Introduction to RTOS m3

Introduction to RTOS About SwiftACT • A Technology services startup company o Under establishment • Areas of specialties: o o Mobile telecommunication services development Embedded systems development • Types of services: o o o o Consultation Managed services Sourcing Training Amr Ali Abdel-Naby@2010 Introduction to RTOS About Me • Graduated 2004 o ECE, ASU: yrs distinction • 5+ years in embedded systems development o SDLC, Apps, MW, DD, Porting, • 3+ years in SW engineering o PSP, CMMI, Systematic reuse, • 3+ years in SW testing o IBM certified, ISTQB certified, Amr Ali Abdel-Naby@2010 Introduction to RTOS Copyright • Materials in this course is the property of Amr Ali Abdel-Naby • Reproduction or transmission of the materials in any manner without the copyright owner permission is a law violation Amr Ali Abdel-Naby@2010 Introduction to RTOS Outline • • • • • • Introduction Tasks Semaphores Message Queues Other Kernel Objects Other Kernel Services Amr Ali Abdel-Naby@2010 Introduction to RTOS Outline • • • • • • Introduction Tasks Semaphores Message Queues Other Kernel Objects Other Kernel Services Amr Ali Abdel-Naby@2010 Introduction to RTOS Critical Sections: Resource Sharing • Resources used by task at a time, not interrupted o Some devices, shared data, algorithms (non-reentrant) • Critical Section == Code for serially reusable resource • If tasks enter same critical section simultaneously  Catastrophe • Mechanisms to avoid such collisions using an RTOS o o o o Resource monitor tasks Semaphores Mutexes … Print “I am #1” I am #I am #21 Print “I am #2” Amr Ali Abdel-Naby@ 2009 Real-Time Operating Systems Defining Semaphores • Binary Semaphore o o Can be used as a lock to protect a critical region In some RTOSes, need to build your own:  Restrict counting semaphore to ≤ • Counting Semaphore o o o Can be used to protect pools of equivalent resources Keeps track of number of free equivalent units of resource Available in many RTOSes • Mutex o o Specialized kind of binary semaphore, for mutual exclusion purposes Available in many modern RTOSes Amr Ali Abdel-Naby@ 2009 Real-Time Operating Systems Binary Semaphore Release (value = 1) Initial value = Initial value = Available Unavailable Acquire (value = 0) Amr Ali Abdel-Naby@2010 Introduction to RTOS Counting Semaphore Release (count++) Release (count = 1) Initial count > Initial count = Available Acquire (count ) Amr Ali Abdel-Naby@2010 Unavailable Acquire (count = 0) Introduction to RTOS Mutual Exclusion Semaphore (Mutex) Acquire recursive (lock count++) Acquire (lock count = 1) Initial Lock count = Locked Release recursive (lock count ) Amr Ali Abdel-Naby@2010 Unlocked Release (lock count = 0) Introduction to RTOS Mutexes vs Semaphores • • • • Mutex ownership Recursive locking Task deletion safety Priority inversion safety o o Ceiling protocol Inheritance protocol Amr Ali Abdel-Naby@2010 Introduction to RTOS Semaphores: Traditional Usages • Resource limit … or use a Resource Monitor Task o o o • Critical Section … or use a Resource Monitor Task or Mutex o o o • Initial count = wait_sem to enter critical section signal_sem to leave critical section Wait / Signal … or use a Message o o o • Initial count = # of equivalent resources wait_sem to gain access to a resource signal_sem to release a resource Initial count = wait_sem to wait signal_sem to signal Credit tracking … or use Messages o o o Initial count = signal_sem to increment (Credit) wait_sem to consume (Debit) Amr Ali Abdel-Naby@ 2009 Real-Time Operating Systems Examples: Wait/Signal, Credit Tracking HW Device ISR SW Task • Wait/Signal o o o Task waits on semaphore Interrupt task (or ISR) signals to semaphore when it runs semaphore counts don’t accumulate • Credit tracking o o Semaphore counts can accumulate Count represents interrupts not yet handled by the task • Alternative: Messages HW Device n Amr Ali Abdel-Naby@ 2009 ISR n Real-Time Operating Systems SW Task Some Problems with Semaphores • Processor boundaries • Difficult to debug (all tokens identical) • When used for multiple equivalent resources o When task is allowed access to a resource, which one is it ?? • Interrupt tasks and ISRs can not take a token (get the semaphore) • Task entrapments / Priority inversions Amr Ali Abdel-Naby@ 2009 Real-Time Operating Systems Typical Semaphore Operations • Semaphores creation and deletion o o Create: binary, counting, or mutex Delete • Acquiring and releasing semaphores o o Acquire: wait for ever, wait with timeout, or not wait Release • Clearing semaphore task-waiting lists o Flush • Obtaining semaphore information o o Show info Show blocked tasks Amr Ali Abdel-Naby@2010 Introduction to RTOS [...]... Release (lock count = 0) Introduction to RTOS Mutexes vs Semaphores • • • • Mutex ownership Recursive locking Task deletion safety Priority inversion safety o o Ceiling protocol Inheritance protocol Amr Ali Abdel-Naby@2010 Introduction to RTOS Semaphores: 4 Traditional Usages • Resource limit … or use a Resource Monitor Task o o o • Critical Section … or use a Resource Monitor Task or Mutex o o o •... count = 1 wait_sem to enter critical section signal_sem to leave critical section Wait / Signal … or use a Message o o o • Initial count = # of equivalent resources wait_sem to gain access to a resource signal_sem to release a resource Initial count = 0 wait_sem to wait signal_sem to signal Credit tracking … or use Messages o o o Initial count = 0 signal_sem to increment (Credit) wait_sem to consume (Debit)... wait for ever, wait with timeout, or do not wait Release • Clearing semaphore task-waiting lists o Flush • Obtaining semaphore information o o Show info Show blocked tasks Amr Ali Abdel-Naby@2010 Introduction to RTOS ... Interrupt task (or ISR) signals to semaphore when it runs semaphore counts don’t accumulate • Credit tracking o o Semaphore counts can accumulate Count represents interrupts not yet handled by the task • Alternative: Messages HW Device n Amr Ali Abdel-Naby@ 2009 ISR n Real-Time Operating Systems SW Task Some Problems with Semaphores • Processor boundaries • Difficult to debug (all tokens identical) • When... Problems with Semaphores • Processor boundaries • Difficult to debug (all tokens identical) • When used for multiple equivalent resources o When task is allowed access to a resource, which one is it ?? • Interrupt tasks and ISRs can not take a token (get the semaphore) • Task entrapments / Priority inversions Amr Ali Abdel-Naby@ 2009 Real-Time Operating Systems Typical Semaphore Operations • Semaphores creation