RT+Linux +Training 10

8/8/2019 RT+Linux +Training 10

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RT LinuxRT Linux

A Real Time Approach of Linux A Real Time Approach of Linux


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W hatW hat isis realreal--time?time?

Correctness of output depends on timingCorrectness of output depends on timingas well as resultas well as result

Hard vs. soft realHard vs. soft real- -timetime

Are W indows and Linux real Are W indows and Linux real- -time?time?


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Real time IntroductionReal time IntroductionDefinitionsDefinitions

Real Time SystemReal Time System is one where theis one where thecorrectness of a computation is acorrectness of a computation is afunction of both the result of thefunction of both the result of thecomputation and the time at which thecomputation and the time at which thecomputation is deliveredcomputation is delivered

Soft RTSoft RT occasionally allowed tooccasionally allowed tomiss deadlines (eg. Updating a videomiss deadlines (eg. Updating a videodisplay)display)H ard RTH ard RT M eeting the deadline isM eeting the deadline isabsolutely critical (eg. Rocketabsolutely critical (eg. Rocketcontroller)controller)


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Typical RTSTypical RTS


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RTOS Vs. GPOSRTOS Vs. GPOS

RTOSRTOSneeds deterministic timing behavior needs deterministic timing behavior

W orks under worst case assumptionsW orks under worst case assumptions

GPOSGPOShigh throughput and fairnesshigh throughput and fairness

Optimizes for the average caseOptimizes for the average case

Hence, the design objectives of the OSs are differentHence, the design objectives of the OSs are different


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Incompatibility withIncompatibility with

Timesharing SystemsTimesharing SystemsTimesharing systems try to optimizeTimesharing systems try to optimizeaverage case but RT systems mustaverage case but RT systems mustconsider worst case.consider worst case.Contradictory design goals for the twoContradictory design goals for the two what is good for the average case tendswhat is good for the average case tendsto deteriorate the worst case. A classicto deteriorate the worst case. A classicexample is virtual memory and demandexample is virtual memory and demandpaging.paging.


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Two Types of Real Time OSTwo Types of Real Time OS

Hard Real TimeHard Real TimeM eeting the deadlineM eeting the deadlineis absolutely criticalis absolutely critical

E.g. Rocket controller E.g. Rocket controller

Soft Real TimeSoft Real TimeOccasionallyOccasionallyallowed to missallowed to missdeadlinesdeadlines

E.g. Updating aE.g. Updating avideo displayvideo display

So what happens in the case a Hard Real Time Systemdoes not meet a deadline?

Failure!


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Hard versus SoftHard versus Soft

HARD:HARD: miss a deadline and youre in trouble!miss a deadline and youre in trouble!(planes, trains, factory control, nuclear facilities, ...)(planes, trains, factory control, nuclear facilities, ...)SOFT:SOFT: try to meet deadlines, but if not, system stilltry to meet deadlines, but if not, system still

works, although with degraded performanceworks, although with degraded performance(multimedia, thermostat, ...)(multimedia, thermostat, ...)F IRM :F IRM : late results are worthless, but you are not inlate results are worthless, but you are not introubletrouble


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Why RTOS?

Interactive embedded systems require realtime schedulingSome legacy OS can provide some of thisfunctionality BUT they are too big to run onan embedded systems


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Real Time OS problemReal Time OS problem

A RTOS needs to be fast and predictable: so itA RTOS needs to be fast and predictable: so itneeds to be small and precisely designed.needs to be small and precisely designed.

Modern RT applications need networking stacks,Modern RT applications need networking stacks,GUI, databases, file systems, developmentGUI, databases, file systems, developmentenvironments, web servers, and so on: So theenvironments, web servers, and so on: So theoperating system needs to have many featuresoperating system needs to have many features H ard to estimate service time.H ard to estimate service time.

Both requirements are hardly satisfied at the sameBoth requirements are hardly satisfied at the sametime.time.


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D esirable Real-Time Kernel Features

MultitaskingPriority-based preemptive scheduling

Inter-task synchronization and communicationSemaphores, mailboxes, event-flags ...Basic memory managementInterrupt handling

Timer handling, time managementImplementable on small deeply embedded systems


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Defects In Linux As An RTOSDefects In Linux As An RTOS


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Rea l ti e I tr ti Rea l ti e I tr ti Li x as g RT pl a tf r Li x as g RT pl a tf r

Excellent stability and efficiencyExcellent stability and efficiencySource code availabilitySource code availability

Active license, and substantial user base. Active license, and substantial user base.Linux can run on the most widely availableLinux can run on the most widely availablecomputerscomputersLinux has all features of a modern UN IX system:Linux has all features of a modern UN IX system:(X W indow, graphical user interface toolkits,(X W indow, graphical user interface toolkits,

networking, databases, programming languages,networking, databases, programming languages,debuggers, and a variety of applications).debuggers, and a variety of applications).It is embeddable and is able to perform efficientlyIt is embeddable and is able to perform efficientlyusing relatively small amounts of RA M and other using relatively small amounts of RA M and other computer resources.computer resources.


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Defects of LinuxDefects of Linux

Linux scheduling algorithms are not designed for realLinux scheduling algorithms are not designed for real- -time tasks.time tasks.

Linux scheduling is unpredictable.Linux scheduling is unpredictable.

Linux Timer resolution is coarse, 10 ms.Linux Timer resolution is coarse, 10 ms.

Linux Kernel is NonLinux Kernel is Non- -preemptible .preemptible .

Linux disables interrupts used for coarseLinux disables interrupts used for coarse- -grainedgrained

synchronization.synchronization.Linux uses virtual memory.Linux uses virtual memory.

Linux reorders requests for efficiency (e.g., for disk I/O).Linux reorders requests for efficiency (e.g., for disk I/O).


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

L inuxL inuxnonnon- -preemptible kernelpreemptible kernel

A system call might take long time to complete A system call might take long time to complete

C oarse timer resolutionC oarse timer resolutionTasks can be released only withTasks can be released only with 10ms10ms precisionprecisionV irtual memoryV irtual memory

Introduces unpredictable amount of delaysIntroduces unpredictable amount of delaysV ariable priorityV ariable priority

Each task is assigned a priority which varies over the time;Each task is assigned a priority which varies over the time;this is to achieve fairnessthis is to achieve fairness


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Unpredictable schedulingUnpredictable scheduling depends on systemdepends on systemload.load.Time resolutionTime resolutionNonNon--preemptible kernelpreemptible kernelDisabling of interrupt during critical sectionsDisabling of interrupt during critical sectionsUse of virtual memoryUse of virtual memory

Rea l tim e Introd ctionWhy is Lin x Uns it ab le for H a rd RT ?


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Real Time LinuxReal Time Linux- - An AnIntroductionIntroduction


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W hat is RTLinux ? ..W hat is RTLinux ? ..

To support hard real timeTo support hard real timeapplications, the Linuxapplications, the Linuxkernel is modified bykernel is modified byintroducing a layer introducing a layer between the hardwarebetween the hardwareand the standard Linuxand the standard Linuxkernel.kernel.RTRT--Linux is a hard realLinux is a hard real- -

time kernel that runstime kernel that runsLinux or BSD as an idleLinux or BSD as an idletask, whenever there aretask, whenever there areno realno real- -time demands.time demands.


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RTRT--LinuxLinux Add iti a l l aye r b e tw een Li nu x Add iti na l l aye r b e tw een Li nu x k e r ne l an d h a r d w a r e.k e r ne l an d h a r d w a r e.

Sm a ll r ea l ti m e k e r ne l e xi s t s withSm a ll r ea l ti m e k e r ne l e xi s t s withLi nu x k e r ne l .Li nu x k e r ne l .

RT RT- - Li nu x i s mo d u l e o ri en t e d .Li nu x i s mo d u l e o ri en t e d .

Dec o u pl es th e m ec h an i s m s o f r ea l Dec o u pl es th e m ec h an i s m s o f r ea l - - ti m e k e r ne l fr om th e g ene r a l p u rp o se ti m e k e r ne l fr om th e g ene r a l p u rp o se k e r ne l .k e r ne l .


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POS IX 1003.1b StandardPOS IX 1003.1b StandardFor soft RT features in UN IX For soft RT features in UN IX It requires the followingIt requires the following

Preemptive priority schedulingPreemptive priority schedulingLocking of virtual pages into memoryLocking of virtual pages into memoryRealReal--time signalstime signalsImproved IPCImproved IPCImproved timers

Improved timersLinux only partially conforms to the above.Linux only partially conforms to the above.

(mlock and setsched system calls)(mlock and setsched system calls)


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RT Li nu x I n tr o d uc ti o nRT Li nu x I n tr o d uc ti o nG o a l sG o a l s

Achieve hard RT performance. Achieve hard RT performance.High level of timer precision in scheduling.High level of timer precision in scheduling.To provide low interrupt latency.To provide low interrupt latency.Customized scheduling.Customized scheduling.M inimal changes to the Linux kernel so that theM inimal changes to the Linux kernel so that thefull range of OS services are available.full range of OS services are available.


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RTRT--Linux is developed and supported byLinux is developed and supported byFS M LabsFS M Labs . RT. RT--Linux began life as a researchLinux began life as a researchproject at the New M exico Institute of project at the New M exico Institute of

Technology (N M T). RTTechnology (N M T). RT- -Linux is an openLinux is an open- -sourcesourceproduct. RTproduct. RT- -LinuxLinux--specific components arespecific components arereleased under the GPL, and Linux componentsreleased under the GPL, and Linux componentsreleased under the standard Linux license. Thereleased under the standard Linux license. The

source code is freely distributed.source code is freely distributed.

RT Li nu x I n tr o d uc ti o nRT Li nu x I n tr o d uc ti o nBac kgr o un d Bac kgr o un d


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The mostThe most- -current stable version of RTLinuxcurrent stable version of RTLinuxV3.0, offers a POS IX style applicationV3.0, offers a POS IX style applicationprogramming interface (AP I), runs on x86,programming interface (AP I), runs on x86,PowerPC and Alpha processors and supportsPowerPC and Alpha processors and supportsS M P (x86 and PPC).S M P (x86 and PPC).

RT Li nu x I n tr o d uc ti o nRT Li nu x I n tr o d uc ti o nBac kgr o n d Bac kgr o n d - - C o n t C o n t


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RTRT--Linux a version of the Linux OperatingLinux a version of the Linux OperatingSystem, its a hard realSystem, its a hard real- -time operating systemtime operating systemthat runs Linux as its lowest priority executionthat runs Linux as its lowest priority execution

thread.thread.The Linux thread is made completelyThe Linux thread is made completelypreemptible so that real time threads andpreemptible so that real time threads andinterrupt handlers are never delayed by noninterrupt handlers are never delayed by non- -realreal

time operationstime operationsPrevents disabling of interrupts by Linux.Prevents disabling of interrupts by Linux.

RT Li nu x I n tr o d uc ti o nRT Li nu x I n tr o d uc ti o n A ppr o ac h A ppr o ac h


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R T Li nu x I n tr o d uc ti o nR T Li nu x I n tr o d uc ti o nS tr uc t u r eS tr uc t u r e

Linux Process 1 Linux Process 2

Linux Kernel An RT - Process

Real Time Kernel


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RealReal--time tasks in RTtime tasks in RT- -Linux canLinux cancommunicate with Linux processes viacommunicate with Linux processes viashared memory or using a F IFO. Thus, realshared memory or using a F IFO. Thus, real- -time applications can make use of all thetime applications can make use of all thepowerful, nonpowerful, non- -realreal--time services of Linux,time services of Linux,including:including:

NetworkingNetworking

GraphicsGraphicsW indowing systemsW indowing systemsData analysis packagesData analysis packagesLinux device drivers, andLinux device drivers, and

Standard POSIX

API.Standard POS

IXAP

I.

RT Li nu x I n tr o d uc ti o nRT Li nu x I n tr o d uc ti o n A ppr o ac h A ppr o ac h C o n t C o n t


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Real Time LinuxReal Time Linux

Real time LinuxReal time LinuxProject:Project:

UT IM EUT IM ERTRT--Linux PlatformLinux PlatformKurt PlatformKurt PlatformRED LinuxRED Linux

RTA IRTA ILinux /RK(fromLinux /RK(fromM ac /RK)M ac /RK)

Embedded LinuxEmbedded LinuxProjectsProjects

M iniRTLM iniRTLExtension to RTExtension to RTLinuxLinux

ELKS ProjectELKS ProjectHAHA--Linux (HighLinux (High

Availability) Availability)VM E Linux ProjectVM E Linux Project


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RTLinux FeaturesRTLinux Features

RealReal--time scheduling algorithm.time scheduling algorithm.Predictable delays.Predictable delays.

By its small size and limited operations.By its small size and limited operations.

Finer timer resolution.Finer timer resolution.Preemptible kernel.Preemptible kernel.

The RTLinux executive is itself nonpreemptible.The RTLinux executive is itself nonpreemptible.Its routine are very small and fast, this does not cause bigIts routine are very small and fast, this does not cause big

delays.delays.Interrupts from Linux are disabled.Interrupts from Linux are disabled.

No virtual memory.No virtual memory.


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A rchitecture of RT Linux


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Linux KernelLinux Kernel

System libraries

Device drivers L inux kernel

Hardware

I/O Hardware Interrupts

User Processes


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RTRT--Linux: Design CriteriaLinux: Design Criteria

To provide timing reliability to the LinuxTo provide timing reliability to the LinuxKernel.Kernel.

Linux Kernel is unpredictable becauseLinux Kernel is unpredictable becauseof:of:

Scheduling algorithms (maximize throughput)Scheduling algorithms (maximize throughput)Device Drivers.Device Drivers.Uninterruptible System Calls.Uninterruptible System Calls.Use of interrupt disabling procedures.Use of interrupt disabling procedures.


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W hen is Hard RealW hen is Hard Real- -TimeTimeneeded?needed?

Hard vs. Soft RealHard vs. Soft Real- -TimeTimeHard RT always guarantees exact timingHard RT always guarantees exact timing

Hard RealHard Real- -Time applicationsTime applicationsClosedClosed- -loop control systemsloop control systemsStimulus /response systemsStimulus /response systems

ExamplesExamples

Robotics, AnimatronicsRobotics, AnimatronicsIndustrial equipmentIndustrial equipmentNeural, fuzzy and adaptive systemsNeural, fuzzy and adaptive systems

However:However:Hard RealHard Real- -Time extensions can degrade OSTime extensions can degrade OSperformance of non timeperformance of non time- -critical applicationscritical applications


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RTLinux: The Real Time LinuxRTLinux: The Real Time Linux

GoalsGoalsmaintain compatibility with GNU /Linuxmaintain compatibility with GNU /Linux

handle tasks with hardhandle tasks with hard- -real time constrainsreal time constrainssimplicity and reliability of the RTsimplicity and reliability of the RTtradeoff: ease of usetradeoff: ease of use

Approach Approachrealreal--time microkerneltime microkernelLinux kernel is running as lowest priority task inLinux kernel is running as lowest priority task inthe realthe real- -time scheduletime schedulerealreal--time applications are written as microkerneltime applications are written as microkernelmodulesmodules

microkernel is transparent to regular applicationmicrokernel is transparent to regular application


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RTLinux: M icrokernelRTLinux: M icrokernel

M icrokernelM icrokernelIS a thin layer of IS a thin layer of abstractionabstraction

PROV IDES anPROV IDES aninterface for hardwareinterface for hardwareinterrupts and AP I for interrupts and AP I for realreal--time taskstime tasksM ED I ATES requestsM ED I ATES requeststo /from hardwareto /from hardware


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RTLinux: M icrokernel (cont.)RTLinux: M icrokernel (cont.)

ComponentsComponentsProcessor clock controlProcessor clock control

RealReal--Time scheduler Time scheduler POS IXPOS IX --like interface to device driverslike interface to device driversRealReal--Time F IFOsTime F IFOsShared memory buffersShared memory buffersPOS IXPOS IX --like IPClike IPC

Blocking mutexesBlocking mutexesSemaphoresSemaphores

SourceSource- -level debugger supportlevel debugger supportSerial port interfaceSerial port interface


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RTLinux: M icrokernel DetailsRTLinux: M icrokernel Details

Scheduler Scheduler Preemptive, priority basedPreemptive, priority based

Hardware context switch isHardware context switch isnot not

usedusedReason: saves too much state, not fast enoughReason: saves too much state, not fast enoughStack is used insteadStack is used instead

RT kernel handles hardware interruptsRT kernel handles hardware interrupts

Linux thread does not directly receive themLinux thread does not directly receive themW hen Linux wants to disable interruptsW hen Linux wants to disable interruptsThe microkernel stops passing them to the LinuxThe microkernel stops passing them to the LinuxthreadthreadHowever, realHowever, real- -time tasks can receive themtime tasks can receive them


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RT+Linux +Training 10