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Commit ad67dc31 authored by Luca Abeni's avatar Luca Abeni Committed by Ingo Molnar
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Documentation/scheduler/sched-deadline.txt: Fix terminology and improve clarity 



Several small changes regarding SCHED_DEADLINE documentation
that fix terminology and improve clarity and readability:

 - "current runtime" becomes "remaining runtime"

 - readablity of an equation is improved by introducing more spacing

 - clarify when admission control will certainly fail

 - new URL for CBS technical report

 - substitue "smallest" with "earliest"

Signed-off-by: default avatarLuca Abeni <luca.abeni@unitn.it>
Signed-off-by: default avatarJuri Lelli <juri.lelli@arm.com>
Reviewed-by: default avatarHenrik Austad <henrik@austad.us>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Dario Faggioli <raistlin@linux.it>
Cc: Juri Lelli <juri.lelli@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1410256636-26171-2-git-send-email-juri.lelli@arm.com


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 8236d907

Documentation/scheduler/sched-deadline.txt

+20 −18
Original line number Diff line number Diff line
@@ -45,17 +45,17 @@ CONTENTS 
 every time the task wakes up, the scheduler computes a "scheduling deadline"

 consistent with the guarantee (using the CBS[2,3] algorithm). Tasks are then

 scheduled using EDF[1] on these scheduling deadlines (the task with the

 smallest scheduling deadline is selected for execution). Notice that this

 earliest scheduling deadline is selected for execution). Notice that this

 guaranteed is respected if a proper "admission control" strategy (see Section

 "4. Bandwidth management") is used.



 Summing up, the CBS[2,3] algorithms assigns scheduling deadlines to tasks so

 that each task runs for at most its runtime every period, avoiding any

 interference between different tasks (bandwidth isolation), while the EDF[1]

 algorithm selects the task with the smallest scheduling deadline as the one

 to be executed first.  Thanks to this feature, also tasks that do not

 strictly comply with the "traditional" real-time task model (see Section 3)

 can effectively use the new policy.

 algorithm selects the task with the earliest scheduling deadline as the one

 to be executed next. Thanks to this feature, tasks that do not strictly comply

 with the "traditional" real-time task model (see Section 3) can effectively

 use the new policy.



 In more details, the CBS algorithm assigns scheduling deadlines to

 tasks in the following way:
@@ -64,45 +64,45 @@ CONTENTS 
    "deadline", and "period" parameters;



  - The state of the task is described by a "scheduling deadline", and

    a "current runtime". These two parameters are initially set to 0;

    a "remaining runtime". These two parameters are initially set to 0;



  - When a SCHED_DEADLINE task wakes up (becomes ready for execution),

    the scheduler checks if



                    current runtime                runtime

         ---------------------------------- > ----------------

                 remaining runtime                  runtime

        ----------------------------------    >    ---------

        scheduling deadline - current time           period



    then, if the scheduling deadline is smaller than the current time, or

    this condition is verified, the scheduling deadline and the

    current budget are re-initialised as

    remaining runtime are re-initialised as



         scheduling deadline = current time + deadline

         current runtime = runtime

         remaining runtime = runtime



    otherwise, the scheduling deadline and the current runtime are

    otherwise, the scheduling deadline and the remaining runtime are

    left unchanged;



  - When a SCHED_DEADLINE task executes for an amount of time t, its

    current runtime is decreased as

    remaining runtime is decreased as



         current runtime = current runtime - t

         remaining runtime = remaining runtime - t



    (technically, the runtime is decreased at every tick, or when the

    task is descheduled / preempted);



  - When the current runtime becomes less or equal than 0, the task is

  - When the remaining runtime becomes less or equal than 0, the task is

    said to be "throttled" (also known as "depleted" in real-time literature)

    and cannot be scheduled until its scheduling deadline. The "replenishment

    time" for this task (see next item) is set to be equal to the current

    value of the scheduling deadline;



  - When the current time is equal to the replenishment time of a

    throttled task, the scheduling deadline and the current runtime are

    throttled task, the scheduling deadline and the remaining runtime are

    updated as



         scheduling deadline = scheduling deadline + period

         current runtime = current runtime + runtime

         remaining runtime = remaining runtime + runtime





3. Scheduling Real-Time Tasks
@@ -147,6 +147,8 @@ CONTENTS 
 and the absolute deadlines (d_j) coincide, so a proper admission control

 allows to respect the jobs' absolute deadlines for this task (this is what is

 called "hard schedulability property" and is an extension of Lemma 1 of [2]).

 Notice that if runtime > deadline the admission control will surely reject

 this task, as it is not possible to respect its temporal constraints.



 References:

  1 - C. L. Liu and J. W. Layland. Scheduling algorithms for multiprogram-
@@ -156,7 +158,7 @@ CONTENTS 
      Real-Time Systems. Proceedings of the 19th IEEE Real-time Systems

      Symposium, 1998. http://retis.sssup.it/~giorgio/paps/1998/rtss98-cbs.pdf

  3 - L. Abeni. Server Mechanisms for Multimedia Applications. ReTiS Lab

      Technical Report. http://xoomer.virgilio.it/lucabe72/pubs/tr-98-01.ps

      Technical Report. http://disi.unitn.it/~abeni/tr-98-01.pdf



4. Bandwidth management

=======================