QP中RTC的理解

1.概念

RTC(Run To Completion)是运行到完成为止的意思。在状态机中，从源状态到目标状态的转换动作要运行到完成。

从字面上来看，这个过程像是不可中断的，但实际并不是，这个过程可以被硬件中断程序打断。

在本状态机中，一个事件处理要是执行了，本状态机的其它事件到来时，就只能在队列中等待，不可中断这个正在处理的过程，也就是要运行到完成(RTC)。这个事件处理完后，才能对队列中的其它事件进行处理。所以在活动对象（Active Object，AO）中总是有一个队列，这个队列用来保存发送到本状态机的事件的。

2. 几种情况

• 更高级任务的事件到来

异步事件
如果在任务L处理事件a时，硬件中断到来了，中断了任务L的事件a处理过程（保存断点），硬件中断中可能产生一个更高级任务H的事件b（异步事件），则在中断返回前执行调度器，处理高级任务H的事件b, 之后，从上边保存的断点返回，继续执行没有完成的任务L的事件a。事件a不是真正的RTC，被事件b中断了。

同步事件
如果在任务L处理事件a时，post了一个事件c（同步事件）, 事件c是更高级任务H的，则post()中也调用调度器，先处理任务H的事件c, 之后继续执行任务L的事件a中post()后的代码。事件a不是真正的RTC，被事件c中断了。

• 同级任务的事件到来

如果在任务L处理事件a时，硬件中断中产生了事件d（异步事件），或处理事件a中post了事件d（同步事件）, 事件d是属于任务L的，则保存事件d到对应的任务L的队列中，先处理完事件a，之后由QK的循环处理事件d。事件a是RTC的。

• 更低级任务的事件到来

如果在任务L处理事件a时，硬件中断中产生了事件d（异步事件），或处理事件a中post了事件d（同步事件）, 事件d是属于比任务L更低的任务LL的，则保存事件d到对应的任务LL的队列中，先处理完事件a，之后由QK的循环处理事件d。事件a是RTC的。

请参考Miro Samek以下的说明：
Miro Samek:"All state machine formalisms, including UML statecharts, universally assume that a state machine completes processing of each event before it can start processing the next event. This model of execution is called run to completion, or RTC.

In the RTC model, the system processes events in discrete, indivisible RTC steps. New incoming events cannot interrupt the processing of the current event and must be stored (typically in an event queue) until the state machine becomes idle again. These semantics completely avoid any internal concurrency issues within a single state machine. The RTC model also gets around the conceptual problem of processing actions associated with transitions, where the state machine is not in a well-defined state(is between two states) for the duration of the action. During event processing, the system is unresponsive (unobservable),so the ill-defined state during that time has no practical significance.

Note, however, that RTC does not mean that a state machine has to monopolize the CPU until the RTC step is complete. The preemption restriction only applies to the task context of the state machine that is already busy processing events. In a multitasking environment, other tasks (not related to the task context of the busy state machine) can be running, possibly preempting the currently executing state machine. As long as other state machines do not share variables or other resources with each other, there are no concurrency hazards.

The key advantage of RTC processing is simplicity. Its biggest disadvantage is that the responsiveness of a state machine is determined by its longest RTC step.4 Achieving short RTC steps can often significantly complicate real-time designs.