c - 增量队列 - 嵌入式调度程序

Question

我正在尝试在小型 Cortex-M3 上实现一个简单的增量队列，以便在将来安排一些任务。我 build 了一些东西，但我不认为它很优雅(我不经常写代码)。它也似乎有点片状由于不正确地使用了 volatile 说明符。

#include "deltaqueue.h"
#include "debug.h"
#include "interrupt.h"

//*****************************************************************************
//
// Define NULL, if not already defined.
//
//*****************************************************************************
#ifndef NULL
#define NULL                    ((void *)0)
#endif

//! Delta queue structure encapsulating a complete process entry into the queue
typedef struct dq{
    struct dq * psPrev;             //Address of previous queue entry
    struct dq * psNext;             //Address of next queue entry
    unsigned long ulDelta;          //Delta ticks (ticks relative to the next/previous process)          
    tProcessObject sProcess;        //Process to be executed
} tDeltaQueueObject;


//! Contains the maximum number of processes in the queue at any one time (health indicator).
static unsigned long g_ulMaximumProcesses=0;
//! Contains the current number of processes in the queue (health indicator).
static unsigned long g_ulCurrentProcesses=0;
//! Contains the current number of executed processes (health indicator).
static unsigned long g_ulExecutedProcesses=0;
//! Contains the total number of processes scheduled since initialized (health indicator).
static unsigned long g_ulTotalProcesses=0;

//! Contains the accumulated tick count.
static volatile unsigned long g_ulSchedulerTickCount;
//! Simple counter used to generate process IDs.
static unsigned long g_ulPID=1;
//! Pointer to the first sleeping process.
static tDeltaQueueObject * volatile psSleeping;
//! Pointer to the processes ready for execution.
static tDeltaQueueObject * psReady;
//! Pointer to an available slot in the queue.
static tDeltaQueueObject * psAvailable;
//! Queue of processes.
static tDeltaQueueObject sDeltaQueue[QUEUE_MAX];


unsigned long SchedulerElapsedTicksCalc(unsigned long, unsigned long);
unsigned long GetProcessID(void);
tDeltaQueueObject * FreeEntry(void);

//****************************************************************************
//
//! Initializes the scheduler.
//!
//! This function resets the queue pointers.
//!
//! \return None.
//
//****************************************************************************
void SchedulerInit(void){

    //Initialize queue pointers
    psAvailable=&sDeltaQueue[0];
    psSleeping=psAvailable;
    psReady=psAvailable;

}


//****************************************************************************
//
//! Inserts supplied process into the queue.
//!
//! This function iterates the queue starting the sleep pointer and looks for
//! the insert location based on the supplied delay. As this is a delta queue,
//! the delay is decremented by the sleeping process' delta until a the delay
//! is less than that of the sleeping process. This then becomes the insertion
//! point. If there are no sleeping processes then the process is inserted 
//! after the last ready process. If there are no sleeping processes or ready
//! processes then it's inserted and becomes the sole sleeping process.
//!
//! \param pf is the process to execute after the supplied delay.
//! \param ulDelay is the number of ticks to wait before executing the supplied
//! process.
//!
//! \return Process ID of inserted process or zero if unable to insert.
//
//****************************************************************************
unsigned long SchedulerInsert(void (*pf)(void),unsigned long ulDelay){

    static unsigned long ulBeginCount; 
    static unsigned long ulEndCount;   

    ASSERT(psSleeping);
    ASSERT(psAvailable);    

    //Pick off current systick count to calculate execution time
    ulBeginCount=(*((volatile unsigned long *)(NVIC_ST_CURRENT)));

    //CRITICAL SECTION BEGIN
    IntMasterDisable();    

    //Begin iterating at the current sleep pointer
    tDeltaQueueObject * p=(void *)psSleeping;
    tDeltaQueueObject * q;

    //Adjust health indicators
    g_ulTotalProcesses++;
    if(++g_ulCurrentProcesses>g_ulMaximumProcesses)
        g_ulMaximumProcesses=g_ulCurrentProcesses;    

    //Loop through each sleeping process starting at the current
    //sleep pointer and ending when the next pointer of any is 
    //equivalent to the available pointer
    while(p!=psAvailable){

        //If the delay is greater than the current queue item delay,
        //compute the delta for the inserted process and move on
        if(p->ulDelta <= ulDelay){
            ulDelay-=p->ulDelta;   
        }
        //Otherwise, this is the point to insert the new process
        else{   

            //Insert the new process before the current queue entry
            q=FreeEntry();
            ASSERT(q);  //TODO: Exit gracefully when no room
            q->psNext=p;
            q->psPrev=p->psPrev;      

            //Adjust previous and next pointers on each side of the new process
            p->psPrev->psNext=q;
            p->psPrev=q;

            //Set deltas for inserted queue entry and the supplied queue entry 
            p->ulDelta-=ulDelay;
            q->ulDelta=ulDelay;

            //Set the function pointer for the new process and obtain a unique
            //process ID
            q->sProcess.pf=pf;
            q->sProcess.ulPID=GetProcessID();             

            //Adjust the sleep pointer if the insert 
            //happens before it
            if(p==psSleeping)
                psSleeping=q;

            //CRITICAL SECTION END
            IntMasterEnable();       

            //Pick off current systick count to calculate execution time
            ulEndCount=(*((volatile unsigned long *)(NVIC_ST_CURRENT)));

            return q->sProcess.ulPID;
        }

        //Move to next
        p=p->psNext;

    }

    //If here, the list is either empty or the delay is larger than the 
    //sum of all the delays in the queue and so it should be appended 
    //to the end of the queue
    psAvailable->ulDelta = ulDelay;
    psAvailable->sProcess.pf=pf;
    psAvailable->sProcess.ulPID=GetProcessID();      
    q=psAvailable;

    //Increment the available pointer 
    psAvailable=FreeEntry();
    ASSERT(psAvailable);
    psAvailable->psPrev=q;
    q->psNext=psAvailable;
    psAvailable->psNext=NULL;

    //CRITICAL SECTION END
    IntMasterEnable();       

    //Pick off current systick count to calculate execution time
    ulEndCount=(*((volatile unsigned long *)(NVIC_ST_CURRENT)));

    return q->sProcess.ulPID; 
}

//****************************************************************************
//
//! Runs any processes which are ready for execution.
//!
//! This function is usually called in the main loop of the application
//! (anywhere NOT within an interrupt handler). It will iterate the queue
//! and execute any processes which are not sleeping (delta is zero).
//!
//! \return None.
//
//****************************************************************************
void SchedulerRunTask(void){

    tDeltaQueueObject * p;

    ASSERT(psReady);

    //Run tasks until we bump up against the sleeping tasks
    while(psReady!=psSleeping){

        //Adjust health indicators
        g_ulCurrentProcesses--;
        g_ulExecutedProcesses++;

    //Execute task      
    if(psReady->sProcess.pf)
            (psReady->sProcess.pf)();

        p=psReady->psNext;

    //Clear task
    psReady->sProcess.pf=NULL;
        psReady->sProcess.ulPID=0;
    psReady->psNext=NULL;
        psReady->psPrev=NULL;
        psReady->ulDelta=0;

        //Increment ready pointer
    psReady=p;

    }   
}

//****************************************************************************
//
//! Manages sleeping processes in the queue.
//!
//! This function is to be called by the system tick interrupt (at a given 
//! interval). When called, the sleeping tasks' delta is decremented and the
//! sleep pointer is adjusted to point at the next sleeping task (if changed). 
//!
//! \return None.
//
//****************************************************************************
void SchedulerTick(void){

    ASSERT(psSleeping);

    //Increment tick counter
    g_ulSchedulerTickCount++;

    //Adjust sleeping task (never roll past zero)
    if(psSleeping->ulDelta)
        psSleeping->ulDelta--;

    //Push the sleep pointer until a non-zero delta.
    //Multiple processes can expire on one tick.
    while(!psSleeping->ulDelta && psSleeping!=psAvailable){
        psSleeping=psSleeping->psNext;
    }

}

//****************************************************************************
//
//! Searches the queue for a free slot.
//!
//! This function iterates the entire queue looking for an open slot.
//!
//! \return Pointer to the next free DeltaQueueObject or 0 if no free space
//! available.
//
//****************************************************************************
tDeltaQueueObject * FreeEntry(){

    unsigned long i;

    //Iterate entire queue
    for(i=0; i<QUEUE_MAX; i++){

        //Look for a free slot by examining the contents
        if(!(sDeltaQueue[i].psNext) && !(sDeltaQueue[i].psPrev) && !(sDeltaQueue[i].sProcess.ulPID) && !(sDeltaQueue[i].ulDelta) && !(sDeltaQueue[i].sProcess.pf))
            return &sDeltaQueue[i];
    }

    //If we are here, there are no free spots in the queue
    ASSERT(1);
    return NULL;

}

//****************************************************************************
//
//! Produces a unique process ID.
//!
//! This function simply returns the next PID available.
//!
//! \todo Keep a list of unexpired PIDs so that it can be guaranteed unique
//! must have before creating remove function
//!
//! \return A unique process ID.
//
//****************************************************************************
unsigned long GetProcessID(void){

    //PID can never be zero, catch this case
    if(!g_ulPID)
        g_ulPID=1;    

    return g_ulPID++;
}

我所拥有的背后的想法是存在一个已填充的静态缓冲区与增量队列对象。每个增量队列对象都有指向前一个/下一个增量队列对象，相对于前一个任务的延迟和一些进程信息(进程 ID 和函数指针)。有3个全局指针、就绪指针、 sleep 指针和可用指针指针。就绪指针指向要执行的任务列表。这sleep 指向任务列表的指针，这些任务......好吧......睡着了但还没有准备好执行。可用指针基本上指向那里的结尾是可用插槽。这些指针只会向前移动。当一个被推与另一个相对，那个“子队列”是空的。例如，当准备指针等于 sleep 指针，没有就绪任务。

所以，一个例子可能是这样的:

最初指针看起来像这样..

Pointers    Slot #    Delta  
RP,SP,AP -> Slot 1    0

一个任务以 50 毫秒的延迟插入，队列现在看起来像...

Pointers    Slot #    Delta  
RP,SP    -> Slot 1    50
AP       -> Slot 2    0

几个滴答声过去，另一个任务被插入，延迟 10 毫秒...

Pointers    Slot #    Delta  
RP,SP    -> Slot 3    10
         -> Slot 1    38
AP       -> Slot 2    0

20 个滴答声过去了，我们有...

Pointers    Slot #    Delta  
RP       -> Slot 3    0
SP       -> Slot 1    18
AP       -> Slot 2    0

SchedulerTick() 由 systick 中断以 1 毫秒的速率调用。SchedulerRun() 从应用程序的主循环中调用(当它没有做任何其他事情)所以我的系统中断很短。SchedulerInsert() 根据需要调用以安排任务。

所以，这就是我使用上面的代码的目的。现在，我的问题...

1) 我将 psSleeping 指定为 volatile 指针，因为它在 SchedulerTick() 中被修改。我确信它是必需的，但我的用法正确吗？指针是声明为 volatile 还是指向的指针声明为 volatile。

2) SchedulerTick() 和 SchedulerRun() 函数非常简单，但 SchedulerInsert() 变得相当困惑。大多数困惑是由于插入的任务可以放在 sleep 指针之前，这意味着 SchedulerTick() 不再专门写入它，所以我必须在这样做时禁用中断.此外，插入中似乎存在一些错误(大概)导致 SchedulerTick() 在 while 循环中停止，因为从未达到 psAvailable。这个错误很少发生......我不能在单步执行时重复它。可能与 volatile 声明有关？

有什么想法吗？

Answer 1

我的建议是您重新考虑您是否真的需要在中断处理程序中进行任何实际的列表处理。

据我所知，您可以通过仅跟踪经过的滴答并使用它们来唤醒您之前在中断之外访问 sleep 尾指针的任何地方的 sleep 任务来获得类似的结果。

例如沿着这些方向的东西:

// Only bumb the tick counter from within interrupts
void SchedulerTick(void) {
    g_ulSchedulerTickCount++;
}

// Use the number of elapsed ticks since the last call wake up processes for execution.
// Returns the first task that's still sleeping
tDeltaQueueObject *SchedulerStillSleeping(void) {
    static unsigned long lastTick;
    unsigned long currentTick = g_ulSchedulerTickCount;
    signed long elapsedTicks = currentTick - lastTick;
    lastTick = currentTick;

    for(; psSleeping != psAvailable; psSleeping = psSleeping->psNext) {
        if(psSleeping->ulDelta > elapsedTicks)
            psSleeping->ulDelta -= elapsedTicks;
            break;
        }
        elapsedTicks -= psSleeping->ulDelta;
        psSleeping->ulDelta = 0;
    }
    return psSleeping;
}

// Reassess the set of sleeping processes by calling the StillSleeping function anywhere
// you would previously have polled the list head
void SchedulerRunTask(void) {
    while(psReady != SchedulerStillSleeping()) {
        .
        .
        .
    }
}

unsigned long SchedulerInsert(...) {
    .
    .
    .
    tDeltaQueueObject *p = SchedulerStillSleeping();
    while(p != psAvailable) {
        .
        .
        .
    }
}