SOS: SOS Operating System

1. SOS:SOS Operating System Updated by Andreas Savvides Nov 2, 2004 Chih-Chieh Han, Ram Rengaswamy, Roy Shea Eddie Kohler, and Mani Srivastava Based on initial tutorial by Sung Park and Andreas Savvides Electrical Engineering Departments University of California, Los Angeles September 17, 2004

2. Recap: A Program without OS int main(void) { Init_All(); for (;;) { IO_Scan(); // bar-code scanner IO_ProcessOutputs(); KBD_Scan(); // keyboard PRN_Print(); // printer LCD_Update(); // display RS232_Receive(); // serial port RS232_Send(); TIMER_Process(); // timer } // should never ever get here }

3. Recap:Task Requirements • Some tasks are periodic • Blink the LCD cursor once every second • Tasks may not need to run at the same frequency • Some tasks may be event triggered • RS-232 receive only needs to execute of there is a character received • PRN_Print() only needs to execute when a receipt is required • Need a method to communicate between tasks

4. Event Driven Tasks & Task Communication Task 2 Task 1 Task3_PutEvent() Task3_PutEvent() Task3_GetEvent() Task 3 Task Event Queue

5. Task Execution Frequency • Different tasks may need to execute at different frequencies OR • Task execution frequency may change during the lifetime of an application • Example: • You may not need to refresh the LCD every 1ms if the information to be displayed does not change • You may not want to delay execution of other tasks too long

6. Software Timers • Timers are needed to provide multitasking ability to your software • Need to schedule a large number of periodic or single shot events • Blinking cursor • Flashing leds • Be able to put your device to sleep after some idle time • Timers in the implementation of communication protocols • You could use hardware timers for some tasks BUT • There is a very limited number of hardware timers compared to the needs of an application

7. Designing a Software Timer Tasks deposit their events in a queue Application handler functions Handler for the hardware 10ms timer check Delta Queue For expired timers Software timer process

8. Using A Software Timer • The timer API function in SOS looks like this int8_t ker_timer_start(sos_pid_t pid, uint8_t tid, uint8_t type, int32_t interval) int8_t ker_timer_stop(uint8_t pid, uint8_t tid) pid – ID of the module(task) that creates the timer pid is unique to the whole OS tid – ID of the timer within tid needs to be unique to a specific module We will return to this in a few slides…

9. SOS: Pseudo-Realtime (soft) • Structure: OS functions and user defined Tasks • Multi-tasking (Event-Driven) • Each tasks is a routine which processes events that are stored in event queues • Supports Inter-task Communication • Implements a messaging model

10. SOS: Pseudo-Realtime (soft) • Instead of hard-realtime, SOS provides soft-realtime guarantee (best-effort) • Tasks cannot be pre-emptied (by other tasks) • Tasks run one after the other • No shared memory protection required • Cooperative task scheduling • Task posts messages to self or other tasks for further execution.

11. Need for Reprogramming • How do you update a deployed system of 100s of nodes? • Changing project needs, surfacing of new applications, repairing bugs needs reprogramming • Different attempts for reprogramming • XNP mechanism on MICA nodes • Image stored on external flash rebooting the nodes • Could use differential patching • Just update the part of the binary image that changed

12. SOS Approach Dynamic Loadable Binary Modules Dynamic Loadable Binary Modules Static SOS Kernel Hardware Abstraction Module Communication Memory Manager

13. SOS Functional Layout

14. SOS features • Message Passing Communication • Virtual Delta Timers • Dynamic Memory Management (heap) • Support Module Insertion • Event driven Sensing Interface • Cross Platform • Memory footprint in Mica2 • FLASH size(Code): 17816 bytes (Total: 128Kbytes) • RAM Size (Memory): 2697 bytes (Total: 4Kbytes) • Includes hardware drivers, radio stack, and heap.

15. Compared to tinyOS • Notion of well-defined tasks • Inter-task communication through the use of message queue • More elaborate scheduling scheme where task has context • Easier to debug • Minimum use of macros • Standard C language => JTAG friendly!

16. SOS Modules • Each module is uniquely identified by its ID or pid. • Module has private state. • Modules can be loaded post-deployment. • Each module is represented by a message handler and has following prototype. int8_t handler(void *private_state, Message *msg) • Each module is a finite state machine that changes state based on messages. • Return value follows errno. • SOS_OK for success. -EINVAL, -ENOMEM, etc for failure.

17. Inter-Module Message Passing Asynchronous communication Messages dispatched by a two-level priority scheduler Suited for services with long latency Module A Module B Module A Module B Indirect Function Call Post Module Function Pointer Table Message Buffer Inter-Module Communication Inter-Module Function Calls • Synchronous communication • Kernel stores pointers to functions registered by modules • Blocking calls with low latency • Type-safe runtime function binding

18. SOS Messaging • Task scheduling • Send message to self • Schedule timer for later message delivery • Inter-module asynchronous communication • Send message to other module • Multi-level queues (currently Two) • High priority Message for timely response. • Network capable (Currently NOT implemented on XYZ) • Same message format for both local message queue and radio send queue. • Receive queue is local message queue.

19. 3 Module A 2 Network Module B 1 4 5 Msg Queue Send Queue SOS Messaging and Modules • Module is active when it is handling the message (2)(4). • Message handling runs to completion and can only be interrupted by hardware interrupts. • Module can send message to another module (3) or send message to the network (5). • Message can come from both network (1) and local host (3).

20. Modules and Memory Dependencies • There are 2 main types of memory dependencies • Function dependencies – when a module needs to make a function call to another module • SOS wraps a function call into a message • Data dependencies – arise when a module needs to store data to memory • SOS provides heap memory. Upon a message arrival, SOS is passed the message along with a pointer to the module’s state maintained by SOS int8_t app_handler(void* state, Message *msg)

21. typedefstruct { sos_pid_t did; // destination module ID sos_pid_t sid; // source module ID uint16_t daddr; // destination node uint16_t saddr; // source node uint8_t type; // message type uint8_t len; // message length uint8_t *data; // payload uint8_t flag; // options } Message; Messages are best-effort by default. No senddone and Low priority Can be changed via flag in runtime Network Capable Messages • Messages are filtered when received. • CRC Check and Non-promiscuous mode • Can turn off filter in runtime

22. SOS Messaging API // send message int8_tpost(Message *msg); // short message struct typedefstruct { uint8_t byte; uint16_t word; } MsgParam; // send short message int8_tpost_short( sos_pid_t did, sos_pid_t sid, uint8_t type, uint8_t byte, uint16_t word, uint8_t flag); // send message over net int8_tpost_net( sos_pid_t did, sos_pid_t sid, uint8_t type, uint8_t length, void *data, uint8_t flag, uint16_t daddr); // send long message int8_tpost_long( sos_pid_t did, sos_pid_t sid, uint8_t type, uint8_t length, void *data, uint8_t flag);

23. Messaging Example: Ping_pong enum { MSG_LONG_BALL = MOD_MSG_START, MSG_SHORT_BALL = (MOD_MSG_START + 1), }; enum { PLAYER1_PID = DFLT_APP_ID0, PLAYER2_PID = DFLT_APP_ID1, }; typedefuint8_t ball_t; typedefstruct { ball_t next_seq; } player_t;

24. Messaging Example: Ping_pong int8_t player(void *state, Message *msg){ player_t *s = (player_t*)state; switch (msg->type){ case MSG_INIT: { //! initialize the state s->next_seq = 0; //! start with short ball if(msg->did == PLAYER1_PID) { post_short(PLAYER2_PID, PLAYER1_PID, MSG_SHORT_BALL, s->next_seq, 0, 0); } return SOS_OK; }

25. Messaging Example: Ping_pong case MSG_SHORT_BALL: { MsgParam *p = (MsgParam*)(msg->data); s->next_seq = p->byte + 1; DEBUG("%dget short ball%d\n", msg->did, p->byte); if(p->byte % 2) { post_short(msg->sid, msg->did, MSG_SHORT_BALL, s->next_seq, 0, 0); } else { post_net(msg->sid, msg->did, MSG_LONG_BALL, sizeof(ball_t), &(s->next_seq), 0, ker_id()); } return SOS_OK; }

26. Messaging Example: Ping_pong case MSG_LONG_BALL: { ball_t *b = (ball_t*)(msg->data); s->next_seq = (*b) + 1; DEBUG("%dget long ball%d\n", msg->did, *b); if((*b) % 2) { post_long(msg->sid, msg->did, MSG_LONG_BALL, sizeof(ball_t), &(s->next_seq), 0); } else { Message m; m.did = msg->sid; m.sid = msg->did; m.daddr = ker_id(); m.saddr = ker_id(); m.type = MSG_SHORT_BALL;m.len = sizeof(ball_t); m.data = &(s->next_seq); m.flag = 0; post(&m); } return SOS_OK; }

27. Messaging Example: Ping_pong default: return-EINVAL; } } void sos_start(void){ ker_register_task(DFLT_APP_ID0, sizeof(player_t), player); ker_register_task(DFLT_APP_ID1, sizeof(player_t), player); }

28. Message Types // msg discription enum { MSG_INIT = (KER_MSG_START + 0), //!< initialization MSG_DEBUG = (KER_MSG_START + 1), //!< debug info request MSG_TIMER_TIMEOUT = (KER_MSG_START + 2), //!< timeout timer id MSG_PKT_SENDDONE = (KER_MSG_START + 3), //!< send done MSG_DATA_READY = (KER_MSG_START + 4), //!< sensor data ready MSG_TIMER3_TIMEOUT = (KER_MSG_START + 5), //!< Timer 3 timeout MSG_FINAL = (KER_MSG_START + 6), //!< process kill MSG_FROM_USER = (KER_MSG_START + 7), //!< user input (gw only) MSG_GET_DATA = (KER_MSG_START + 8), //!< sensor get data MSG_SEND_PACKET = (KER_MSG_START + 9), //!< send packet //! XXX probably not a good idea to put I2C stuff here MSG_I2C_SENDSTARTDONE = (KER_MSG_START + 10), //!< I2C send Start done MSG_I2C_SENDENDDONE = (KER_MSG_START + 11), //!< I2C send End done MSG_I2C_READDONE = (KER_MSG_START + 12), //!< I2C Read Done MSG_I2C_WRITEDONE = (KER_MSG_START + 13), //!< I2C Write Done //! MAXIMUM is 31 for now }; //! PLEASE add name string to kernel/message.c Applications can create their own messages, that need to be added here – include/message_types.h

29. Module A Module B 3 2 1 Module Function Pointer Table Synchronous Communication • Module can register function for low latency blocking call (1). • Modules which need such function can subscribe it by getting function pointer pointer (i.e. **func) (2). • When service is needed, module dereferences the function pointer pointer (3).

30. Synchronous Communcation API typedefint8_t (*fn_ptr_t)(void); // register function int8_t ker_register_fn( sos_pid_t pid, // function owner uint8_t fid, // function id char *prototype, // function prototype fn_ptr_t func); // function // subscribe function fn_ptr_t* ker_get_handle( sos_pid_t req_pid, // function owner uint8_t req_fid, // function id char* prototype) // function prototype

31. Memory Management • Modules need memory to store state information • Problems with static memory allocation • Worst case memory allocation – every variable is global • Single packet in the radio stack – can lead to race conditions • Problems with general purpose memory allocation • Non-deterministic execution delay • Suffers from external fragmentation • Use fixed-partition dynamic memory allocation • Memory allocated in blocks of fixed sizes • Constant allocation time • Low overhead • Memory management features • Guard bytes for run-time memory over-flow checks • Semi-auto ownership tracking of memory blocks • Automatic free-up upon completion of usage

32. SOS Memory API // allocate memory to id void *ker_malloc(uint16_t size, sos_pid_t id); // de-allocate memory void ker_free(void* ptr);

33. Messaging and Dynamic Memory • Messaging is asynchronous operation. Attaching dynamic memory in post() results transfer of ownership. • Bit Flag is used to tell SOS kernel the existence of dynamic memory. • SOS_MSG_DYM_ALLOC -- data is dynamically allocated • SOS_MSG_FREE_ON_FAIL -- free memory when post fail. • SOS_DYM_MANAGED = SOS_MSG_DYM_ALLOC | SOS_MSG_FREE_ON_FAIL • Dynamically allocated message payload will be automatically freed after module handling. • This is the default. You can change it by return SOS_TAKEN instead of SOS_OK to take the memory. • Message header belongs to the kernel, and it will be recycled. If you need them, make a deep copy.

34. Module A System Call System Messages High Priority Message Buffer System Jump Table SOS Kernel Interrupt HW Specific API Hardware Asynchronous Module Kernel Interaction • Kernel provides system services and access to hardware • Kernel jump table re-directs system calls from modules to kernel handlers • Hardware interrupts and messages from the kernel to modules are dispatched through a high priority message buffer • Low latency • Concurrency safe operation

35. Schedule Message with Software Timer Module A • Two priority: high and low. • Normal timer has high priority while slow timer is not. • Two types: periodic and one shot Timer syscall Timer Messages High Priority Message Buffer System Jump Table SOS Kernel Interrupt Timer API Delta Timer

36. SOS Timer API enum { TIMER_REPEAT = 0, // high priority, periodic TIMER_ONE_SHOT = 1, // high priority, one shot SLOW_TIMER_REPEAT = 2, // low priority, periodic SLOW_TIMER_ONE_SHOT = 3, // low priority, one shot }; int8_t ker_timer_start( sos_pid_t pid, // module id uint8_t tid, // timer id uint8_t type, // timer type int32_t interval // binary interval ); int8_t ker_timer_stop( sos_pid_t pid, // module id uint8_t tid // timer id );

37. Timer Example: Blink #include<sos.h> #defineMY_IDDFLT_APP_ID0 int8_t blink(void *state, Message *msg) { switch (msg->type) { case MSG_INIT: //!< initial message from SOS //! 256 ticks is 250 milliseconds ker_timer_start(MY_ID, 0, TIMER_REPEAT, 256); return SOS_OK; case MSG_TIMER_TIMEOUT: //!< timeout message arrived ker_led(LED_RED_TOGGLE); return SOS_OK; default: return -EINVAL; } } void sos_start() { ker_register(MY_ID, 0, blink); }

38. Module A Module B Polled Access Periodic Access Data Sensor Manager Data Request Data Policy Sensor 1 Sensor 2 Sensor Manager • Enables sharing of sensor data between multiple modules • Presents a uniform data access API to many diverse sensors • Underlying device specific drivers register with the sensor manager • Device specific sensor drivers control • Calibration • Data interpolation • Sensordrivers are loadable • Enables post-deployment configuration of sensors • Enables hot-swapping of sensors on a running node

39. SOS Directory Layout sos/apps Application directory blank_sos Application with just SOS core blink Blink application sender Periodic packet sending ping_pong ping-pong example sos/dev Device specific directory mica2 mica2 hardware device drivers micaz micaz hardware device drivers gw Gateway(PC) hardware device drivers sim Simulated hardware device drivers xyz XYZ device driver template sample hardware template sos/doc SOS Documentation directory sos/include Include files modules include files for loadable modules sos/kernel Portable kernel sos/modules Loadable module directory gw Modules for Gateway(PC) class device mc Modules for microcontroller class device

40. CVS Access % export CVSROOT= anon@cvs.nesl.ucla.edu: /Volumes/Vol1/neslcvs/CVS % export CVS_RSH=ssh % cvs co sos password = ‘anon’ % echo “happy hacking”

41. References • Michael Melkonian, “Get by Without an RTOS”, Embedded Systems Programming Mag, vol 3. No. 10 Sept., 2000 • Jack W. Crenshaw, “Mea Culpa (Is RTOS needed?)”, http://www.embedded.com/story/OEG20020222S0023 • Karl Fogel, “Open Source Development with CVS”, http://cvsbook.red-bean.com/ • CVS FAQ, http://www.cs.utah.edu/dept/old/texinfo/cvs/FAQ.txt