RTLinux Lab Introduction Cpre 558

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RTLinux Lab IntroductionCpre 558

• Anil
• anil_at_iastate.edu

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Contents

• Real-Time Operating System (RTOS) Vs. General
  Purpose OS (GPOS)
• Can Linux provide real-time guarantees?
• Commercial RTOSs
• RTLinux Vs. Linux Architectural comparison
• RTLinux Vs. Linux Code perspective
• Get the RTLinux setup ready
• Things to Issue and Handling the hard disk
• Lab 1 Detailed discussion

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RTOS Vs. GPOS

• RTOS
• needs deterministic timing behavior
• Works under worst case assumptions
• GPOS
• high throughput and fairness
• Optimizes for the average case
• Hence, the design objectives of the OSs are
  different

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Can Linux provide real-time guarantees?-- No!!

• Linux
• non-preemptible kernel
• A system call might take long time to complete
• Coarse timer resolution
• Tasks can be released only with 10ms precision
• Virtual memory
• Introduces unpredictable amount of delays
• Variable priority
• Each task is assigned a priority which varies
  over the time this is to achieve fairness

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Can Linux provide real-time guarantees?-- No!!
(contd..)

• Linux
• Linux will batch several operations for efficient
  use of H/W delaying all tasks
• Linux also reorders requests from multiple
  processes for H/W efficiency
• Linux does not preempt low priority task in a
  system call, instead queues the high priority
  task
• Linux makes high priority tasks to wait for low
  priority tasks to release resources
• Therefore, Linux cannot provide real-time
  guarantees

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Commercial RTOS

• RTLinux Monolithic kernel with loadable modules
• QNX Microkernel approach
• VxWorks Monolithic kernel
• LynxOS Microkernel
• ARTS, Spring, KURT, RTAI, etc.

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RTLinux Overview

• Open source Linux project
• Supports x86, PowerPC, Alpha
• Available as a patch to the regular Linux kernel
• Provides an RT API for developers
• Runs Linux kernel as lowest priority process

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Linux Kernel
User Processes
System libraries
Linux kernel
Device drivers
I/O
Hardware Interrupts
Hardware
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RTLinux Kernel
Linux is executed in the background
User Processes
Real Time Tasks
System libraries
Linux kernel
Device drivers
I/O
Software Interrupts
Direct h/w access
RTLinux Plug-in
RT-Scheduler
I/O
Hardware Interrupts
Hardware
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Linux Kernel code perspective
THE controller
User Process
Kernel Loadable Modules
Module M2 service (system call)
M1
M2
M2
Linux Monolithic kernel core
M3
M4
RAM
DISK
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RTLinux Kernel code perspective
THE controller
RT Load command
RT core
Kernel Loadable Modules
Load RT core
RTM2
RTM3
Linux Monolithic kernel core
M1
M2
RT-Core Module
M3
RAM
DISK
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Getting RTLinux ready

• Configure and compile a fresh Linux kernel
  (2.4.29)
• Download the kernel from
• http//www.kernel.org/pub/linux/kernel/v2.4/linux-
  2.4.29.tar.gz
• Patch the RTLinux to the Linux kernel
• Recompile the kernel and reboot the system into
  the new kernel
• Configure the RTLinux kernel and compile it

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So, How do we run a RT-application program ?

• Step 1 Write the RT-application program as a
  kernel module
• (You are still in Linux)
• Step 2 Compile the module and check for errors
• (You are still in Linux)
• Step 3 Load the RT-Core (or RT Scheduler, etc..)
  and the RT-application program module
• (After this we will be in RTLinux)
• Step 4 Now you are in RTLinux!!!!

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Homework

• Submission 1
• Give a ten line write-up comparing RTLinux with
  Linux with at least five differences along with
  the references.
• Submission 2
• Give a ten line write-up comparing RTLinux with
  two other commercial RTOSs with at least five
  differences for each of them along with the
  references.

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Lab Policies

• Six assignments in total
• First three assignments are mandatory
• One of the last three assignments is mandatory
• You will be working with a removable hard disk
  which can be issued from CSG

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Things to Issue

• Issue the following from the CSG
• Hard disk (one per group)
• Key (one per group)
• RHEL WS3 installation DVD (one per group)
• You need to download the lab01 tar file
  (lab01.tgz) from the course website.
• The RTLinux lab is in Coover - 1331C

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Handling the hard disk (1)

• Shutdown the PC
• After the power is off, unlock the existing hard
  disk and take it out
• Insert the hard disk given to you and lock it
• Restart the comp
• Follow the manual you have downloaded from the
  course website

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Handling the hard disk (2)

• After you are done with the lab
• Shutdown the PC
• After the power is off, unlock the hard disk and
  remove it
• Insert the original hard disk and lock it
• Leave the Lab

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RTLinux Lab 1Cpre 558

• Anil
• anil_at_iastate.edu

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Lab 1 Objectives

• Part 1
• Getting the RTLinux lab setup ready
• Part 2
• Run a few simple hello-world modules

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Part 1

• Install the RHEL-WS3 using the DVD given to you
• Go for the auto-partitioning option
• Rest everything is straightforward
• Configure and compile fresh Linux kernel (2.4.29)
• Patch the RTLinux to the Linux kernel
• Recompile the kernel and reboot the system into
  the new kernel
• Load RT-modules
• Instructions are provided in the Lab manual

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Part 2

• Boot into the RTLinux kernel compiled in part-1
• The objectives of the part 2 are
• Understanding a simple RTLinux kernel module
• With one thread (hello-1) and two threads
  (hello-2)
• Printing from a RTLinux module
• rtl_printf (hello-3)
• Using rtf_put (RT-FIFO) (hello-4)

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A kernel module Vs. Program

• All RT-Tasks are kernel modules. Therefore, we
  write kernel modules in this lab.
• A user program runs in user space and the kernel
  module runs in the kernel space
• A user program starts running at its main
  function
• A kernel module starts running at the
  init_module function and exits via
  cleanup_module function
• Therefore, for a kernel module init_module and
  cleanup_module are as necessary as main is
  necessary for a C program

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A typical kernel module
Init_module() . Perform your module
initiation stuff etc.. Cleanup_module() . P
erform cleanup stuff like destroying the threads,
freeing memory, etc..
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Hello world Module

• Go through the hello world module
• Realize the importance of init_module and
  cleanup_module
• Notice the way the thread is created and how it
  is made periodic
• Understand the pthread_make_periodic and
  pthread_wait and other interfacing functions

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A typical RTLinux module
This function tells the scheduler that I am a
periodic task with period p
Init_module() For each thread Choose the
priority function //pthread_set_priority(..) Ass
ign the task_function to the thread Create the
thread //pthread_create(.., task_function,..)
Task_function() Pthread_make_periodic_np(p)
while(1) Pthread_wait_np() //perform
the task function //for this instance
Cleanup_module() . Delete the
thread //pthread_delete_np(..)
This function tells the scheduler that I am done
for this instance and schedule me after p units
of time
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Hello World 2

• This module creates two threads using a nice
  thread_struct structure to make the
  initialization easy and compact.
• Once again go through the interfacing funcitons
• This time put more rtl_printf statements and
  open /var/log/messages file to find your output

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RTL_PRINTF how does it work?
Might overflow!!
printk
Rtl_printf
Printk buffer
Kernel ring buffer
Log file
Might overflow!!
syslodg
console
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Other ways of printing RTL-FIFO

• RTLinux FIFO provides communication mechanisms
  between RTL threads and Linux task.
• A linux process creates the RTL-FIFO buffer which
  the RTL threads access and write and read from
  them

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RTL-FIFO

• Creating and destroying FIFOS
• Using functions rtf_create and rtf_destroy
• These functions are called from linux process
  open(), read(), write()
• Accessing FIFOs from RTL threads
• using functions rtf_put and rtf_get

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Using RTL_FIFO
Linux Task my_read task
RT Task my_put task
rtf_create()
open()
rtf_put()
Time
read()
rtf_destroy()
RTL FIFO
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Submissions

• Submission 1
• Submit a five line write-up explaining the
  working of the two RTL interfacing functions
  including
• Pthread_make_periodic_np
• Pthread_wait_np
• Rtf_create,rtf_destroy, rtf_put, rtf_get
• Submission 2
• Submit a five line write-up giving at least five
  differences between a kernel module and a user
  program.

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Thank You!!