目录
默认FreeRTOS调度策略（单核）
FreeRTOS调度策略的实现
任务创建
任务调度的4种情景：
1.第一次启动任务调度器
2.任务主动触发调度
3.SystemTick时钟触发调度
4.因为中断而引起的任务调度
默认情况下，FreeRTOS使用固定优先级抢占式调度策略，并对同等优先级的任务进行循环时间切片：
list是是FreeRTOS核心的数据结构，它在list.c文件中实现为一个双向循环的链表。而FreeRTOS的抢占式的调度策略就是基于此实现的。
在task.c中定义了一个pxReadyTasksLists[ configMAX_PRIORITIES ] 链表数组，其中
configMAX_PRIORITIES 是定义在FreeRTOSConfig.h中的配置参数表示task的最大优先级。所以有几个优先级就有几个ReadyList，这是一一对应的。如下图：
而当我们创建task时就会在创建时，把创建的task插入对应优先级的pxReadyTasksLists中，如下图：
同时创建任务时也会使pxCurrentTCB指针指向优先级最高的TCB，如下图：
任务创建完成之后，启动任务调度器，调度器就会使能SVC中断，如下图：（至于为什么要进入SVC中断，因为在只有在特权级模式下，程序才以访问一些内核的寄存器，和特殊功能寄存器，且我们希望在task正常运行时芯片工作在线程模式，而要启动第一个task，需要访问这些寄存器，具体的可以参考这篇文件章：【RTX操作系统教程】第9章 任务运行在特权级或非特权级模式_硬汉Eric2013_新浪博客 (sina.com.cn)）
之后会跳转到SVC中断，而在SVC中会初始化一些通用寄存器，psp指针，打开BASEPRI中断屏蔽寄存器，并切换到线程模式跳转到第一个task的函数入口，之后退出中断就会执行第一个task了，如下图：
当任务执行一些系统提供的API函数时，就可能触发任务调度，例如执行vTaskDelay函数，执行接收或发送队列函数且设置了阻塞时间，执行信号量和锁相关的函数导致阻塞等等。
例如：调用xQueueReceive队列接收函数时，如果队列为空且设置了阻塞时间，则会把当前的task从pxReadyTasksLists插入到delay链表中，然后调用portYIELD_WITHIN_API 使能PendSV中断,然后在PendSV中断里保存当前任务的上下文，并切换到下一个优先级task
整个os的时间依赖于SystemTick，而SystemTick时间来源于内核硬件的滴答定时器。当某个task运行时，SystemTick时间到了会进入SystemTick的中断，在此中断里会调用xTaskIncrementTick，先判断是否有任务的阻塞时间到了，如果有然后把任务从delayList中放入pxReadyTasksLists中，同时判断新加入的task优先级是否大于当前正在运行的优先级，如果是就置一个pdTRUE的标志位。
如果设置了时间片轮转的宏configUSE_TIME_SLICING，就会再判断当前task所在的pxReadyTasksLists中是否还有其他同优先级的任务，有的话就置标志位相当于一个时间片调度一次同优先级的任务。
BaseType_t xTaskIncrementTick( void ) { TCB_t * pxTCB; TickType_t xItemValue; BaseType_t xSwitchRequired = pdFALSE; /* Called by the portable layer each time a tick interrupt occurs. * Increments the tick then checks to see if the new tick value will cause any * tasks to be unblocked. */ traceTASK_INCREMENT_TICK( xTickCount ); if( uxSchedulerSuspended == ( UBaseType_t ) pdFALSE ) { /* Minor optimisation. The tick count cannot change in this * block. */ const TickType_t xConstTickCount = xTickCount + ( TickType_t ) 1; /* Increment the RTOS tick, switching the delayed and overflowed * delayed lists if it wraps to 0. */ xTickCount = xConstTickCount; //判断tick是溢出，是则交换delaylist if( xConstTickCount == ( TickType_t ) 0U ) /*lint !e774 'if' does not always evaluate to false as it is looking for an overflow. */ { taskSWITCH_DELAYED_LISTS(); } else { mtCOVERAGE_TEST_MARKER(); } /* See if this tick has made a timeout expire. Tasks are stored in * the queue in the order of their wake time - meaning once one task * has been found whose block time has not expired there is no need to * look any further down the list. */ if( xConstTickCount >= xNextTaskUnblockTime ) { for( ; ; ) { //判断延时链表是否为空，为空就是没有要解阻塞的任务，当前任务就一直运行 if( listLIST_IS_EMPTY( pxDelayedTaskList ) != pdFALSE ) { /* The delayed list is empty. Set xNextTaskUnblockTime * to the maximum possible value so it is extremely * unlikely that the * if( xTickCount >= xNextTaskUnblockTime ) test will pass * next time through. */ xNextTaskUnblockTime = portMAX_DELAY; /*lint !e961 MISRA exception as the casts are only redundant for some ports. */ break; } else { /* The delayed list is not empty, get the value of the * item at the head of the delayed list. This is the time * at which the task at the head of the delayed list must * be removed from the Blocked state. */ //找出阻塞优先级最高的task，查看其阻塞时间是否到了 pxTCB = listGET_OWNER_OF_HEAD_ENTRY( pxDelayedTaskList ); /*lint !e9079 void * is used as this macro is used with timers and co-routines too. Alignment is known to be fine as the type of the pointer stored and retrieved is the same. */ xItemValue = listGET_LIST_ITEM_VALUE( &( pxTCB->xStateListItem ) ); if( xConstTickCount < xItemValue ) { /* It is not time to unblock this item yet, but the * item value is the time at which the task at the head * of the blocked list must be removed from the Blocked * state - so record the item value in * xNextTaskUnblockTime. */ xNextTaskUnblockTime = xItemValue; break; /*lint !e9011 Code structure here is deedmed easier to understand with multiple breaks. */ } else { mtCOVERAGE_TEST_MARKER(); } //时间到了从阻塞链表中移除 /* It is time to remove the item from the Blocked state. */ ( void ) uxListRemove( &( pxTCB->xStateListItem ) ); /* Is the task waiting on an event also? If so remove * it from the event list. */ if( listLIST_ITEM_CONTAINER( &( pxTCB->xEventListItem ) ) != NULL ) { ( void ) uxListRemove( &( pxTCB->xEventListItem ) ); } else { mtCOVERAGE_TEST_MARKER(); } //插入reday链表中 /* Place the unblocked task into the appropriate ready * list. */ prvAddTaskToReadyList( pxTCB ); /* A task being unblocked cannot cause an immediate * context switch if preemption is turned off. */ //如果支持抢占式调度 #if ( configUSE_PREEMPTION == 1 ) { /* Preemption is on, but a context switch should * only be performed if the unblocked task has a * priority that is equal to or higher than the * currently executing task. */ //判断插入的task优先级是否大于当前的task if( pxTCB->uxPriority >= pxCurrentTCB->uxPriority ) { xSwitchRequired = pdTRUE; } else { mtCOVERAGE_TEST_MARKER(); } } #endif /* configUSE_PREEMPTION */ } } } /* Tasks of equal priority to the currently running task will share * processing time (time slice) if preemption is on, and the application * writer has not explicitly turned time slicing off. */ //是否支持时间片调度 #if ( ( configUSE_PREEMPTION == 1 ) && ( configUSE_TIME_SLICING == 1 ) ) { //同优先级任务不止一个 if( listCURRENT_LIST_LENGTH( &( pxReadyTasksLists[ pxCurrentTCB->uxPriority ] ) ) > ( UBaseType_t ) 1 ) { xSwitchRequired = pdTRUE; } else { mtCOVERAGE_TEST_MARKER(); } } #endif /* ( ( configUSE_PREEMPTION == 1 ) && ( configUSE_TIME_SLICING == 1 ) ) */ #if ( configUSE_TICK_HOOK == 1 ) { /* Guard against the tick hook being called when the pended tick * count is being unwound (when the scheduler is being unlocked). */ if( xPendedTicks == ( TickType_t ) 0 ) { vApplicationTickHook(); } else { mtCOVERAGE_TEST_MARKER(); } } #endif /* configUSE_TICK_HOOK */ #if ( configUSE_PREEMPTION == 1 ) { //是否有挂起请求 if( xYieldPending != pdFALSE ) { xSwitchRequired = pdTRUE; } else { mtCOVERAGE_TEST_MARKER(); } } #endif /* configUSE_PREEMPTION */ } else { ++xPendedTicks; /* The tick hook gets called at regular intervals, even if the * scheduler is locked. */ #if ( configUSE_TICK_HOOK == 1 ) { vApplicationTickHook(); } #endif } return xSwitchRequired; }
最后会判断返回的标志位，来确定是否进行调度，如果是pdTRUE则会启动pendSV中断在此中断里切换上下文调度到新的task。
中断里调用系统API导致更高优先级的task唤醒，如果不显示的调用调度的函数，则唤醒的任务不会在中断退出后立马调度，它会等到下一个SystemTick中断来时才切换task，这样会造成中断的响应延时，在某些实时性要求高的地方是不能满足需求的。所以可以在中断里直接在条件满足时，先切换task，这样在中断退出时就会直接返回到高优先级的task了。见如下例子：
void ISR(void)
{
BaseType_t xHigherPriorityTaskWoken = pdFALSE, tmpHPT=pdFALSE;
xQueueSendFromISR(rx_handle, &buffer, tmpHPT);
xHigherPriorityTaskWoken = xHigherPriorityTaskWoken || tmpHPT;
/* If lHigherPriorityTaskWoken is now equal to pdTRUE, then a context
switch should be performed before the interrupt exists. That ensures the
unblocked (higher priority) task is returned to immediately. */
portEND_SWITCHING_ISR( xHigherPriorityTaskWoken );
}（如有描述不对烦请指正谢谢!）