時(shí)鐘概念

時(shí)間是非常重要的概念，我們整個(gè)學(xué)生階段有個(gè)東西很重要,就是校園鈴聲. 它控制著上課,下課,吃飯,睡覺(jué)的節(jié)奏.沒(méi)有它學(xué)校的管理就亂套了,老師拖課想拖多久就多久,那可不行,下課鈴聲一響就是在告訴老師時(shí)間到了,該停止了讓學(xué)生HAPPY去了.

操作系統(tǒng)也一樣，需要通過(guò)時(shí)間來(lái)規(guī)范其任務(wù)的執(zhí)行，操作系統(tǒng)中最小的時(shí)間單位是時(shí)鐘節(jié)拍 (OS Tick)。任何操作系統(tǒng)都需要提供一個(gè)時(shí)鐘節(jié)拍，以供系統(tǒng)處理所有和時(shí)間有關(guān)的事件，如線程的延時(shí)、線程的時(shí)間片輪轉(zhuǎn)調(diào)度以及定時(shí)器超時(shí)等。時(shí)鐘節(jié)拍是特定的周期性中斷，這個(gè)中斷可以看做是系統(tǒng)心跳，中斷之間的時(shí)間間隔取決于不同的應(yīng)用，一般是 1ms–100ms，時(shí)鐘節(jié)拍率越快，系統(tǒng)的實(shí)時(shí)響應(yīng)越快，但是系統(tǒng)的額外開(kāi)銷就越大，從系統(tǒng)啟動(dòng)開(kāi)始計(jì)數(shù)的時(shí)鐘節(jié)拍數(shù)稱為系統(tǒng)時(shí)間。

在鴻蒙內(nèi)核中，時(shí)鐘節(jié)拍的長(zhǎng)度可以根據(jù) LOSCFG_BASE_CORE_TICK_PER_SECOND 的定義來(lái)調(diào)整，等于 1/LOSCFG_BASE_CORE_TICK_PER_SECOND 秒。

時(shí)鐘節(jié)拍的實(shí)現(xiàn)方式

時(shí)鐘節(jié)拍由配置為中斷觸發(fā)模式的硬件定時(shí)器產(chǎn)生，當(dāng)中斷到來(lái)時(shí)，將調(diào)用一次：void OsTickHandler(void)，通知操作系統(tǒng)已經(jīng)過(guò)去一個(gè)系統(tǒng)時(shí)鐘；不同硬件定時(shí)器中斷實(shí)現(xiàn)都不同，

/**
 * @ingroup los_config
 * Number of Ticks in one second
 */
#ifndef LOSCFG_BASE_CORE_TICK_PER_SECOND
#define LOSCFG_BASE_CORE_TICK_PER_SECOND 100 //默認(rèn)每秒100次觸發(fā)，當(dāng)然這是可以改的
#endif

每秒100個(gè)tick，時(shí)間單位為10毫秒, 即每秒調(diào)用時(shí)鐘中斷處理程序100次.

/*
 * Description : Tick interruption handler
 *///節(jié)拍中斷處理函數(shù) ,鴻蒙默認(rèn)10ms觸發(fā)一次
LITE_OS_SEC_TEXT VOID OsTickHandler(VOID)
{
    //...
    OsTimesliceCheck();//進(jìn)程和任務(wù)的時(shí)間片檢查
    OsTaskScan(); /* task timeout scan *///任務(wù)掃描
#if (LOSCFG_BASE_CORE_SWTMR == YES)
    OsSwtmrScan();//定時(shí)器掃描,看是否有超時(shí)的定時(shí)器
#endif
}

它主要干了三件事情

第一:檢查當(dāng)前任務(wù)的時(shí)間片,任務(wù)執(zhí)行一次分配多少時(shí)間呢？答案是2個(gè)時(shí)間片，即 20ms.

#ifndef LOSCFG_BASE_CORE_TIMESLICE_TIMEOUT
#define LOSCFG_BASE_CORE_TIMESLICE_TIMEOUT 2 //2個(gè)時(shí)間片,20ms
#endif
//檢查進(jìn)程和任務(wù)的時(shí)間片,如果沒(méi)有時(shí)間片了直接調(diào)度
LITE_OS_SEC_TEXT VOID OsTimesliceCheck(VOID)
{
    LosTaskCB *runTask = NULL;
    LosProcessCB *runProcess = OsCurrProcessGet();//獲取當(dāng)前進(jìn)程
    if (runProcess->policy != LOS_SCHED_RR) {//進(jìn)程調(diào)度算法是否是搶占式
        goto SCHED_TASK;//進(jìn)程不是搶占式調(diào)度直接去檢查任務(wù)的時(shí)間片
    }

    if (runProcess->timeSlice != 0) {//進(jìn)程還有時(shí)間片嗎?
        runProcess->timeSlice--;//進(jìn)程時(shí)間片減少一次
        if (runProcess->timeSlice == 0) {//沒(méi)有時(shí)間片了
            LOS_Schedule();//進(jìn)程時(shí)間片用完,發(fā)起調(diào)度
        }
    }

SCHED_TASK:
    runTask = OsCurrTaskGet();//獲取當(dāng)前任務(wù)
    if (runTask->policy != LOS_SCHED_RR) {//任務(wù)調(diào)度算法是否是搶占式
        return;//任務(wù)不是搶占式調(diào)度直接結(jié)束檢查
    }

    if (runTask->timeSlice != 0) {//任務(wù)還有時(shí)間片嗎?
        runTask->timeSlice--;//任務(wù)時(shí)間片也減少一次
        if (runTask->timeSlice == 0) {//沒(méi)有時(shí)間片了
            LOS_Schedule();//任務(wù)時(shí)間片用完,發(fā)起調(diào)度
        }
    }
}

第二:掃描任務(wù),主要是檢查被阻塞的任務(wù)是否可以被重新調(diào)度

LITE_OS_SEC_TEXT VOID OsTaskScan(VOID)
{
    SortLinkList *sortList = NULL;
    LosTaskCB *taskCB = NULL;
    BOOL needSchedule = FALSE;
    UINT16 tempStatus;
    LOS_DL_LIST *listObject = NULL;
    SortLinkAttribute *taskSortLink = NULL;

    taskSortLink = &OsPercpuGet()->taskSortLink;//獲取任務(wù)的排序鏈表
    taskSortLink->cursor = (taskSortLink->cursor + 1) & OS_TSK_SORTLINK_MASK;
    listObject = taskSortLink->sortLink + taskSortLink->cursor;//只處理這個(gè)游標(biāo)上的鏈表,因?yàn)橄到y(tǒng)對(duì)超時(shí)任務(wù)都已經(jīng)規(guī)鏈表了.
 //當(dāng)任務(wù)因超時(shí)而掛起時(shí)，任務(wù)塊處于超時(shí)排序鏈接上,（每個(gè)cpu）和ipc（互斥鎖、掃描電鏡等）的塊同時(shí)被喚醒
    /*不管是超時(shí)還是相應(yīng)的ipc，它都在等待?，F(xiàn)在使用synchronize sortlink precedure，因此整個(gè)任務(wù)掃描需要保護(hù)，防止另一個(gè)核心同時(shí)刪除sortlink。
     * When task is pended with timeout, the task block is on the timeout sortlink
     * (per cpu) and ipc(mutex,sem and etc.)'s block at the same time, it can be waken
     * up by either timeout or corresponding ipc it's waiting.
     *
     * Now synchronize sortlink preocedure is used, therefore the whole task scan needs
     * to be protected, preventing another core from doing sortlink deletion at same time.
     */
    LOS_SpinLock(&g_taskSpin);

    if (LOS_ListEmpty(listObject)) {
        LOS_SpinUnlock(&g_taskSpin);
        return;
    }
    sortList = LOS_DL_LIST_ENTRY(listObject->pstNext, SortLinkList, sortLinkNode);//拿本次Tick對(duì)應(yīng)鏈表的SortLinkList的第一個(gè)節(jié)點(diǎn)sortLinkNode
    ROLLNUM_DEC(sortList->idxRollNum);//滾動(dòng)數(shù)--

    while (ROLLNUM(sortList->idxRollNum) == 0) {//找到時(shí)間到了節(jié)點(diǎn),注意這些節(jié)點(diǎn)都是由定時(shí)器產(chǎn)生的,
        LOS_ListDelete(&sortList->sortLinkNode);
        taskCB = LOS_DL_LIST_ENTRY(sortList, LosTaskCB, sortList);//拿任務(wù),這里的任務(wù)都是超時(shí)任務(wù)
        taskCB->taskStatus &= ~OS_TASK_STATUS_PEND_TIME;
        tempStatus = taskCB->taskStatus;
        if (tempStatus & OS_TASK_STATUS_PEND) {
            taskCB->taskStatus &= ~OS_TASK_STATUS_PEND;
#if (LOSCFG_KERNEL_LITEIPC == YES)
            taskCB->ipcStatus &= ~IPC_THREAD_STATUS_PEND;
#endif
            taskCB->taskStatus |= OS_TASK_STATUS_TIMEOUT;
            LOS_ListDelete(&taskCB->pendList);
            taskCB->taskSem = NULL;
            taskCB->taskMux = NULL;
        } else {
            taskCB->taskStatus &= ~OS_TASK_STATUS_DELAY;
        }

        if (!(tempStatus & OS_TASK_STATUS_SUSPEND)) {
            OS_TASK_SCHED_QUEUE_ENQUEUE(taskCB, OS_PROCESS_STATUS_PEND);
            needSchedule = TRUE;
        }

        if (LOS_ListEmpty(listObject)) {
            break;
        }

        sortList = LOS_DL_LIST_ENTRY(listObject->pstNext, SortLinkList, sortLinkNode);
    }

    LOS_SpinUnlock(&g_taskSpin);

    if (needSchedule != FALSE) {//需要調(diào)度
        LOS_MpSchedule(OS_MP_CPU_ALL);//核間通訊,給所有CPU發(fā)送調(diào)度信號(hào)
        LOS_Schedule();//開(kāi)始調(diào)度
    }
}

第三:定時(shí)器掃描,看是否有超時(shí)的定時(shí)器

/*
 * Description: Tick interrupt interface module of software timer
 * Return     : LOS_OK on success or error code on failure
 *///OsSwtmrScan 由系統(tǒng)時(shí)鐘中斷處理函數(shù)調(diào)用
LITE_OS_SEC_TEXT VOID OsSwtmrScan(VOID)//掃描定時(shí)器,如果碰到超時(shí)的,就放入超時(shí)隊(duì)列
{
    SortLinkList *sortList = NULL;
    SWTMR_CTRL_S *swtmr = NULL;
    SwtmrHandlerItemPtr swtmrHandler = NULL;
    LOS_DL_LIST *listObject = NULL;
    SortLinkAttribute* swtmrSortLink = &OsPercpuGet()->swtmrSortLink;//拿到當(dāng)前CPU的定時(shí)器鏈表

    swtmrSortLink->cursor = (swtmrSortLink->cursor + 1) & OS_TSK_SORTLINK_MASK;
    listObject = swtmrSortLink->sortLink + swtmrSortLink->cursor;
 //由于swtmr是在特定的sortlink中，所以需要很小心的處理它,但其他CPU Core仍然有機(jī)會(huì)處理它，比如停止計(jì)時(shí)器
    /*
     * it needs to be carefully coped with, since the swtmr is in specific sortlink
     * while other cores still has the chance to process it, like stop the timer.
     */
    LOS_SpinLock(&g_swtmrSpin);

    if (LOS_ListEmpty(listObject)) {
        LOS_SpinUnlock(&g_swtmrSpin);
        return;
    }
    sortList = LOS_DL_LIST_ENTRY(listObject->pstNext, SortLinkList, sortLinkNode);
    ROLLNUM_DEC(sortList->idxRollNum);

    while (ROLLNUM(sortList->idxRollNum) == 0) {
        sortList = LOS_DL_LIST_ENTRY(listObject->pstNext, SortLinkList, sortLinkNode);
        LOS_ListDelete(&sortList->sortLinkNode);
        swtmr = LOS_DL_LIST_ENTRY(sortList, SWTMR_CTRL_S, stSortList);

        swtmrHandler = (SwtmrHandlerItemPtr)LOS_MemboxAlloc(g_swtmrHandlerPool);//取出一個(gè)可用的軟時(shí)鐘處理項(xiàng)
        if (swtmrHandler != NULL) {
            swtmrHandler->handler = swtmr->pfnHandler;
            swtmrHandler->arg = swtmr->uwArg;

            if (LOS_QueueWrite(OsPercpuGet()->swtmrHandlerQueue, swtmrHandler, sizeof(CHAR *), LOS_NO_WAIT)) {
                (VOID)LOS_MemboxFree(g_swtmrHandlerPool, swtmrHandler);
            }
        }

        if (swtmr->ucMode == LOS_SWTMR_MODE_ONCE) {
            OsSwtmrDelete(swtmr);

            if (swtmr->usTimerID < (OS_SWTMR_MAX_TIMERID - LOSCFG_BASE_CORE_SWTMR_LIMIT)) {
                swtmr->usTimerID += LOSCFG_BASE_CORE_SWTMR_LIMIT;
            } else {
                swtmr->usTimerID %= LOSCFG_BASE_CORE_SWTMR_LIMIT;
            }
        } else if (swtmr->ucMode == LOS_SWTMR_MODE_NO_SELFDELETE) {
            swtmr->ucState = OS_SWTMR_STATUS_CREATED;
        } else {
            swtmr->ucOverrun++;
            OsSwtmrStart(swtmr);
        }

        if (LOS_ListEmpty(listObject)) {
            break;
        }

        sortList = LOS_DL_LIST_ENTRY(listObject->pstNext, SortLinkList, sortLinkNode);
    }

    LOS_SpinUnlock(&g_swtmrSpin);
}

最后看調(diào)度算法的實(shí)現(xiàn)

//調(diào)度算法的實(shí)現(xiàn)
VOID OsSchedResched(VOID)
{
    LosTaskCB *runTask = NULL;
    LosTaskCB *newTask = NULL;
    LosProcessCB *runProcess = NULL;
    LosProcessCB *newProcess = NULL;
    LOS_ASSERT(LOS_SpinHeld(&g_taskSpin));//必須持有任務(wù)自旋鎖,自旋鎖是不是進(jìn)程層面去搶鎖,而是CPU各自核之間去爭(zhēng)奪鎖

    if (!OsPreemptableInSched()) {//是否置了重新調(diào)度標(biāo)識(shí)位
        return;
    }
    runTask = OsCurrTaskGet();//獲取當(dāng)前任務(wù)
    newTask = OsGetTopTask();//獲取優(yōu)先級(jí)最最最高的任務(wù)
    /* always be able to get one task */
    LOS_ASSERT(newTask != NULL);//不能沒(méi)有需調(diào)度的任務(wù)
    if (runTask == newTask) {//當(dāng)前任務(wù)就是最高任務(wù),那還調(diào)度個(gè)啥的,直接退出.
        return;
    }
    runTask->taskStatus &= ~OS_TASK_STATUS_RUNNING;//當(dāng)前任務(wù)狀態(tài)位置成不在運(yùn)行狀態(tài)
    newTask->taskStatus |= OS_TASK_STATUS_RUNNING;//最高任務(wù)狀態(tài)位置成在運(yùn)行狀態(tài)
    runProcess = OS_PCB_FROM_PID(runTask->processID);//通過(guò)進(jìn)程ID索引拿到進(jìn)程實(shí)體
    newProcess = OS_PCB_FROM_PID(newTask->processID);//同上
    OsSchedSwitchProcess(runProcess, newProcess);//切換進(jìn)程,里面主要涉及進(jìn)程空間的切換,也就是MMU的上下文切換.
#if (LOSCFG_KERNEL_SMP == YES)//CPU多核的情況
    /* mask new running task's owner processor */
    runTask->currCpu = OS_TASK_INVALID_CPUID;//當(dāng)前任務(wù)不占用CPU
    newTask->currCpu = ArchCurrCpuid();//讓新任務(wù)占用CPU
#endif
    (VOID)OsTaskSwitchCheck(runTask, newTask);//切換task的檢查
#if (LOSCFG_KERNEL_SCHED_STATISTICS == YES)
    OsSchedStatistics(runTask, newTask);
#endif
    if ((newTask->timeSlice == 0) && (newTask->policy == LOS_SCHED_RR)) {//沒(méi)有時(shí)間片且是搶占式調(diào)度的方式,注意 非搶占式都不需要時(shí)間片的.
        newTask->timeSlice = LOSCFG_BASE_CORE_TIMESLICE_TIMEOUT;//給新任務(wù)時(shí)間片 默認(rèn) 20ms
    }
    OsCurrTaskSet((VOID*)newTask);//設(shè)置新的task為CPU核的當(dāng)前任務(wù)
    if (OsProcessIsUserMode(newProcess)) {//用戶模式下會(huì)怎么樣?
        OsCurrUserTaskSet(newTask->userArea);//設(shè)置task?？臻g
    }
    /* do the task context switch */
    OsTaskSchedule(newTask, runTask);//切換任務(wù)上下文,注意OsTaskSchedule是一個(gè)匯編函數(shù) 見(jiàn)于 los_dispatch.s
}

編輯：hfy