Real-Time Kernel (Part 1)

Operating Systems Design and Implementation

Basic Real-Time Concepts

• Soft versus Hard real-time– soft--as fast as possible but missing

deadline is tolerable– hard--correct and on time

Foreground/Background Systems

• Often referred to as super-loops

Background(task level)

Foreground(interrupt level)

ISR ISR

ISR

HandleAsynchronous

Events

Infinite loopcalling modules

Interruptionoccurred

Interrupt Service Routines

• Handle critical operations– can take longer than they should– make data available for background

routines• processing of such information is referred to

task-level response

Example: EFI System(Electronic Fuel Injection)

• What are the components?

Throttle Body

Air-flow meter

Injectors

ECU

Distributor sensors

Cold start solenoid

High pressure fuel pump

Water temperature sensorO2 sensor

manifold sensor

EFI System

• How does it work?– fundamentally, it manages three

necessities to start and maintain operation of a gasoline engine

• fuel• spark• air

EFI System

• What happen to the EFI system when you start a car?

• What happen to the EFI system when you drive a car?

Dissecting EFI System

• Tasks • ISRs

EFI System

• Critical section (atomic or indivisible)– any possible critical regions in our tasks?

• Mutual exclusion?

• Reentrant code?– functions can be used by multiple tasks

without causing data corruption

• Priority inversion problem?

Priority Inversion

• Assume task 3 has lower priority than task 1.– Task 1 is doing I/O so Task 3 gets to run– Task 3 is in the middle of accessing a

shared resource (obtain semaphore)– Task 1 finishes so it preempts Task 3– Task 1 wants to access the same resource

but can’t since Task 3 has the semaphore

Priority Inversion

• In this scenario, the priority of Task 1 is reduced to that of Task 3.

• What is a good solution?

Priority Inheritance

Priority Inversion

• Priority Inheritance– Task 1 is doing I/O so Task 3 gets to run– Task 3 is in the middle of accessing a shared

resource (obtain semaphore)– Task 1 finishes so it preempts Task 3– Task 1 wants to access the same resource but can’t

since Task 3 has the semaphore; thus the kernel raises the priority of Task 3 to the same as Task 1

– Task 3 gets to finish and releases the resource. The priority is reset to the original value

– Task 1 is selected if it still has the highest priority

Assigning Task Priority

• Rate monotonic scheduling– tasks with the highest rate of execution are given

the highest priority• Assume all tasks are periodic• Tasks do not synchronize with another, share resources,

and exchange data• Preemptive scheduling is used

€

E iTi

≤ n(21

n

i

∑ −1)

Assigning Task PriorityNumber of Tasks n(21/n - 1)

1 1

2 0.82

3 0.77

4 0.75

5 0.74

…

- 0.69

Providing Mutual Exclusion

• Disabling interrupt

• Test and Set operation– hardware support (TSL operation)

• Disabling scheduler

• Semaphores– how is semaphore implemented?

Disabling Interrupt

• X86– CLI (disable interrupt)– STI (enable interrupt)

Busy Waiting

Busy Waiting

Semaphore

• Is a type that has a counter and a delay queue– require OS support as processes in the

delay queue are blocked– implementation often requires other

primitive support (disabling interrupt, etc.)

SemaphoreProcedure DOWN( S : semaphore): Downb(mutex) S := S - 1 If (S < 0) then upb(mutex) downb(delay) endif upb(mutex) end DOWN

procedure UP( S : semaphore): downb(mutex) S := S + 1 if (S ≤ 0) then upb(delay) else upb(mutex) endif end UP

assumes the existence of binary semaphore operations upb and downb implemented with a test-and-set instruction and busy waiting

Intertask Communiciation

• Message mailbox

• Message queues– often use to process interrupt

Interrupts

• A hardware mechanism used to notify the CPU that asynchronous events have occurred

• Upon completion, the programs return to:– background for a foreground/background system– the interrupted task for non-premptive kernel– the higest priority task ready to run for premptive

kernel

Example: Interrupt in NIOS

IE bit to enabling interruptPRI bits for priorityMISC bits for interrupt control

Source of Exceptions (NIOS)• External Hardware interrupt Sources

– External logic for producing the 6-bits interruptnumber & asserting the IRQ input pin is automatically generated by the SOPC builder and is included in the Peripheral Bus Module (PBM).

• Internal Exception Sources– 2 sources of internal exceptions

• Register window-overflow, Register window-underflow

• Direct Software Exceptions – Software can request that control be transferred to an

exception handler by issuing a TRAP instruction.

External Hardware Interrupts• Active-high interrupt signal: irq

– Level sensitive– Sampled synchronously at the rising edge of Nios clock– Should stay asserted until the interrupt is acknowledged

by software

• 6-bit Input Interrupt Number: irq_number[5:0]– Identifies the highest priority interrupt currently requested

• Highest priority = 0 (irq #0 to #15 are reserved)• Lowest priority = 63

External Hardware Interrupts

• Nios will process the indicated exception if– IE= 1 – i.e. external interrupts & internal

exceptions are enabled, AND– The interrupt number is smaller (lower or

equal) than the IPRI field value

Interrupt Service Routine Handler

nr_installuserisrnr_installuserisr( int trapNumber,

void *ISRProcedure, int context)

trapNumber is the exception number to be associated with a service routine

ISRProcedure is a routine which has a prototype oftypedef void (*Nios_isrhandlerproc) (int context);

context is a value that will be passed to the routine specified by isrProcedure

ISR Handler

• This routine installs an interrupt service routine for a specific exception number

• If nr_installuserisr() is used to set up the exception handler, then the exception handler can be an ordinary C routine

ISR Process

• Interrupt occurs• Current state is saved

(Context)• ISR address is retrieved from

the vector table based on the interrupt number

• Jump to ISR routineRuns to completion

• Context is restored• Program resumes

Memory

Vector Table

ISR

Main ProgramSave

Context

Restore

Context

ISR ImplementationSpecify your # IRQ

Declare your IRQ Declare your IRQ subroutinessubroutines

Update the ISR Update the ISR vector tablevector table

Write your IRQ Subroutine

Write your IRQ Subroutine

ROM instruction

RAM

stack

@irq_subroutine 0

…

@clock_adj_ISR

@RealTime_ISR

…

@irq_subroutine 63

Vector Table

0xFFFF

…

0xFF0F

0xFF0E

…

0xFFC0