[PATCH v3 1/4] Documentation/scheduler/sched-deadline.txt: fix terminology and improve clarity

Juri Lelli
2014-08-28T10:01:17Z

From: Luca Abeni <luca.abeni@unitn.it>

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 "closest"

Signed-off-by: Luca Abeni <luca.abeni@unitn.it>
Signed-off-by: Juri Lelli <juri.lelli@arm.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Henrik Austad <henrik@austad.us>
Cc: Dario Faggioli <raistlin@linux.it>
Cc: Juri Lelli <juri.lelli@gmail.com>
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
---
Documentation/scheduler/sched-deadline.txt | 32 ++++++++++++++++--------------
1 file changed, 17 insertions(+), 15 deletions(-)

diff --git a/Documentation/scheduler/sched-deadline.txt b/Documentation/scheduler/sched-deadline.txt
index 18adc92..dce6d63 100644
--- a/Documentation/scheduler/sched-deadline.txt
+++ b/Documentation/scheduler/sched-deadline.txt
@@ -45,14 +45,14 @@ 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
+ closest 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
+ algorithm selects the task with the closest 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.
@@ -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
=======================
--
2.0.4