t* queue) { std::lock_guard<std::mutex> lock(alarms_mutex); alarm->creation_time = now(); alarm->period = period; alarm->queue = queue; alarm->callback = cb; alarm->data = data; schedule_next_instance(alarm); alarm->stats.scheduled_count++; }
上面是对定时器进行封装以及赋值,然后调用schedule_next_instance 来启动 定时器:
// Must be called with |alarms_mutex| held static void schedule_next_instance(alarm_t* alarm) { // If the alarm is currently set and it's at the start of the list, // we'll need to re-schedule since we've adjusted the earliest deadline. bool needs_reschedule = (!list_is_empty(alarms) && list_front(alarms) == alarm);//如果alarms 队列第一个元素就是这个定时器,那么需要重启schedule if (alarm->callback) remove_pending_alarm(alarm);//取出所有的pending,重复的alarm // Calculate the next deadline for this alarm period_ms_t just_now = now(); period_ms_t ms_into_period = 0; if ((alarm->is_periodic) && (alarm->period != 0)) ms_into_period = ((just_now - alarm->creation_time) % alarm->period); alarm->deadline = just_now + (alarm->period - ms_into_period); // Add it into the timer list sorted by deadline (earliest deadline first).//以下是给alarm排序,插入到某个合适的问题,最近的alarm 排在第一个 if (list_is_empty(alarms) || ((alarm_t*)list_front(alarms))->deadline > alarm->deadline) { list_prepend(alarms, alarm); } else { for (list_node_t* node = list_begin(alarms); node != list_end(alarms); node = list_next(node)) { list_node_t* next = list_next(node); if (next == list_end(alarms) || ((alarm_t*)list_node(next))->deadline > alarm->deadline) { list_insert_after(alarms, node, alarm); break; } } } // If the new alarm has the earliest deadline, we need to re-evaluate our // schedule. if (needs_reschedule || (!list_is_empty(alarms) && list_front(alarms) == alarm)) { reschedule_root_alarm(); } }
上面主要就是将alarm插入到 alarms 列表中,等待schedule,如果当前这个alarm 就是最紧迫的alarm,那么就会立即进行 schedule.
我们看看其实现reschedule_root_alarm;
// NOTE: must be called with |alarms_mutex| held static void reschedule_root_alarm(void) { CHECK(alarms != NULL); const bool timer_was_set = timer_set; alarm_t* next; int64_t next_expiration; // If used in a zeroed state, disarms the timer. struct itimerspec timer_time; memset(&timer_time, 0, sizeof(timer_time)); if (list_is_empty(alarms)) goto done; next = static_cast<alarm_t*>(list_front(alarms)); next_expiration = next->deadline - now(); if (next_expiration < TIMER_INTERVAL_FOR_WAKELOCK_IN_MS) {//如果deadline<3s> timer_time.it_value.tv_sec = (next->deadline / 1000); timer_time.it_value.tv_nsec = (next->deadline % 1000) * 1000000LL; /*下面设置最长的wake_up 是为了减少删除该timer的开销,可以略过*/ struct itimerspec end_of_time; memset(&end_of_time, 0, sizeof(end_of_time)); end_of_time.it_value.tv_sec = (time_t)(1LL << (sizeof(time_t) * 8 - 2)); timer_settime(wakeup_timer, TIMER_ABSTIME, &end_of_time, NULL); } else { // WARNING: do not attempt to use relative timers with *_ALARM clock IDs // in kernels before 3.17 unless you have the following patch: // https://lkml.org/lkml/2014/7/7/576 struct itimerspec wakeup_time; memset(&wakeup_time, 0, sizeof(wakeup_time)); wakeup_time.it_value.tv_sec = (next->deadline / 1000); wakeup_time.it_value.tv_nsec = (next->deadline % 1000) * 1000000LL; if (timer_settime(wakeup_timer, TIMER_ABSTIME, &wakeup_time, NULL) == -1) LOG_ERROR(LOG_TAG, "%s unable to set wakeup timer: %s", __func__, strerror(errno)); } done: timer_set = timer_time.it_value.tv_sec != 0 || timer_time.it_value.tv_nsec != 0; if (timer_was_set && !timer_set) { wakelock_release(); } if (timer_settime(timer, TIMER_ABSTIME, &timer_time, NULL) == -1) LOG_ERROR(LOG_TAG, "%s unable to set timer: %s", __func__, strerror(errno));//设置定时器 // If next expiration was in the past (e.g. short timer that got context // switched) then the timer might have diarmed itself. Detect this case and // work around it by manually signalling the |alarm_expired| semaphore. // // It is possible that the timer was actually super short (a few // milliseconds) and the timer expired normally before we called // |timer_gettime|. Worst case, |alarm_expired| is signaled twice for that // alarm. Nothing bad should happen in that case though since the callback // dispatch function checks to make sure the timer at the head of the list // actually expired. if (timer_set) { struct itimerspec time_to_expire; timer_gettime(timer, &time_to_expire); if (time_to_expire.it_value.tv_sec == 0 && time_to_expire.it_value.tv_nsec == 0) { semaphore_post(alarm_expired);//如果定时器的时机已经到了,那么直接发送信号量 } } }
代码是实现是 在离expire 不到3s的时候启动定时器.
当定时器时间到的时候,发动alarm_expired的信号.
我们接下来看看 定时器的 已经到期的处理流程:上面我们已经知道,线程dispatcher_thread一直轮询,我们看看其实现:
// Function running on |dispatcher_thread| that performs the following: // (1) Receives a signal using |alarm_exired| that the alarm has expired // (2) Dispatches the alarm callback for processing by the corresponding // thread for that alarm. static void callback_dispatch(UNUSED_ATTR void* context) { while (true) { semaphore_wait(alarm_expired);//等待expire的信号量 if (!dispatcher_thread_active) break; std::lock_guard<std::mutex> lock(alarms_mutex); alarm_t* alarm; // Take into account that the alarm may get cancelled before we get to it. // We're done here if there are no alarms or the alarm at the front is in // the future. Exit right away since there's nothing left to do. if (list_is_empty(alarms) || (alarm = static_cast<alarm_t*>(list_front(alarms)))->deadline > now()) { reschedule_root_alarm(); continue; } list_remove(alarms, alarm);//remove 该alarm 从队列中 if (alarm->is_periodic) { alarm->prev_deadline = alarm->deadline; schedule_next_instance(alarm); alarm->stats.rescheduled_count++; } reschedule_root_alarm();//去启动下一个定时器 // Enqueue the alarm for processing fixed_queue_enqueue(alarm->queue, alarm);//将该expire的定时器放到相应队列等待处理 } LOG_DEBUG(LOG_TAG, "%s Callback thread exited", __func__); }
这个函数做的事情,就像其名字一样,收到expire的信号之后,做一个dispatch的动作,我们接下来看看放置到队列之后如何处理的.
这里我们还要看一下当时队列和线程绑定的情况:
void alarm_register_processing_queue(fixed_queue_t* queue, thread_t* thread) { CHECK(queue != NULL); CHECK(thread != NULL); fixed_queue_register_dequeue(queue, thread_get_reactor(thread), alarm_queue_ready, NULL); }
我们看看alarm_queue_ready:
static void alarm_queue_ready(fixed_queue_t* queue, UNUSED_ATTR void* context) { CHECK(queue != NULL); std::unique_lock<std::mutex> lock(alarms_mutex); alarm_t* alarm = (alarm_t*)fixed_queue_try_dequeue(queue); if (alarm == NULL) { return; // The alarm was probably canceled } // // If the alarm is not periodic, we've fully serviced it now, and can reset // some of its internal state. This is useful to distinguish between expired // alarms and active ones. // alarm_callback_t callback = alarm->callback; void* data = alarm->data; period_ms_t deadline = alarm->deadline; if (alarm->is_periodic) { // The periodic alarm has been rescheduled and alarm->deadline has been // updated, hence we need to use the previous deadline. deadline = alarm->prev_deadline; } else { alarm->deadline = 0; alarm->callback = NULL; alarm->data = NULL; alarm->queue = NULL; } std::lock_guard<std::recursive_mutex> cb_lock(*alarm->callback_mutex); lock.unlock(); period_ms_t t0 = now(); callback(data);//执行callback 函数,并reset alarm period_ms_t t1 = now(); // Update the statistics CHECK(t1 >= t0); period_ms_t delta = t1 - t0; update_scheduling_stats(&alarm->stats, t0, deadline, delta); }
上面流程的核心 就是 取出队列中的alarm,并执行其中的callback,也就是我们开定时器的时候的回调函数.
关于定时器的介绍,就到这里.
