Linux Potential for Embedded and Real Time Applications

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• Linux Potential for Embedded and Real Time
  Applications
• K. Sebi Kumar
• Wipro Technologies

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AGENDA

• What is Real Time?
• Linux Barriers
• How to make Linux Real Time?
• Challenges In the Existing Version.

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Hard real time
If the deadline is missed, data is permanently
lost or people get hurt Often, these systems
are fully autonomous Examples Air bag
deployment Satellite telemetry Anti- lock
brakes
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Soft real-time

• With soft real- time
• systems
• Missed deadlines are not Critical
• Often have a human in the loop
• (HITL)
• Example
• An automatic teller machine
• (ATM)
• If the ATM takes 30 seconds
• longer than the ideal, the user still wont walk
  away .

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Real-time Linux Barriers

• Task scheduling is fair share
• The Kernel is non-preemptive
• Long interrupt disable times
• Interrupt servicing is unbounded
• Unbounded priority inversions in the kernel

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How to make Linux Real-time
4 Choices to implement Real-time with Linux
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Kernel Co-existence
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RT Linux
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Benefits

• Can support hard real-time applications
• All Linux applications still run
• Moving to a new revision of the Linux kernel
  relatively is easy
• Cost effective
• RTLinux's worst case interrupt latency on a
  modest, reasonably configured x86 P is less than
  15 microseconds

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Challenges

• Real-time parts of the application do not run in
  a Linux environment
• No memory protection for real-time programs
• Some applications are very difficult to split
  into real-time/non-real-time parts
• Real-time programs cannot use Linux drivers
• Some Linux drivers may not work
• Market pressures will make the executive more
  complex over time

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Kernel Modification

• Create Linux patches that
• Replace the fair share scheduler with a scheduler
  that supports real-time tasks
• Add preemption points in the kernel
• In short enable Linuxs SMP locking mechanisms
  to work on a single processor.

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Adding preemption points
Check for rescheduling the CPU
Standard Linux Kernel
System call start
System call end
Linux with Preemption Points added
System call start
System call end
Check for rescheduling the CPU
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Comparisons
Standard Kernel
Preempted Kernel
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Benefits

• Can definitely improve the response of the Linux
  kernel to support soft real-time applications
• Result is an open source solution
• Real-time applications run in a full Linux
  environment

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Challenges

• Takes a lot of work to keep patches current
• Some device drivers have long interrupt off times
• Interrupt and mutual exclusion priority inversion
  problems are not solved

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Kernel Replacement

• Take an operating system kernel that was already
  designed for real-time
• Have it support the Linux application programming
  interface (API)
• Add support for the Linux application binary
  interface (ABI)
• Use on systems where Linux does not meet the
  real-time requirements

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Linux as an embedded platform
3rd party software for Linux
Customer application sw
pick kernel that meets real-time requirements
hardware
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Benefits

• Task scheduling is strict priority preemptive
  (more than 64 priorities)
• The Kernel is fully preemptive
• No long interrupt disable times
• Interrupt servicing tightly bounded even for
  devices that can produce an unbounded number of
  interrupt requests
• No unbounded priority inversions in the kernel

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Differences In Design
Standard RTOS Design
Linux with Preemption added
Preemptible Region
Non-Preemptible Region
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But Is it Linux?

• Complete Linux API and ABI
• Linux user interface
• Most of Linux application will run
• Complete networking support
• Same process model and memory protection as Linux
• Smaller memory footprint

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Benefits

• Even hard real-time requirements can be met
• Real-time tasks run in a Linux environment
• Smaller footprint possible - may save flash
  memory
• Kernel can be certified for critical applications
• Decision can be postponed until it is clear that
  standard Linux wont meet requirements

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Challenges

• Not an open source solution
• Software purchase costs are higher
• Some Linux applications may not run
• Cannot currently use Linux device drivers
• Not available on all the processors

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Conclusion

• If a system has real-time requirements there are
  three ways Linux can be used
• Kernel coexistence
• Kernel modification
• Kernel replacement
• Each has its advantages and disadvantages

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Thank You sebikumar.kuruvilla_at_wipro.com
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