05. Real-Time Multitasking

8/16/2019 05. Real-Time Multitasking

http://slidepdf.com/reader/full/05-real-time-multitasking 1/53

5-1®

Real-Time MultitaskingReal-Time Multitasking

5.1 Introduction

Task Basics

Task Control

Error Status

System Tasks



5-2®

What is Real-TimeWhat is Real-Time

Real-time denotes the a!ility o" a control system to kee#u# $ith the system !eing controlled.

% system is descri!ed as !eing deterministic i" itsres#onse time is #redicta!le.

The lag time !et$een the occurrence o" an e&ent and theres#onse to that e&ent is called latency.

'eterministic res#onse is the key to real-time

#er"ormance.



5-3®

Re(uirements o" a Real-time SystemRe(uirements o" a Real-time System

%!ility to control many e)ternal com#onents* – Inde#endent com#onents

– %synchronous com#onents

– Synchronous com#onents

+igh s#eed e)ecution*

– ,ast res#onse

– o$ o&erhead

'eterministic o#eration*

– % late ans$er is a $rong ans$er.



5-4®

'esign #tions'esign #tions



5-5®

/nitasking %##roach/nitasking %##roach

ne task controlling all the com#onents in a loo#.

arm (){for (;;)

{if (shoulder needs moving) moveShoulder();

if (elbow needs moving) moveElbow();

if (wrist needs moving)

moveWrist();...}

}



5-6®

Multitasking %##roachMultitasking %##roach

Create a se#arate task to mani#ulate each 0oint*

Each task alternates !et$een ready2 and $aiting2. 3)Works allo$s a task to $ait "or*

– % s#eci"ied time delay

– %n e&ent 4e.g. an interru#t

joint (){for (;;)

{

waitForMove(); ! "ntil joint needs moving ! move#oint();}

}


http://slidepdf.com/reader/full/05-real-time-multitasking 7/535-7®

Task StatesTask States



Multitasking 6ernelMultitasking 6ernel

The Wind kernel is that #art o" 3)Works $hich directlymanages tasks.

%llocates the C7/ to tasks according to the 3)Worksscheduling algorithm 4to !e discussed.

/ses Task Control Blocks 4TCBs to kee# track o" tasks.

– ne #er task

– 'eclared as WI8'9TCB data structure in taski!.htaski!.h

– .S. control in"ormation e.g. state: task #riority:delay timer: !reak#oint list: error status: I; redirections: etc.

– C7/ Conte)t In"ormation e.g. 7C: S7: C7/ registers: ,7/registers



6ernel #eration6ernel #eration



Conte)t S$itchConte)t S$itch

When one task sto#s e)ecuting and a ne$ task starts: acontext switch or reschedule has occurred.

To schedule a ne$ task to run: the kernel must*

1. Sa&e conte)t o" currently e)ecuting task into its TCB.

<. Restore conte)t o" ne)t task to e)ecute "rom its TCB.

Com#lete conte)t s$itch must !e &ery "ast.



Ty#es o" Conte)t S$itchesTy#es o" Conte)t S$itches

Synchronous conte)t s$itches occur !ecause thee)ecuting task

– #ends: delays: or sus#ends itsel".

– makes a higher #riority task ready to run.

– 4less "re(uently lo$ers its o$n #riority: or e)its.

%synchronous conte)t s$itches occur $hen an ISR*

– makes a higher #riority task ready to run.

– 4less "re(uently sus#ends the current task or lo$ers its #riority.

Synchronous conte)t s$itches re(uire sa&ing "e$erregisters than asynchronous conte)t s$itches: and so are"aster.



7riority Scheduling7riority Scheduling

'i""erent a##lication 0o!s may ha&e di""erent#recedences: $hich should !e o!ser&ed $hen allocatingthe C7/.

7reem#ti&e scheduling is !ased on task #riorities chosen

to re"lect 0o! #recedence.

The highest #riority task ready to run 4not #ended ordelayed is allocated the C7/.

Rescheduling can occur at any time as a result o"* – 6ernel calls

– Interru#ts 4e.g.: system clock tick

The conte)t s$itch is not delayed until the ne)t systemclock tick.



7riority Based 7reem#tion7riority Based 7reem#tion

E(ual #riority tasks $on=t #reem#t each other 4!y de"ault.



Round-Ro!in SchedulingRound-Ro!in Scheduling


http://slidepdf.com/reader/full/05-real-time-multitasking 15/535-15

®

Slicing TimeSlicing Time

To allo$ e(ual #riority tasks to #reem#t each other: timeslicing must !e turned on*

kernelTimeSlice(ticks)

I" ticks > ?: time slicing is turned o"".

7riority scheduling al$ays takes #recedence.

– Round-ro!in only a##lies to tasks o" the same #riority.

7riority-!ased rescheduling can ha##en any time.

– Round-ro!in rescheduling can only ha##en e&ery "e$ clock ticks.



®

7er"ormance Enhancements7er"ormance Enhancements

%ll tasks reside in a common address s#ace.

%ll tasks run in su#er&isor 4#ri&ileged mode.



®

Multitasking ,acilitiesMultitasking ,acilities

+ 3 W k ti S t M t+ 3 W k ti S t M t



®

+o$ 3)Works #erating System Meets+o$ 3)Works #erating System Meets

Real-time Re(uirementsReal-time Re(uirements

%!le to control many e)ternal com#onents – Multitasking allo$s solution to mirror the #ro!lem.

– Inde#endent "unctions are assigned to di""erent tasks.

– Intertask communication allo$s tasks to coo#erate.

+igh s#eed e)ecution

– Tasks are light-$eight: ena!ling "ast conte)t s$itch.

– 8o system call2 o&erhead.

'eterministic o#eration

– 7reem#ti&e #riority scheduling assures res#onse "or high #rioritytasks.



®


Introduction

5.< Task Basics

Task Control

Error Status

System Tasks



®

&er&ie$&er&ie$

o$ le&el routines to create and mani#ulate tasks are"ound in taski!.

% 3)Works task consists o"*

– % stack 4"or local storage o" automatic &aria!les and #arameters

#assed to routines.

– % TCB 4"or S control.

'o not con"use e)ecuta!le code $ith the task4s $hiche)ecute it*

– Code is do$nloaded !e"ore tasks are s#a$ned.

– Se&eral tasks can e)ecute the same code4e.g.: printf( ).



5-21®

Creating a TaskCreating a Task



5-22®

Creating a TaskCreating a Task

int taskSpawn (name, priority, options,stackSize, entryPt, arg1, …, arg10)

name Task name: i" 8/ gi&es a de"ault name.

#riority Task #riority ?-<55.

o#tions Task o#tions e.g. 3@9/8BRE%6%BE.

stackSiAe SiAe o" stack to !e allocated in !ytes.

entry7t %ddress o" code to start e)ecuting 4initial 7C.

arg1: ...: arg1? /# to 1? arguments to entry-#oint routine.

Returns a task id or ERRR i" unsuccess"ul.



5-23®

Task I'=sTask I'=s

%ssigned !y kernel $hen task is created.

/ni(ue to each task.

E""icient <-!it handle "or task.

May !e reused a"ter task e)its.

% task id o" Aero re"ers to task making call 4sel".

Rele&anttaski!taski! routines*

taskIdSelf( ) et I' o" calling task.

taskIdListGet( ),ill array $ith I'=s o" all e)isting tasks.

taskIdVerify( ) 3eri"y a task I' is &alid.



5-24®

Task 8amesTask 8ames

7ro&ided "or human con&enience. – Ty#ically used only "rom the shell 4during de&elo#ment.

– Within #rograms: use task Ids.

By 4o"ten ignored con&ention: task names start $ith a tt.

– 7romotes inter#retation as a task name.

– 'e"ault name is a tt "ollo$ed !y an ascending integer.

'oesn=t ha&e to !e uni(ue 4!ut usually is.

Rele&ant taski!taski! routines*

taska!e( ) et name "rom tid.

taska!eToId( ) et tid "rom task name.



5-25®

Task 7rioritiesTask 7riorities

Range "rom ? 4highest to <55 4lo$est.

'etermining ho$ to set task #riorities is a di""icultsu!0ect: and !eyond the sco#e o" this course. +o$e&er*

– Timing re(uirements rather than haAy ideas a!out task im#ortance

should go&ern #riorities.

– % su!stantial !ody o" theory e)ists.

ne can mani#ulate #riorities dynamically $ith*

taskPriorityGet (tid, &priority)taskPrioritySet (tid, priority)

– 'oing so may make your a##lication=s !eha&ior more di""icult toanalyAe.



5-26®

Task StacksTask Stacks

%llocated "rom system memory #ool $hen task is created.

,i)ed siAe a"ter creation.

The kernel reser&es some s#ace "rom the stack: making

the stack s#ace actually a&aila!le slightly less than thestack s#ace re(uested.

E)ceeding stack siAe 4stack crash2 causesun#redicta!le system !eha&ior.



5-27®

Stack &er"lo$sStack &er"lo$s

To check "or a stack o&er"lo$ use the Bro$ser* – 7ress the check-stack !utton*

– E)amine the high $ater mark indicator in the stack dis#lay$indo$.

High water mark indicator

(Uni) !"rrent "sage shown #yshaded #ar and n"m#er inside$

High water mark shown #yshaded #ar$ (%indows)!"rrent "sage shown #y n"m#erinside #ar$



5-28®

Task #tionsTask #tions

Can !e !it$ise or=ed together $hen the task is created*3@9,79T%S6 %dd "loating #oint su##ort.

3@989ST%C69,I 'on=t "ill stack $ith ?)ee=s.

3@9/8BRE%6%BE 'isa!le !reak#oints.

3@9'E%C9ST%C6 'eallocate stack and TCB $hen task e)its4automatically set "or you.

/se task"ptionsGet( ) to in(uire a!out a tasks o#tions.

/se task"ptionsSet( ) to unset 3@9'E%C9ST%C6.



5-29®

Task CreationTask Creation

'uring time critical code: task creation can !eunacce#ta!ly time consuming.

To reduce creation time: a task can !e s#a$ned $ith the3@989ST%C69,I o#tion !it set.

%lternati&ely: s#a$n a task at system start-u# $hich!locks immediately: and $aits to !e made ready $henneeded.



5-30®

Task 'eletionTask 'eletion

taske'ete (tid) 'eletes the s#eci"ied task.

'eallocates the TCB and stack.

eit(code)

E(ui&alent to a task#elete( ) o" sel".

code #arameter is stored in the TCB "ield exitCode.

TCB may !e e)amined "or #ost-mortem de!ugging !y

– /nsetting the 3@9'E%C9ST%C6 o#tion or:

– /sing a delete hook.



5-31®

Resource ReclamationResource Reclamation

Contrary to the #hiloso#hy o" sharing system resourcesamong all tasks.

Can !e an e)#ensi&e #rocess: $hich must !e thea##lication=s res#onsi!ility.

TCB and stack are the only resources automaticallyreclaimed.

Tasks are res#onsi!le "or cleaning u# a"ter themsel&es.

– 'eallocating memory. – Releasing locks on shared resources.

– Closing "iles $hich are o#en.

– 'eleting child tasks $hen #arent task e)its.



5-32®


Introduction

Task Basics

5. Task Control

Error Status

System Tasks



5-33®

Task RestartTask Restart

taskestart (tid)

Task is terminated and res#a$ned $ith originalarguments and tid.

/sually used "or error reco&ery.



5-34®

Task Sus#end;ResumeTask Sus#end;Resume

taskS"spend (tid)

Makes task ineligi!le to e)ecute.

Can !e added to #ended or delayed state.

It is sa"est to ha&e a task sus#end itsel".

taskes"me (tid)

Remo&es sus#ension.

/sually taskSuspend 4 and task$esu!e4 are used "orde!ugging and de&elo#ment #ur#oses.



5-35®

Task 'elayTask 'elay

To delay a task "or a s#eci"ied num!er o" system clockticks*

S*US taske'ay (tics)

To #oll e&ery 1;D second*

F$%E&E% {tas'ela (ss*l'%ate+et( ) ,)...}

– %ccurate only i" clock rate is a multi#le o" se&en ticks;second.

– Can su""er "rom dri"t.2

/se sysClk$ateSet( ) to change the clock rate.



5-36®

Reentrancy and Task 3aria!lesReentrancy and Task 3aria!les

I" tasks access the same glo!al or static &aria!les: theresource can !ecome corru#ted 4called a race condition.

7ossi!le Solutions*

– /se only stack &aria!les in a##lications.

– 7rotect the resource $ith a sema#hore.

– /se task &aria!les to make the &aria!le #ri&ate to a task.

Task 3aria!les cause a <-!it &alue to !e sa&ed andrestored on conte)t s$itches: like a register.

Ca&eat* task &aria!les increase conte)t s$itch times.

See the task3ari!task3ari! manual #ages "or details.





5-38®

Task In"ormationTask In"ormation

ti (task+amer-d)

ike i4 : !ut also dis#lays*

– Stack in"ormation

– Task o#tions – C7/ registers

– ,7/ registers 4i" the 3@9,79T%S6 o#tion !it is set.

Can also use sho$ 4 *

- show (t/et0as'1 2)show (t/et0as'1 2)



5-39®

Task Bro$ser Task Bro$ser

To o!tain in"ormationa!out a s#eci"ic task:dou!le-click on the task=ssummary line in the maintask !ro$ser dis#lay: or

enter the task=s I' in theSho$Sho$ !o).



5-40®

What is 7SI@What is 7SI@

riginally: an IEEE committee con&ened to create astandard inter"ace to /8I@ "or*

– Increased #orta!ility.

– Con&enience.

3)Works su##orts almost all o" the 1??.1! 7SI@ Real-time E)tensions.

The 7SI@ real-time e)tensions are !ased on im#licitassum#tions a!out the /8I@ #rocess model $hich do not

al$ays hold in 3)Works. In 3)Works: – Conte)t s$itch times are &ery "ast.

– Te)t: data: and !ss segments are stored in a common: glo!aladdress s#ace.



5-41®

What does 3)Works Su##ortWhat does 3)Works Su##ort

Library DescriptionaioPxLib Asynchronous I/O

semPxLib POSIX Semaphores

mqPxLib POSIX Message Queues

mmanPxLib POSIX Memory Management

schedPxLib POSIX Scheduler Interface

sigLib POSIX Signals

timerLib, clockLibPOSIX Timer/Clock Interface

dirLib File/Directory Information



5-42®


Introduction

Task Basics

Task Control5. Error Status

System Tasks

S



5-43®

Error StatusError Status

lo!al integer errno is used to #ro#agate errorin"ormation*

– o$ le&el routine detecting an error sets errno.

– Calling routine can e)amine errno to disco&er $hy the routine"ailed.

E d C t t S it h



5-44®

Errno and Conte)t S$itchesErrno and Conte)t S$itches

%t each conte)t s$itch: the kernel sa&es and restores the&alue o" errno.

SS tti E



5-45®

Setting ErrnoSetting Errno

o$est le&el routine to detect an error sets errno andreturns ERRR*

S030"S m%outine(){...if (m/umFlurbishes 4 M356F7"%89S:)

{errno 4 S6m7ib60$$6M3/6F7"%89S:ES;return (E%%$%);}

... <Mem 4 mallo= (si>eof(mStru=t));if (<Mem 44 /"77)

{! mallo=() sets errno - do?nt redefine it

!return (E%%$%)}

...

}

E i i EE i i E



5-46®

E)amining ErrnoE)amining Errno

E)amine errno to "ind out $hy a routine "ailed.

errno is only &alid a"ter an error occurs.

if ( rea=tor$'( ) 44 E%%$% ){swit=h (errno)

{=ase S6r=tor7ib60EM@63/+E%6A$/EB

startShutdown( );

brea';=ase S6r=tor7ib60EM@6*%909*376A$/EB

logMsg(C%unD); brea';

=ase S6r=tor7ib67E36@$SS987EB=he='&essel( );

brea';defaultB startEmerg@ro=( );}

}

I t ti EI t ti E



5-47®

Inter#reting ErrnoInter#reting Errno

3)Works uses the <-!it &alue errno as "ollo$s*

1 15 ?

– 3)Works module num!ers are de"ined in &$Mod8um.h&$Mod8um.h.

– Each module de"ines its o$n error num!ers in its header "ile.

,or e)am#le: an errno o" ?)11???1 $ould !e*

– Module num!er ?)11 4de"ined in &$Mod8um.h&$Mod8um.h to !e memi!memi! and

– Error num!er ?)?1 4de"ined in memi!.hmemi!.h to !e not enoughmemory2.

module error num!er

E ME M



5-48®

Error MessagesError Messages

3)Works uses an error sym!ol ta!le 4statSymT!l tocon&ert error num!ers to error messages.

To o!tain the error string corres#onding to errno*

To #rint the error message associated $ith an errornum!er to the WindSh console*

- <rintErrno (GH22GGG2)

S9memi!98T9E8/+9MEMRF

{

=har errStr I/3ME6M35J;strerror6r (errno1 errStr);...}

/ ' "i d E C d/ ' "i d E C d



5-49®

/ser-'e"ined Error Codes/ser-'e"ined Error Codes

To allo$ strerror'r 4 to su##ort your error messages*1. Create a user header "ile directory.

<. Create a "ile ))Mod8um.h))Mod8um.h in the user header directory*

Kdefine M6m7ib (LG2 2N)

. 'e"ine error macros in your header "iles 4$hich must !e in theuser header directory*

Kdefine S6m7ib6836S0"FF (M6m7ibO2)

. Re!uild the system error ta!le: statT!l.ostatT!l.o.

5. Include the com#onent de&elo#ment tool com#onents G sym!ol ta!lede&elo#ment tool com#onents G sym!ol ta!le

com#onents G error status ta!lecom#onents G error status ta!le in 3)Works. %dd statTlo to theE@TR%9M'/ES macro.

H. Re!uild 3)Works.

R l Ti M ltit kiR l Ti M ltit ki



5-50®


Introduction

Task Basics

Task ControlError Status

5.5 System Tasks

S t T kS t T k



5-51®

System TasksSystem Tasks

Task 8ame 7riority ,unction

t*sr$oot ? ,irst task. InitialiAes included

"acilities: s#a$ns user a##lication:

and e)its.

tLo+Task ? Message logging.

t,xcTask ? Ser&er $hich e)ecutes miscellaneous

task-le&el "unctions at high #riority.

tWdTask W'B run-time agent.

tetTask 5? Task-le&el net$ork "unctions.

t-tpdTask 55 ,T7 ser&er.

SS



5-52®

SummarySummary

Real-time multitasking re(uirements* – 7reem#ti&e #riority-!ased scheduler

– o$ o&erhead

Task #ro#erties stored in task=s TCB.

– S control in"ormation 4#riority: stack siAe: o#tions: state: ....

– Sa&ed C7/ conte)t 47C: S7: C7/ registers: ....

taskSpawn( ) lets you s#eci"y intrinsic #ro#erties*

– 7riority

– Stack siAe

– #tions

SS



SummarySummary

Task mani#ulation routines in taski!* – taskSpawn.task#elete

– task#elay

– taskSuspend.task$esu!e

Task in"ormation

– ti.show

– Task Bro$ser

%dditional task conte)t* – errno

– Task &aria!les

– Task hooks

Documents

05. Real-Time Multitasking