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 scannerIO_ProcessOutputs(); KBD_Scan(); // keyboardPRN_Print(); // printerLCD_Update(); // displayRS232_Receive(); // serial portRS232_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 3

Task 1

Task 2

Task Event Queue

Task3_PutEvent()

Task3_PutEvent()

Task3_GetEvent()

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 TimerTasks deposit theirevents in a queue

Application handler functions

Software timer processHandler for the hardware 10ms timer check Delta Queue

For expired timers

8

Using A Software Timer

The timer API function in SOS looks like thisint8_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

Hardware Abstraction

ModuleCommunication

MemoryManager

Static SOS Kernel

Dynamic LoadableBinary Modules

Dynamic LoadableBinary Modules

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 Communication

Inter-Module Message Passing

Asynchronous communication

Messages dispatched by a two-level priority scheduler

Suited for services with long latency

Inter-Module Function Calls

Synchronous communication

Kernel stores pointers to functions registered by modules

Blocking calls with low latency

Type-safe runtime function binding

PostMessage

Buffer

Module A Module B

Module FunctionPointer Table

Indirect Function Call

Module A Module B

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

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).

Module A

Module BMsg Queue

2

3

Send Queue4 5

Network1

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

Network Capable Messages

typedef struct { 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

Messages are filtered when received.

CRC Check and Non-promiscuous mode

Can turn off filter in runtime

22

SOS Messaging API

// send message over netint8_t post_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 messageint8_t post_long( sos_pid_t did, sos_pid_t sid, uint8_t type, uint8_t length, void *data, uint8_t flag);

// send message

int8_t post(Message *msg);

// short message struct

typedef struct {

uint8_t byte;

uint16_t word;

} MsgParam;

// send short message

int8_t post_short(

sos_pid_t did,

sos_pid_t sid,

uint8_t type,

uint8_t byte,

uint16_t word,

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,

};

typedef uint8_t ball_t;

typedef struct {

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("%d get 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("%d get 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 hereMSG_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

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).

Module FunctionPointer Table

Module A Module B

3

12

30

Synchronous Communcation API

typedef int8_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

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

Module A

SystemJump Table

Hardware

System Call

High PriorityMessage

Buffer

HW Specific API Interrupt

System Messages

SOS Kernel

35

Schedule Message with Software Timer

Two priority: high and low.• Normal timer has high priority while slow timer is not.

Two types: periodic and one shot

Module A

SystemJump Table

Delta Timer

Timer syscall

High PriorityMessage

Buffer

Timer API Interrupt

Timer Messages

SOS Kernel

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>#define MY_ID DFLT_APP_ID0int8_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

Sensor Manager

PeriodicAccess

Request

Sensor Manager

Module A Module B

Sensor 2

Data Policy

Sensor 1

Data

DataPolledAccess

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

Sensor drivers are loadable• Enables post-deployment configuration of

sensors• Enables hot-swapping of sensors on a running

node

39

SOS Directory Layoutsos/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