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    Subject[tip:sched/core] Documentation/scheduler/sched-deadline.txt: Fix terminology and improve clarity
    Commit-ID:  ad67dc316f000df4756b027f3559ad0491497d9e
    Gitweb: http://git.kernel.org/tip/ad67dc316f000df4756b027f3559ad0491497d9e
    Author: Luca Abeni <luca.abeni@unitn.it>
    AuthorDate: Tue, 9 Sep 2014 10:57:12 +0100
    Committer: Ingo Molnar <mingo@kernel.org>
    CommitDate: Tue, 16 Sep 2014 10:23:00 +0200

    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: Luca Abeni <luca.abeni@unitn.it>
    Signed-off-by: Juri Lelli <juri.lelli@arm.com>
    Reviewed-by: Henrik 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: Ingo Molnar <mingo@kernel.org>
    ---
    Documentation/scheduler/sched-deadline.txt | 38 ++++++++++++++++--------------
    1 file changed, 20 insertions(+), 18 deletions(-)

    diff --git a/Documentation/scheduler/sched-deadline.txt b/Documentation/scheduler/sched-deadline.txt
    index 18adc92..a029891 100644
    --- a/Documentation/scheduler/sched-deadline.txt
    +++ b/Documentation/scheduler/sched-deadline.txt
    @@ -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
    - ---------------------------------- > ----------------
    - scheduling deadline - current time period
    + 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
    =======================

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