Programming in Real-Time OS (uc/os-ii)

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1 Programming in Real-Time OS (uc/os-ii) 경희대학교컴퓨터공학과 조진성 Embedded Software

2 Taxonomy [2] Library only Non-multitasking AVR studio Real-time OS (Embedded OS) Preemptive multitasking Micro-kernel approach VxWorks uc/os-ii Full-function OS Preemptive multitasking Embedded Linux WinCE ios Embedded Software 2

3 Taxonomy [1] Non-multitasking Single task with polling I/O Single task with ISR Foreground/background Cooperative multitasking Non-preemptive kernel + multiple tasks Preemptive multitasking Preemptive kernel + multiple tasks Embedded Software 3

4 Multitasking Multitasking (Multiprocessing) An application is sub-divided into logical tasks in a system. Each task processes its own function and can be implemented with ease. OS handles multitasking with scheduling and context-switching. Synchronization issues may occur and should be handled with semaphores and mutexes. Ex) ADSL Router PPP(point-to-point) task IP(Internet Protocol) task UDP(User Datagram Protocol) task TCP(Transmission Control Protocol) task RIP(Routing Information Protocol) task ATM(Asynchronous Transfer Mode) task Embedded Software 4

5 Multitasking (Cont d) Task Smallest execution unit Similar to `Thread Has its priority, state, stack, context (CPU register sets) Task state diagram terminate WATING get something wait something(events) start context switch interrupt DORMANT READY RUNNING ISR terminate context switch return Embedded Software 5

6 Multitasking (Cont d) Task priority Task-specific parameter which indicates its importance The more importance, the higher value (0 is the highest or the lowest) Static priority Not changed while running Given in compile-time Rate-Monotonic Scheduling (RMS) Dynamic priority Can be changed and given in run-time Earliest Deadline First (EDF) Embedded Software 6

7 Multitasking (Cont d) Multitasking Embedded Software 7

8 Multitasking (Cont d) Context switch TASK1 TASK2 TASK3 stack stack stack.. push registers Image into the each TCB context switch pop registers image from the TCB into CPU registers TCB TCB TCB Priority Status Priority Status Priority Status 1 Stack Base Address CPU register 4 2 Stack Base Address CPU register 3 Stack Base Address CPU register context MEMORY CPU CPU register Embedded Software 8

9 Multitasking (Cont d) Scheduler Selects which task should be executed among tasks in ready list In general, Priority & RR scheduling in embedded systems Dispatcher Allocates CPU to the scheduled task Ready list High priority Priority based scheduler TASK A 100 TASK B 80 TASK C 50 TASK D 30 TASK E 5 Scheduler TASK A win!! Embedded Software 9

10 Cooperative Multitasking Multitasking (multiple tasks) but non-preemptive kernel Context switch only on explicit system call, e.g. yield() So, programmers should control the flow of tasks, i.e. context switching of tasks. After an interrupt, CPU returns to the same task before the interrupt Example of non-preemptive kernel Pros Cons Original Windows 3.X, Macintosh Low interrupt latency Relatively small synchronization problem Large latency of high-priority tasks System fault due to a malfunctioning task (e.g. indefinite loop) Embedded Software 10

11 Cooperative Multitasking (Cont d) Non-preemptive kernel (non-preemptive scheduling) Low priority task (1) (2) ISR (3) (5) (4) (6) High priority task time (7) Embedded Software 11

12 Preemptive Multitasking Preemptive kernel Fast response of higher priority tasks The highest priority ready task takes control of CPU. On ISR return, OS schedules the highest priority ready task. Low priority task (1) (2) ISR (3) High priority task (4) (5) time (6) (7) Embedded Software 12

13 Preemptive Multitasking Preemptive kernel Current commercial OSs take this approach Embedded Software 13

14 Taxonomy [2] Library only Non-multitasking AVR studio Real-time OS (Embedded OS) Preemptive multitasking Micro-kernel approach VxWorks uc/os-ii Full-function OS Preemptive multitasking Embedded Linux WinCE ios Embedded Software 14

15 uc/os-ii Real-Time OS implemented by Jean J. Labrosse in 1992 Free open source for education Easy to study source codes Used on a lot of commercial products Avionics, medical devices, mobile handsets, consumer electronics, etc. License required for commercial purpose Official site Embedded Software 15

16 uc/os-ii Features Portable 8Bit, 16Bit, 32Bit, 64Bit CPU & DSP Reliable Approved by FAA(Federal Aviation Administration) in July, 2000 ROMable Very small footprint Preemptive real-time kernel Fast response The highest priority task preempts other tasks Multitasking Up to 64 tasks 0 is the highest priority Embedded Software 16 16

17 uc/os-ii File Structure OS_MUTEX.C OS_FLAG.C Embedded Software 17

18 Task Multitasking (Revisited) An application is sub-divided into logical tasks in a system. Each task processes its own function and can be implemented with ease. OS handles multitasking with scheduling and context-switching. Synchronization issues may occur and should be handled with semaphores and mutexes. Ex) ADSL Router PPP(point-to-point) task IP(Internet Protocol) task UDP(User Datagram Protocol) task TCP(Transmission Control Protocol) task RIP(Routing Information Protocol) task ATM(Asynchronous Transfer Mode) task Embedded Software 18

19 Task in uc/os-ii Task ID & priority Max. 64 tasks Task ID = Task priority Static priority Task 0 is the highest priority task The lowest priority task (e.g. task 63) is reserved as the idle task The lowest-1 priority task (e.g. task 62) is reserved as the statistics task Scheduling Priority scheduling algorithm Very fast: O(1) Embedded Software 19

20 Task in uc/os-ii (Cont d) Task state diagram in uc/os-ii Waiting OSFlagPend () OSMbocPend () OSTakDel () OSTask Creat () OSTask CreatExt () OSFlagPost () OSMboxPost () OSMboxPostOpt () OSMutexPost () OSQPost () OSQPostFront () OSQPostOpy () OSSemPost OSTaskResume OSTimeDlyResume OSTimetick () OSSStart () OSInt Exit () OS_TASK_SW () Dormant Ready Running OSMutexPend () OSQPend () OSSemPend OSTaskSuspend () OSTimeDLy () OSTimeDLyHMSM () Interrupt ISR Running OSTakDel () Task Preemption OSInt Exit() OSTakDel () Embedded Software 20

21 uc/os-ii System Call Task INT8U OSTaskCreate(void (*task)(void *pd), void *pdata, OS_STK *ptos, INT8U prio); INT8U OSTaskCreateExt(void (*task)(void *pd), void *pdata, OS_STK *ptos, INT8U prio, INT16U id, OS_STK *pbos, INT32U stk_size, void *pext, INT16U opt); INT8U OSTaskStkChk(INT8U prio, OS_STK_DATA *pdata); INT8U OSTaskDel(INT8U prio); INT8U OSTaskDelReq(INT8U prio); INT8U OSTaskChangePrio(INT8U oldprio, INT8U newprio); INT8U OSTaskSuspend(INT8U prio); INT8U OSTaskResume(INT8U prio); INT8U OSTaskQuery(INT8U prio, OS_TCB *pdata); Embedded Software 21

22 uc/os-ii System Call Time Void INT8U OSTimeDly(INT16U ticks); OSTimeDlyHMSM(INT8U hours, INT8U minutes, INT8U seconds, INT16U milli); INT8U INT32U Void OSTimeDlyResume(INT8U prio); OSTimeGet(void); OSTimeSet(INT32U ticks); Embedded Software 22

23 Synchronization Tasks cooperate in multitasking systems To coordinate their execution To share resources, and to access shared data structures Definitions Resource Entities which process/thread/task may utilize Physical entities such as CPU, memory, I/O devices (display, keyboard, printer, etc.) Logical entities such as variables, arrays, data structures (queue, stack, tree, etc.) Shared Resource Resource which one or more process/thread/task share Process/thread/task needs exclusive access on shared resources to avoid data corruption through synchronization mechanisms. Embedded Software 23

24 Synchronization (Cont d) Definitions Race condition A situation where several tasks access and manipulate shared resources concurrently The result is non-deterministic and depends on timing Critical section Code segment in which shared resources are accessed Mutually exclusive access should be guaranteed in critical sections (mutual exclusion, mutex) Critical section problem or Synchronization problem Shared resources may be corrupted in multitasking systems Embedded Software 24

25 Synchronization Mechanisms Primitive mechanisms for OS Software algorithms Disabling interrupts Spinlocks Hardware atomic instruction Busy waiting High-level mechanisms for application task/process/thread Semaphores Basic, easy to get the hang of, hard to program with Binary semaphore = mutex ( lock) Counting semaphore Monitors High-level, requires language support, implicit operations Easy to program with: Java synchronized Embedded Software 25

26 Synchronization Examples To coordinate executions Execute B in P j after A executed in P i P i A signal(sem) synchronized P j wait(sem) B To share resources Protect critical section C between P i and P j P i wait(sem) wait(sem) C C signal(sem) signal(sem) synchronized Embedded Software 26 P j

27 uc/os-ii System Call Semaphore OS_EVENT INT16U Void INT8U INT8U OS_EVENT *OSSemCreate(INT16U cnt); OSSemAccept(OS_EVENT *prevent); OSSemPend(OS_EVENT *prevent, INT16U timeout, INT8U *err); OSSemPost(OS_EVENT *prevent); OSSemQuery(OS_EVENT *prevent, OS_SEM_DATA *pdata); OSSemDel(OS_EVENT *prevent, INT8U opt, INT8U *err); Embedded Software 27

28 uc/os-ii System Call Mutex (Mutually Exclusive Lock) OS_EVENT *OSMutexCreate(INT8U prio, INT8U *err); Void OSMutexPend(OS_EVENT *prevent, INT16U timeout, INT8U *err); INT8U OSMutexAccept(OS_EVENT *prevent, INT8U *err); INT8U OSMutexPost(OS_EVENT *prevent); INT8U OSMutexQuery(OS_EVENT *prevent, OS_MUTEX_DATA *pdata); OS_EVENT OSMutexDel(OS_EVENT *prevent, INT8U opt, INT8U *err); Embedded Software 28

29 Deadlock Definition A set of blocked tasks/processes each holding a resource and waiting to acquire a resource held by another task/process in the set Example Semaphores A and B, initialized to 1 P i wait (A); wait (B); P j wait (B) wait (A) deadlock Embedded Software 29

30 Handling Deadlocks Deadlock prevention Restrain how requests are made Ensure that at least one necessary condition cannot hold Deadlock avoidance Require additional information about how resources are to be requested Decide to approve or disapprove requests on the fly Deadlock detection and recovery Allow the system to enter a deadlock state and then recover Just ignore the problem altogether! Embedded Software 30

31 Deadlock Prevention Deadlock can arise if four conditions hold simultaneously Mutual exclusion: only one process at a time can use a resource Hold and wait: a process holding at least one resource is waiting to acquire additional resources held by other processes No preemption: a resource can be released only voluntarily by the process holding it, after that process has completed its task Circular wait: there exists a set {P 0, P 1,, P n, P 0 } of waiting processes such that P 0 is waiting for a resource that is held by P 1, P 1 is waiting for a resource that is held by P 2,, P n 1 is waiting for a resource that is held by P n, and P n is waiting for a resource that is held by P 0 Embedded Software 31

32 Inter-Task Communication Two types of IPC Message passing P i SendMsg(data) P j RecvMsg(data) synchronized (by kernel) Shared memory P i write msg. on data wait(sem) signal(sem) read msg. from data synchronized (by application via semaphore) P j Embedded Software 32

33 uc/os-ii System Call Message Mailbox OS_EVENT Void Void INT8U INT8U INT8U OS_EVENT *OSMboxCreate(void *msg); *OSMboxPend(OS_EVENT *pevent, INT16U timeout, INT8U *err); *OSMboxAccept(OS_EVENT *pevent); OSMboxPost(OS_EVENT *pevent, void *msg); OSMboxPostOpt(OS_EVENT *pevent, void *msg, INT8U opt); OSMboxQuery(OS_EVENT *pevent, OS_MBOX_DATA *pdata); *OSMboxDel(OS_EVENT *pevent, INT8U opt, INT8U *err); Embedded Software 33

34 uc/os-ii System Call Message Queue OS_EVENT void INT8U *err); void INT8U INT8U opt); INT8U INT8U INT8U OS_EVENT *err); *OSQCreate(void **start, INT16U size); *OSQPend(OS_EVENT *pevent, INT16U timeout, *OSQAccept(OS_EVENT *pevent); OSQPost(OS_EVENT *pevent, void *msg); OSQPostOpt(OS_EVENT *pevent, void *msg, INT8U OSQPostFront(OS_EVENT *pevent, void *msg); OSQQuery(OS_EVENT *pevent, OS_Q_DATA *pdata); OSQFlush(OS_EVENT *pevent); *OSQDel(OS_EVENT *pevent, INT8U opt, INT8U Embedded Software 34

µc/os-ii, The Real-Time Kernel

µc/os-ii, The Real-Time Kernel Semaphores (OS_SEM.C) INT16U OSSemAccept(OS_EVENT *pevent); OS_EVENT *OSSemCreate(INT16U cnt); OS_EVENT *OSSemDel(OS_EVENT *pevent, INT8U opt, INT8U *err); void OSSemPend(OS_EVENT *pevent, INT16U timeout,