[PATCH v3 0/4] SCHED_DEADLINE documentation fixes and improvements

[PATCH v3 0/4] SCHED_DEADLINE documentation fixes and improvements

[Juri Lelli]

Hello everyone,

This is version 3 of a small patchset that fixes and improves SCHED_DEADLINE
documentation.

Patch 1/4 fixes and clarifies terminology; patch 2/4 aligns Section 4 to
the current interface; patch 3/4 improves and clarifies what admission
control means on UP an SMP systems; patch 4/4 introduces an appendix about
testing.

Changes since v1:

 - fixed typos spotted by Randy and Peter (thanks!)

Changes since v2:

 - fixed typos and rewrote paragraphs as suggested by Ingo (thanks!)

Best Regards,

- Juri

Juri Lelli (2):
  Documentation/scheduler/sched-deadline.txt: Rewrite section 4 intro
  Documentation/scheduler/sched-deadline.txt: add tests suite appendix

Luca Abeni (2):
  Documentation/scheduler/sched-deadline.txt: fix terminology and
    improve clarity
  Documentation/scheduler/sched-deadline.txt: improve and clarify AC
    bits

 Documentation/scheduler/sched-deadline.txt | 204 ++++++++++++++++++++++-------
 1 file changed, 159 insertions(+), 45 deletions(-)

-- 
2.0.4

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

[Juri Lelli]

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

[PATCH v3 2/4] Documentation/scheduler/sched-deadline.txt: Rewrite section 4 intro

[Juri Lelli]

Section 4 intro was still describing the old interface. Rewrite it.

Signed-off-by: Juri Lelli <juri.lelli@arm.com>
Signed-off-by: Luca Abeni <luca.abeni@unitn.it>
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 | 51 +++++++++++++++---------------
 1 file changed, 25 insertions(+), 26 deletions(-)

diff --git a/Documentation/scheduler/sched-deadline.txt b/Documentation/scheduler/sched-deadline.txt
index dce6d63..0aff2d5 100644
--- a/Documentation/scheduler/sched-deadline.txt
+++ b/Documentation/scheduler/sched-deadline.txt
@@ -165,39 +165,38 @@ CONTENTS
 
  In order for the -deadline scheduling to be effective and useful, it is
  important to have some method to keep the allocation of the available CPU
- bandwidth to the tasks under control.
- This is usually called "admission control" and if it is not performed at all,
- no guarantee can be given on the actual scheduling of the -deadline tasks.
-
- Since when RT-throttling has been introduced each task group has a bandwidth
- associated, calculated as a certain amount of runtime over a period.
- Moreover, to make it possible to manipulate such bandwidth, readable/writable
- controls have been added to both procfs (for system wide settings) and cgroupfs
- (for per-group settings).
- Therefore, the same interface is being used for controlling the bandwidth
- distrubution to -deadline tasks.
-
- However, more discussion is needed in order to figure out how we want to manage
- SCHED_DEADLINE bandwidth at the task group level. Therefore, SCHED_DEADLINE
- uses (for now) a less sophisticated, but actually very sensible, mechanism to
- ensure that a certain utilization cap is not overcome per each root_domain.
-
- Another main difference between deadline bandwidth management and RT-throttling
+ bandwidth to the tasks under control. This is usually called "admission
+ control" and if it is not performed at all, no guarantee can be given on
+ the actual scheduling of the -deadline tasks.
+
+ The interface used to control the fraction of CPU bandwidth that can be
+ allocated to -deadline tasks is similar to the one already used for -rt
+ tasks with real-time group scheduling (a.k.a. RT-throttling - see
+ Documentation/scheduler/sched-rt-group.txt), and is based on readable/
+ writable control files located in procfs (for system wide settings).
+ Notice that per-group settings (controlled through cgroupfs) are still not
+ defined for -deadline tasks, because more discussion is needed in order to
+ figure out how we want to manage SCHED_DEADLINE bandwidth at the task group
+ level.
+
+ A main difference between deadline bandwidth management and RT-throttling
  is that -deadline tasks have bandwidth on their own (while -rt ones don't!),
- and thus we don't need an higher level throttling mechanism to enforce the
- desired bandwidth.
+ and thus we don't need a higher level throttling mechanism to enforce the
+ desired bandwidth. Therefore, using this simple interface we can put a cap
+ on total utilization of -deadline tasks (i.e., \Sum (runtime_i / period_i) <
+ some_desired_value).
 
 4.1 System wide settings
 ------------------------
 
  The system wide settings are configured under the /proc virtual file system.
 
- For now the -rt knobs are used for dl admission control and the -deadline
- runtime is accounted against the -rt runtime. We realise that this isn't
- entirely desirable; however, it is better to have a small interface for now,
- and be able to change it easily later. The ideal situation (see 5.) is to run
- -rt tasks from a -deadline server; in which case the -rt bandwidth is a direct
- subset of dl_bw.
+ For now the -rt knobs are used for -deadline admission control and the
+ -deadline runtime is accounted against the -rt runtime. We realise that this
+ isn't entirely desirable; however, it is better to have a small interface for
+ now, and be able to change it easily later. The ideal situation (see 5.) is to
+ run -rt tasks from a -deadline server; in which case the -rt bandwidth is a
+ direct subset of dl_bw.
 
  This means that, for a root_domain comprising M CPUs, -deadline tasks
  can be created while the sum of their bandwidths stays below:
-- 
2.0.4

[PATCH v3 3/4] Documentation/scheduler/sched-deadline.txt: improve and clarify AC bits

[Juri Lelli]

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

Admission control is of key importance for SCHED_DEADLINE, since it guarantees
system schedulability (or tells us something about the degree of guarantees
we can provide to the user).

This patch improves and clarifies bits and pieces regarding AC, both for UP
and SMP systems.

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 | 89 +++++++++++++++++++++++++-----
 1 file changed, 75 insertions(+), 14 deletions(-)

diff --git a/Documentation/scheduler/sched-deadline.txt b/Documentation/scheduler/sched-deadline.txt
index 0aff2d5..641395e 100644
--- a/Documentation/scheduler/sched-deadline.txt
+++ b/Documentation/scheduler/sched-deadline.txt
@@ -38,16 +38,17 @@ CONTENTS
 ==================
 
  SCHED_DEADLINE uses three parameters, named "runtime", "period", and
- "deadline" to schedule tasks. A SCHED_DEADLINE task is guaranteed to receive
+ "deadline", to schedule tasks. A SCHED_DEADLINE task should receive
  "runtime" microseconds of execution time every "period" microseconds, and
  these "runtime" microseconds are available within "deadline" microseconds
  from the beginning of the period.  In order to implement this behaviour,
  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
- 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.
+ closest scheduling deadline is selected for execution). Notice that the
+ task actually receives "runtime" time units within "deadline" if a proper
+ "admission control" strategy (see Section "4. Bandwidth management") is used
+ (clearly, if the system is overloaded this guarantee cannot be respected).
 
  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
@@ -134,6 +135,50 @@ CONTENTS
  A real-time task can be periodic with period P if r_{j+1} = r_j + P, or
  sporadic with minimum inter-arrival time P is r_{j+1} >= r_j + P. Finally,
  d_j = r_j + D, where D is the task's relative deadline.
+ The utilisation of a real-time task is defined as the ratio between its
+ WCET and its period (or minimum inter-arrival time), and represents
+ the fraction of CPU time needed to execute the task.
+
+ If the total utilisation sum_i(WCET_i/P_i) is larger than M (with M equal
+ to the number of CPUs), then the scheduler is unable to respect all the
+ deadlines.
+ Note that total utilisation is defined as the sum of the utilisations
+ WCET_i/P_i over all the real-time tasks in the system. When considering
+ multiple real-time tasks, the parameters of the i-th task are indicated
+ with the "_i" suffix.
+ Moreover, if the total utilisation is larger than M, then we risk starving
+ non- real-time tasks by real-time tasks.
+ If, instead, the total utilisation is smaller than M, then non real-time
+ tasks will not be starved and the system might be able to respect all the
+ deadlines.
+ As a matter of fact, in this case it is possible to provide an upper bound
+ for tardiness (defined as the maximum between 0 and the difference
+ between the finishing time of a job and its absolute deadline).
+ More precisely, it can be proven that using a global EDF scheduler the
+ maximum tardiness of each task is smaller or equal than
+	((M − 1) · WCET_max − WCET_min)/(M − (M − 2) · U_max) + WCET_max
+ where WCET_max = max_i{WCET_i} is the maximum WCET, WCET_min=min_i{WCET_i}
+ is the minimum WCET, and U_max = max_i{WCET_i/P_i} is the maximum utilisation.
+
+ If M=1 (uniprocessor system), or in case of partitioned scheduling (each
+ real-time task is statically assigned to one and only one CPU), it is
+ possible to formally check if all the deadlines are respected.
+ If D_i = P_i for all tasks, then EDF is able to respect all the deadlines
+ of all the tasks executing on a CPU if and only if the total utilisation
+ of the tasks running on such a CPU is smaller or equal than 1.
+ If D_i != P_i for some task, then it is possible to define the density of
+ a task as C_i/min{D_i,T_i}, and EDF is able to respect all the deadlines
+ of all the tasks running on a CPU if the sum sum_i C_i/min{D_i,T_i} of the
+ densities of the tasks running on such a CPU is smaller or equal than 1
+ (notice that this condition is only sufficient, and not necessary).
+
+ On multiprocessor systems with global EDF scheduling (non partitioned
+ systems), a sufficient test for schedulability can not be based on the
+ utilisations (it can be shown that task sets with utilisations slightly
+ larger than 1 can miss deadlines regardless of the number of CPUs M).
+ However, as previously stated, enforcing that the total utilisation is smaller
+ than M is enough to guarantee that non real-time tasks are not starved and
+ that the tardiness of real-time tasks has an upper bound.
 
  SCHED_DEADLINE can be used to schedule real-time tasks guaranteeing that
  the jobs' deadlines of a task are respected. In order to do this, a task
@@ -163,14 +208,22 @@ CONTENTS
 4. Bandwidth management
 =======================
 
- In order for the -deadline scheduling to be effective and useful, it is
- important to have some method to keep the allocation of the available CPU
- bandwidth to the tasks under control. This is usually called "admission
- control" and if it is not performed at all, no guarantee can be given on
- the actual scheduling of the -deadline tasks.
-
- The interface used to control the fraction of CPU bandwidth that can be
- allocated to -deadline tasks is similar to the one already used for -rt
+ As previously mentioned, in order for -deadline scheduling to be
+ effective and useful (that is, to be able to provide "runtime" time units
+ within "deadline"), it is important to have some method to keep the allocation
+ of the available fractions of CPU time to the various tasks under control.
+ This is usually called "admission control" and if it is not performed, then
+ no guarantee can be given on the actual scheduling of the -deadline tasks.
+
+ As already stated in Section 3, a necessary condition to be respected to
+ correctly schedule a set of real-time tasks is that the total utilisation
+ is smaller than M. When talking about -deadline tasks, this requires to
+ impose that the sum of the ratio between runtime and period for all tasks
+ is smaller than M. Notice that the ratio runtime/period is equivalent to
+ the utilisation of a "traditional" real-time task, and is also often
+ referred to as "bandwidth".
+ The interface used to control the CPU bandwidth that can be allocated
+ to -deadline tasks is similar to the one already used for -rt
  tasks with real-time group scheduling (a.k.a. RT-throttling - see
  Documentation/scheduler/sched-rt-group.txt), and is based on readable/
  writable control files located in procfs (for system wide settings).
@@ -232,8 +285,16 @@ CONTENTS
  950000. With rt_period equal to 1000000, by default, it means that -deadline
  tasks can use at most 95%, multiplied by the number of CPUs that compose the
  root_domain, for each root_domain.
-
- A -deadline task cannot fork.
+ This means that non -deadline tasks will receive at least 5% of the CPU time,
+ and that -deadline tasks will receive their runtime with a guaranteed
+ worst-case delay respect to the "deadline" parameter. If "deadline" = "period"
+ and the cpuset mechanism is used to implement partitioned scheduling (see
+ Section 5), then this simple setting of the bandwidth management is able to
+ deterministically guarantee that -deadline tasks will receive their runtime
+ in a period.
+
+ Finally, notice that in order not to jeopardize this admission control a
+ -deadline task cannot fork.
 
 5. Tasks CPU affinity
 =====================
-- 
2.0.4

[PATCH v3 4/4] Documentation/scheduler/sched-deadline.txt: add tests suite appendix

[Juri Lelli]

Add an appendix briefly describing tools that can be used to test SCHED_DEADLINE
(and the scheduler in general). Links to where source code of the tools is hosted
are also provided.

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 | 52 ++++++++++++++++++++++++++++++
 1 file changed, 52 insertions(+)

diff --git a/Documentation/scheduler/sched-deadline.txt b/Documentation/scheduler/sched-deadline.txt
index 641395e..2f5b174 100644
--- a/Documentation/scheduler/sched-deadline.txt
+++ b/Documentation/scheduler/sched-deadline.txt
@@ -15,6 +15,7 @@ CONTENTS
  5. Tasks CPU affinity
    5.1 SCHED_DEADLINE and cpusets HOWTO
  6. Future plans
+ A. Test suite
 
 
 0. WARNING
@@ -341,3 +342,54 @@ CONTENTS
  throttling patches [https://lkml.org/lkml/2010/2/23/239] but we still are in
  the preliminary phases of the merge and we really seek feedback that would
  help us decide on the direction it should take.
+
+Appendix A. Test suite
+======================
+
+ The SCHED_DEADLINE policy can be easily tested using two applications that
+ are part of a wider Linux Scheduler validation suite. The suite is
+ available as a GitHub repository: https://github.com/scheduler-tools.
+
+ The first testing application is called rt-app and can be used to
+ start multiple threads with specific parameters. rt-app supports
+ SCHED_{OTHER,FIFO,RR,DEADLINE} scheduling policies and their related
+ parameters (e.g., niceness, priority, runtime/deadline/period). rt-app
+ is a valuable tool, as it can be used to synthetically recreate certain
+ workloads (maybe mimicking real use-cases) and evaluate how the scheduler
+ behaves under such workloads. In this way, results are easily reproducible.
+ rt-app is available at: https://github.com/scheduler-tools/rt-app.
+
+ Thread parameters can be specified from the command line, with something like
+ this:
+
+  # rt-app -t 100000:10000:d -t 150000:20000:f:10 -D5
+
+ The above creates two threads. The first one, scheduled by SCHED_DEADLINE,
+ executes for 10ms every 100ms and the second one, scheduled at RT priority 10
+ with SCHED_FIFO, executes for 20ms every 150ms. The configuration runs
+ for 5 seconds.
+
+ More interestingly, configurations can be described with a json file that
+ can be passed as input to rt-app with something like this:
+
+  # rt-app my_config.json
+
+ The parameters that can be specified with the second method are a superset
+ of the command line options. Please refer to rt-app documentation for more
+ details.
+
+ The second testing application is a modification of schedtool, called
+ schedtool-dl, which can be used to setup SCHED_DEADLINE parameters for a
+ certain pid/application. schedtool-dl is available at:
+ https://github.com/scheduler-tools/schedtool-dl.git.
+
+ The usage is straightforward:
+
+  # schedtool -E -t 10000000:100000000 -e ./my_cpuhog_app
+
+ With this, my_cpuhog_app is put to run inside a SCHED_DEADLINE reservation
+ of 10ms every 100ms (note that parameters are expressed in microseconds).
+ You can also use schedtool to create a reservation for an already running
+ application, given that you know its pid:
+
+  # schedtool -E -t 10000000:100000000 my_app_pid
-- 
2.0.4

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

[Henrik Austad]

On Thu, Aug 28, 2014 at 11:00:26AM +0100, Juri Lelli wrote:
> 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"

I'm tempted to say "earliest" (being part of the algorithm's name and all 
;)

> 
> 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

s/first/next/

Also, next sentence does not make much sense, I would drop the also;

"Thanks to this feature, tasks that do not strictly comply with the ..."

>   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

Apart from the minor nitpick, it looks good!

-- 
Henrik

Re: [PATCH v3 2/4] Documentation/scheduler/sched-deadline.txt: Rewrite section 4 intro

[Henrik Austad]

On Thu, Aug 28, 2014 at 11:00:27AM +0100, Juri Lelli wrote:
> Section 4 intro was still describing the old interface. Rewrite it.
> 
> Signed-off-by: Juri Lelli <juri.lelli@arm.com>
> Signed-off-by: Luca Abeni <luca.abeni@unitn.it>
> 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 | 51 +++++++++++++++---------------
>  1 file changed, 25 insertions(+), 26 deletions(-)
> 
> diff --git a/Documentation/scheduler/sched-deadline.txt b/Documentation/scheduler/sched-deadline.txt
> index dce6d63..0aff2d5 100644
> --- a/Documentation/scheduler/sched-deadline.txt
> +++ b/Documentation/scheduler/sched-deadline.txt
> @@ -165,39 +165,38 @@ CONTENTS
>  
>   In order for the -deadline scheduling to be effective and useful, it is
>   important to have some method to keep the allocation of the available CPU
> - bandwidth to the tasks under control.
> - This is usually called "admission control" and if it is not performed at all,
> - no guarantee can be given on the actual scheduling of the -deadline tasks.
> -
> - Since when RT-throttling has been introduced each task group has a bandwidth
> - associated, calculated as a certain amount of runtime over a period.
> - Moreover, to make it possible to manipulate such bandwidth, readable/writable
> - controls have been added to both procfs (for system wide settings) and cgroupfs
> - (for per-group settings).
> - Therefore, the same interface is being used for controlling the bandwidth
> - distrubution to -deadline tasks.
> -
> - However, more discussion is needed in order to figure out how we want to manage
> - SCHED_DEADLINE bandwidth at the task group level. Therefore, SCHED_DEADLINE
> - uses (for now) a less sophisticated, but actually very sensible, mechanism to
> - ensure that a certain utilization cap is not overcome per each root_domain.
> -
> - Another main difference between deadline bandwidth management and RT-throttling
> + bandwidth to the tasks under control. This is usually called "admission
> + control" and if it is not performed at all, no guarantee can be given on
> + the actual scheduling of the -deadline tasks.
> +
> + The interface used to control the fraction of CPU bandwidth that can be
> + allocated to -deadline tasks is similar to the one already used for -rt
> + tasks with real-time group scheduling (a.k.a. RT-throttling - see
> + Documentation/scheduler/sched-rt-group.txt), and is based on readable/
> + writable control files located in procfs (for system wide settings).
> + Notice that per-group settings (controlled through cgroupfs) are still not
> + defined for -deadline tasks, because more discussion is needed in order to
> + figure out how we want to manage SCHED_DEADLINE bandwidth at the task group
> + level.
> +
> + A main difference between deadline bandwidth management and RT-throttling
>   is that -deadline tasks have bandwidth on their own (while -rt ones don't!),
> - and thus we don't need an higher level throttling mechanism to enforce the
> - desired bandwidth.
> + and thus we don't need a higher level throttling mechanism to enforce the
> + desired bandwidth. Therefore, using this simple interface we can put a cap
> + on total utilization of -deadline tasks (i.e., \Sum (runtime_i / period_i) <
> + some_desired_value).

s/some_desired_value/global_dl_utilization_cap/  perhaps?

>  4.1 System wide settings
>  ------------------------
>  
>   The system wide settings are configured under the /proc virtual file system.
>  
> - For now the -rt knobs are used for dl admission control and the -deadline
> - runtime is accounted against the -rt runtime. We realise that this isn't
> - entirely desirable; however, it is better to have a small interface for now,
> - and be able to change it easily later. The ideal situation (see 5.) is to run
> - -rt tasks from a -deadline server; in which case the -rt bandwidth is a direct
> - subset of dl_bw.
> + For now the -rt knobs are used for -deadline admission control and the
> + -deadline runtime is accounted against the -rt runtime. We realise that this
> + isn't entirely desirable; however, it is better to have a small interface for
> + now, and be able to change it easily later. The ideal situation (see 5.) is to
> + run -rt tasks from a -deadline server; in which case the -rt bandwidth is a
> + direct subset of dl_bw.

Isn't forking forbidden for _dl tasks? Or did I miss a memo somewhere?

>   This means that, for a root_domain comprising M CPUs, -deadline tasks
>   can be created while the sum of their bandwidths stays below:
> -- 
> 2.0.4
> 
> 

All in all, looks pretty good to me!

-- 
Henrik

Re: [PATCH v3 3/4] Documentation/scheduler/sched-deadline.txt: improve and clarify AC bits

[Henrik Austad]

On Thu, Aug 28, 2014 at 11:00:28AM +0100, Juri Lelli wrote:
> From: Luca Abeni <luca.abeni@unitn.it>
> 
> Admission control is of key importance for SCHED_DEADLINE, since it guarantees
> system schedulability (or tells us something about the degree of guarantees
> we can provide to the user).
> 
> This patch improves and clarifies bits and pieces regarding AC, both for UP
> and SMP systems.
> 
> 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 | 89 +++++++++++++++++++++++++-----
>  1 file changed, 75 insertions(+), 14 deletions(-)
> 
> diff --git a/Documentation/scheduler/sched-deadline.txt b/Documentation/scheduler/sched-deadline.txt
> index 0aff2d5..641395e 100644
> --- a/Documentation/scheduler/sched-deadline.txt
> +++ b/Documentation/scheduler/sched-deadline.txt
> @@ -38,16 +38,17 @@ CONTENTS
>  ==================
>  
>   SCHED_DEADLINE uses three parameters, named "runtime", "period", and
> - "deadline" to schedule tasks. A SCHED_DEADLINE task is guaranteed to receive
> + "deadline", to schedule tasks. A SCHED_DEADLINE task should receive
>   "runtime" microseconds of execution time every "period" microseconds, and
>   these "runtime" microseconds are available within "deadline" microseconds
>   from the beginning of the period.  In order to implement this behaviour,
>   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
> - 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.
> + closest scheduling deadline is selected for execution). Notice that the
> + task actually receives "runtime" time units within "deadline" if a proper
> + "admission control" strategy (see Section "4. Bandwidth management") is used
> + (clearly, if the system is overloaded this guarantee cannot be respected).
>  
>   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
> @@ -134,6 +135,50 @@ CONTENTS
>   A real-time task can be periodic with period P if r_{j+1} = r_j + P, or
>   sporadic with minimum inter-arrival time P is r_{j+1} >= r_j + P. Finally,
>   d_j = r_j + D, where D is the task's relative deadline.
> + The utilisation of a real-time task is defined as the ratio between its
> + WCET and its period (or minimum inter-arrival time), and represents
> + the fraction of CPU time needed to execute the task.
> +
> + If the total utilisation sum_i(WCET_i/P_i) is larger than M (with M equal
> + to the number of CPUs), then the scheduler is unable to respect all the
> + deadlines.
> + Note that total utilisation is defined as the sum of the utilisations
> + WCET_i/P_i over all the real-time tasks in the system. When considering
> + multiple real-time tasks, the parameters of the i-th task are indicated
> + with the "_i" suffix.
> + Moreover, if the total utilisation is larger than M, then we risk starving
> + non- real-time tasks by real-time tasks.
> + If, instead, the total utilisation is smaller than M, then non real-time
> + tasks will not be starved and the system might be able to respect all the
> + deadlines.
> + As a matter of fact, in this case it is possible to provide an upper bound
> + for tardiness (defined as the maximum between 0 and the difference
> + between the finishing time of a job and its absolute deadline).
> + More precisely, it can be proven that using a global EDF scheduler the
> + maximum tardiness of each task is smaller or equal than
> +	((M − 1) · WCET_max − WCET_min)/(M − (M − 2) · U_max) + WCET_max
> + where WCET_max = max_i{WCET_i} is the maximum WCET, WCET_min=min_i{WCET_i}
> + is the minimum WCET, and U_max = max_i{WCET_i/P_i} is the maximum utilisation.
> +
> + If M=1 (uniprocessor system), or in case of partitioned scheduling (each
> + real-time task is statically assigned to one and only one CPU), it is
> + possible to formally check if all the deadlines are respected.
> + If D_i = P_i for all tasks, then EDF is able to respect all the deadlines
> + of all the tasks executing on a CPU if and only if the total utilisation
> + of the tasks running on such a CPU is smaller or equal than 1.
> + If D_i != P_i for some task, then it is possible to define the density of
> + a task as C_i/min{D_i,T_i}, and EDF is able to respect all the deadlines
> + of all the tasks running on a CPU if the sum sum_i C_i/min{D_i,T_i} of the
> + densities of the tasks running on such a CPU is smaller or equal than 1
> + (notice that this condition is only sufficient, and not necessary).
> +
> + On multiprocessor systems with global EDF scheduling (non partitioned
> + systems), a sufficient test for schedulability can not be based on the
> + utilisations (it can be shown that task sets with utilisations slightly
> + larger than 1 can miss deadlines regardless of the number of CPUs M).
> + However, as previously stated, enforcing that the total utilisation is smaller
> + than M is enough to guarantee that non real-time tasks are not starved and
> + that the tardiness of real-time tasks has an upper bound.

I'd _really_ appreciate a link to a paper where all of this is presented 
and proved!

>   SCHED_DEADLINE can be used to schedule real-time tasks guaranteeing that
>   the jobs' deadlines of a task are respected. In order to do this, a task
> @@ -163,14 +208,22 @@ CONTENTS
>  4. Bandwidth management
>  =======================
>  
> - In order for the -deadline scheduling to be effective and useful, it is
> - important to have some method to keep the allocation of the available CPU
> - bandwidth to the tasks under control. This is usually called "admission
> - control" and if it is not performed at all, no guarantee can be given on
> - the actual scheduling of the -deadline tasks.
> -
> - The interface used to control the fraction of CPU bandwidth that can be
> - allocated to -deadline tasks is similar to the one already used for -rt
> + As previously mentioned, in order for -deadline scheduling to be
> + effective and useful (that is, to be able to provide "runtime" time units
> + within "deadline"), it is important to have some method to keep the allocation
> + of the available fractions of CPU time to the various tasks under control.
> + This is usually called "admission control" and if it is not performed, then
> + no guarantee can be given on the actual scheduling of the -deadline tasks.
> +
> + As already stated in Section 3, a necessary condition to be respected to
> + correctly schedule a set of real-time tasks is that the total utilisation
> + is smaller than M. When talking about -deadline tasks, this requires to
> + impose that the sum of the ratio between runtime and period for all tasks
> + is smaller than M.

"This requires to impose that .." uhm, what? Drop 'to impose'.

> [...] Notice that the ratio runtime/period is equivalent to
> + the utilisation of a "traditional" real-time task, and is also often
> + referred to as "bandwidth".
> + The interface used to control the CPU bandwidth that can be allocated
> + to -deadline tasks is similar to the one already used for -rt
>   tasks with real-time group scheduling (a.k.a. RT-throttling - see
>   Documentation/scheduler/sched-rt-group.txt), and is based on readable/
>   writable control files located in procfs (for system wide settings).
> @@ -232,8 +285,16 @@ CONTENTS
>   950000. With rt_period equal to 1000000, by default, it means that -deadline
>   tasks can use at most 95%, multiplied by the number of CPUs that compose the
>   root_domain, for each root_domain.
> -
> - A -deadline task cannot fork.
> + This means that non -deadline tasks will receive at least 5% of the CPU time,
> + and that -deadline tasks will receive their runtime with a guaranteed
> + worst-case delay respect to the "deadline" parameter. If "deadline" = "period"
> + and the cpuset mechanism is used to implement partitioned scheduling (see
> + Section 5), then this simple setting of the bandwidth management is able to
> + deterministically guarantee that -deadline tasks will receive their runtime
> + in a period.

The whole 950000 / 1000000, is at least 50 *consecutive* ms given to non 
rt/dl tasks every second, or is this more finegrained now?

If the 50ms can be given in a single go, then I don't think you can 
guarantee that deadline-tasks will receive their runtime in a period - a 
period can be <50ms, no?

> +
> + Finally, notice that in order not to jeopardize this admission control a
> + -deadline task cannot fork.

s/this/the
(there aren't any other admission controls in the kernel)

>  
>  5. Tasks CPU affinity
>  =====================
> -- 
> 2.0.4
> 
> 

-- 
Henrik

Re: [PATCH v3 4/4] Documentation/scheduler/sched-deadline.txt: add tests suite appendix

[Henrik Austad]

On Thu, Aug 28, 2014 at 11:00:29AM +0100, Juri Lelli wrote:
> Add an appendix briefly describing tools that can be used to test SCHED_DEADLINE
> (and the scheduler in general). Links to where source code of the tools is hosted
> are also provided.
> 
> 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 | 52 ++++++++++++++++++++++++++++++
>  1 file changed, 52 insertions(+)
> 
> diff --git a/Documentation/scheduler/sched-deadline.txt b/Documentation/scheduler/sched-deadline.txt
> index 641395e..2f5b174 100644
> --- a/Documentation/scheduler/sched-deadline.txt
> +++ b/Documentation/scheduler/sched-deadline.txt
> @@ -15,6 +15,7 @@ CONTENTS
>   5. Tasks CPU affinity
>     5.1 SCHED_DEADLINE and cpusets HOWTO
>   6. Future plans
> + A. Test suite
>  
>  
>  0. WARNING
> @@ -341,3 +342,54 @@ CONTENTS
>   throttling patches [https://lkml.org/lkml/2010/2/23/239] but we still are in
>   the preliminary phases of the merge and we really seek feedback that would
>   help us decide on the direction it should take.
> +
> +Appendix A. Test suite
> +======================
> +
> + The SCHED_DEADLINE policy can be easily tested using two applications that
> + are part of a wider Linux Scheduler validation suite. The suite is
> + available as a GitHub repository: https://github.com/scheduler-tools.
> +
> + The first testing application is called rt-app and can be used to
> + start multiple threads with specific parameters. rt-app supports
> + SCHED_{OTHER,FIFO,RR,DEADLINE} scheduling policies and their related
> + parameters (e.g., niceness, priority, runtime/deadline/period). rt-app
> + is a valuable tool, as it can be used to synthetically recreate certain
> + workloads (maybe mimicking real use-cases) and evaluate how the scheduler
> + behaves under such workloads. In this way, results are easily reproducible.
> + rt-app is available at: https://github.com/scheduler-tools/rt-app.
> +
> + Thread parameters can be specified from the command line, with something like
> + this:
> +
> +  # rt-app -t 100000:10000:d -t 150000:20000:f:10 -D5
> +
> + The above creates two threads. The first one, scheduled by SCHED_DEADLINE,
> + executes for 10ms every 100ms and the second one, scheduled at RT priority 10
> + with SCHED_FIFO, executes for 20ms every 150ms. The configuration runs
> + for 5 seconds.

I'd prefer

The above creates 2 threads, T1 and T2. T1 is scheduled by SCHED_DEADLINE 
with a 100ms period and 10ms WCET. T2 is handled by SCHED_FIFO priority 10, 
150ms period and 20ms WCET. The test will run for a total of 5 seconds.

One can expect the eager reader to have at least a grasp of the terminology 
at this stage, using WCET should be acceptable.

> +
> + More interestingly, configurations can be described with a json file that
> + can be passed as input to rt-app with something like this:
> +
> +  # rt-app my_config.json
> +
> + The parameters that can be specified with the second method are a superset
> + of the command line options. Please refer to rt-app documentation for more
> + details.

This can be found that this url: ....

> +
> + The second testing application is a modification of schedtool, called
> + schedtool-dl, which can be used to setup SCHED_DEADLINE parameters for a
> + certain pid/application. schedtool-dl is available at:
> + https://github.com/scheduler-tools/schedtool-dl.git.
> +
> + The usage is straightforward:
> +
> +  # schedtool -E -t 10000000:100000000 -e ./my_cpuhog_app
> +
> + With this, my_cpuhog_app is put to run inside a SCHED_DEADLINE reservation
> + of 10ms every 100ms (note that parameters are expressed in microseconds).
> + You can also use schedtool to create a reservation for an already running
> + application, given that you know its pid:
> +
> +  # schedtool -E -t 10000000:100000000 my_app_pid
> -- 
> 2.0.4

Would it make sense to add an appendix B with a minimal SCHED_DEADLINE 
main() ? I for one like to invent my own wheels.

Just my $0.02

-- 
Henrik

Re: [PATCH v3 0/4] SCHED_DEADLINE documentation fixes and improvements

[Henrik Austad]

On Thu, Aug 28, 2014 at 11:00:25AM +0100, Juri Lelli wrote:
> Hello everyone,
> 
> This is version 3 of a small patchset that fixes and improves SCHED_DEADLINE
> documentation.
> 
> Patch 1/4 fixes and clarifies terminology; patch 2/4 aligns Section 4 to
> the current interface; patch 3/4 improves and clarifies what admission
> control means on UP an SMP systems; patch 4/4 introduces an appendix about
> testing.

For patch 1 & 2, please consider adding a Reviewed-by from me (there's 
some nitpick, but nothing serious).

For patch 3 & 4 I have some comments, but again, nothing severe. I'd like a 
short feedback before adding a reviewed-by though.

All in all, nice work! :D

> 
> Changes since v1:
> 
>  - fixed typos spotted by Randy and Peter (thanks!)
> 
> Changes since v2:
> 
>  - fixed typos and rewrote paragraphs as suggested by Ingo (thanks!)
> 
> Best Regards,
> 
> - Juri
> 
> Juri Lelli (2):
>   Documentation/scheduler/sched-deadline.txt: Rewrite section 4 intro
>   Documentation/scheduler/sched-deadline.txt: add tests suite appendix
> 
> Luca Abeni (2):
>   Documentation/scheduler/sched-deadline.txt: fix terminology and
>     improve clarity
>   Documentation/scheduler/sched-deadline.txt: improve and clarify AC
>     bits
> 
>  Documentation/scheduler/sched-deadline.txt | 204 ++++++++++++++++++++++-------
>  1 file changed, 159 insertions(+), 45 deletions(-)
> 
> -- 
> 2.0.4
> 

-- 
Henrik

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

[Luca Abeni]

Hi,

On 09/02/2014 11:10 PM, Henrik Austad wrote:
> On Thu, Aug 28, 2014 at 11:00:26AM +0100, Juri Lelli wrote:
>> 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"
>
> I'm tempted to say "earliest" (being part of the algorithm's name and all
> ;)
Well, AFAIR "closest" was suggested during the initial review some months ago...
Anyway, if now there is agreement on "earliest" I can change to it; let me know.

[...]
>>    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
>
> s/first/next/
>
> Also, next sentence does not make much sense, I would drop the also;
>
> "Thanks to this feature, tasks that do not strictly comply with the ..."
I agree with these changes, but they are in text that is not changed by my
patch, right?
What should I do? Add these changes to the patch, or send an additional incremental
patch with these changes?



				Thanks,
					Luca

Re: [PATCH v3 3/4] Documentation/scheduler/sched-deadline.txt: improve and clarify AC bits

[Luca Abeni]

Hi,

On 09/02/2014 11:45 PM, Henrik Austad wrote:
[...]
>> + On multiprocessor systems with global EDF scheduling (non partitioned
>> + systems), a sufficient test for schedulability can not be based on the
>> + utilisations (it can be shown that task sets with utilisations slightly
>> + larger than 1 can miss deadlines regardless of the number of CPUs M).
>> + However, as previously stated, enforcing that the total utilisation is smaller
>> + than M is enough to guarantee that non real-time tasks are not starved and
>> + that the tardiness of real-time tasks has an upper bound.
>
> I'd _really_ appreciate a link to a paper where all of this is presented
> and proved!
Well, my original plan was to add the bibliography in the next round of patches...
Is this ok?

[...]
>> + As already stated in Section 3, a necessary condition to be respected to
>> + correctly schedule a set of real-time tasks is that the total utilisation
>> + is smaller than M. When talking about -deadline tasks, this requires to
>> + impose that the sum of the ratio between runtime and period for all tasks
>> + is smaller than M.
>
> "This requires to impose that .." uhm, what? Drop 'to impose'.
Ok. I'll send an updated patch to Juri in few days


>> [...] Notice that the ratio runtime/period is equivalent to
>> + the utilisation of a "traditional" real-time task, and is also often
>> + referred to as "bandwidth".
>> + The interface used to control the CPU bandwidth that can be allocated
>> + to -deadline tasks is similar to the one already used for -rt
>>    tasks with real-time group scheduling (a.k.a. RT-throttling - see
>>    Documentation/scheduler/sched-rt-group.txt), and is based on readable/
>>    writable control files located in procfs (for system wide settings).
>> @@ -232,8 +285,16 @@ CONTENTS
>>    950000. With rt_period equal to 1000000, by default, it means that -deadline
>>    tasks can use at most 95%, multiplied by the number of CPUs that compose the
>>    root_domain, for each root_domain.
>> -
>> - A -deadline task cannot fork.
>> + This means that non -deadline tasks will receive at least 5% of the CPU time,
>> + and that -deadline tasks will receive their runtime with a guaranteed
>> + worst-case delay respect to the "deadline" parameter. If "deadline" = "period"
>> + and the cpuset mechanism is used to implement partitioned scheduling (see
>> + Section 5), then this simple setting of the bandwidth management is able to
>> + deterministically guarantee that -deadline tasks will receive their runtime
>> + in a period.
>
> The whole 950000 / 1000000, is at least 50 *consecutive* ms given to non
> rt/dl tasks every second, or is this more finegrained now?
>
> If the 50ms can be given in a single go, then I don't think you can
> guarantee that deadline-tasks will receive their runtime in a period - a
> period can be <50ms, no?
Uhmm... Maybe there is something I am missing in how the SCHED_DEADLINE admission
control is implemented, but I do not know about any "50 consecutive ms to non dl
tasks" rule. I agree that if there is such a rule then deadline tasks are screwed.
Juri?


>> + Finally, notice that in order not to jeopardize this admission control a
>> + -deadline task cannot fork.
>
> s/this/the
> (there aren't any other admission controls in the kernel)
Ok; this will go in my updated patch



			Thanks,
				Luca

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

[Luca Abeni]

On 09/03/2014 09:45 AM, Henrik Austad wrote:
[...]
>>      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
>>>>
>>>
>>> s/first/next/
>>>
>>> Also, next sentence does not make much sense, I would drop the also;
>>>
>>> "Thanks to this feature, tasks that do not strictly comply with the ..."
>>>
>> I agree with these changes, but they are in text that is not changed by my
>> patch, right?
>> What should I do? Add these changes to the patch, or send an additional
>> incremental
>> patch with these changes?
>>
>
> Patch is about clarity, right? I'd just add it to this patch.
Ok; I'll send an updated patch to Juri.



			Thanks,
				Luca

Re: [PATCH v3 3/4] Documentation/scheduler/sched-deadline.txt: improve and clarify AC bits

[Luca Abeni]

On 09/03/2014 09:48 AM, Henrik Austad wrote:
> On Wed, Sep 3, 2014 at 8:49 AM, Luca Abeni <luca.abeni@unitn.it> wrote:
>
>> Hi,
>>
>> On 09/02/2014 11:45 PM, Henrik Austad wrote:
>> [...]
>>
>>   + On multiprocessor systems with global EDF scheduling (non partitioned
>>>> + systems), a sufficient test for schedulability can not be based on the
>>>> + utilisations (it can be shown that task sets with utilisations slightly
>>>> + larger than 1 can miss deadlines regardless of the number of CPUs M).
>>>> + However, as previously stated, enforcing that the total utilisation is
>>>> smaller
>>>> + than M is enough to guarantee that non real-time tasks are not starved
>>>> and
>>>> + that the tardiness of real-time tasks has an upper bound.
>>>>
>>>
>>> I'd _really_ appreciate a link to a paper where all of this is presented
>>> and proved!
>>>
>> Well, my original plan was to add the bibliography in the next round of
>> patches...
>> Is this ok?
>>
>
> Sure, but I don't see why you cannot add this now
Well, there are multiple papers to be cited here: the one showing that
in a pathological case you can have missed deadlines on M CPUs with utilisation
slightly larger than 1 (Dhall's effect), one or more papers presenting a more
advanced admission control for global EDF, one or more papers showing the tardiness
bound, ...
So, I'd like to spend some time doing some research to properly cite the most
appropriate references.

> you (or Juri?) did update one of the papers in this series, right?
In patch 1, I updated the URL for a publically available technical report I wrote
in 1998 (the old URL was in a web site I cannot access anymore, so I changed the
URL to point to a web site I can control).

> As long as an article with all the headache ends up here, I'm happy :)
As said, if possible I'd like to do it in the next batch of patches, so that
these updates are not delayed... If this is not ok, I'll update the patch
adding some references, but this will take some time.



			Thanks,
				Luca

Re: [PATCH v3 0/4] SCHED_DEADLINE documentation fixes and improvements

[Ingo Molnar]

* Henrik Austad <henrik@austad.us> wrote:

> On Thu, Aug 28, 2014 at 11:00:25AM +0100, Juri Lelli wrote:
> > Hello everyone,
> > 
> > This is version 3 of a small patchset that fixes and improves SCHED_DEADLINE
> > documentation.
> > 
> > Patch 1/4 fixes and clarifies terminology; patch 2/4 aligns Section 4 to
> > the current interface; patch 3/4 improves and clarifies what admission
> > control means on UP an SMP systems; patch 4/4 introduces an appendix about
> > testing.
> 
> For patch 1 & 2, please consider adding a Reviewed-by from me 
> (there's some nitpick, but nothing serious).
> 
> For patch 3 & 4 I have some comments, but again, nothing 
> severe. I'd like a short feedback before adding a reviewed-by 
> though.

I had these queued up, but I'll wait for the next round of 
patches before applying them.

Thanks,

	Ingo

Re: [PATCH v3 3/4] Documentation/scheduler/sched-deadline.txt: improve and clarify AC bits

[Juri Lelli]

Hi,

On 03/09/14 07:49, Luca Abeni wrote:
> Hi,
> 
> On 09/02/2014 11:45 PM, Henrik Austad wrote:
> [...]
>>> + On multiprocessor systems with global EDF scheduling (non partitioned
>>> + systems), a sufficient test for schedulability can not be based on the
>>> + utilisations (it can be shown that task sets with utilisations slightly
>>> + larger than 1 can miss deadlines regardless of the number of CPUs M).
>>> + However, as previously stated, enforcing that the total utilisation is smaller
>>> + than M is enough to guarantee that non real-time tasks are not starved and
>>> + that the tardiness of real-time tasks has an upper bound.
>>
>> I'd _really_ appreciate a link to a paper where all of this is presented
>> and proved!
> Well, my original plan was to add the bibliography in the next round of patches...
> Is this ok?
> 
> [...]
>>> + As already stated in Section 3, a necessary condition to be respected to
>>> + correctly schedule a set of real-time tasks is that the total utilisation
>>> + is smaller than M. When talking about -deadline tasks, this requires to
>>> + impose that the sum of the ratio between runtime and period for all tasks
>>> + is smaller than M.
>>
>> "This requires to impose that .." uhm, what? Drop 'to impose'.
> Ok. I'll send an updated patch to Juri in few days
> 
> 
>>> [...] Notice that the ratio runtime/period is equivalent to
>>> + the utilisation of a "traditional" real-time task, and is also often
>>> + referred to as "bandwidth".
>>> + The interface used to control the CPU bandwidth that can be allocated
>>> + to -deadline tasks is similar to the one already used for -rt
>>>    tasks with real-time group scheduling (a.k.a. RT-throttling - see
>>>    Documentation/scheduler/sched-rt-group.txt), and is based on readable/
>>>    writable control files located in procfs (for system wide settings).
>>> @@ -232,8 +285,16 @@ CONTENTS
>>>    950000. With rt_period equal to 1000000, by default, it means that -deadline
>>>    tasks can use at most 95%, multiplied by the number of CPUs that compose the
>>>    root_domain, for each root_domain.
>>> -
>>> - A -deadline task cannot fork.
>>> + This means that non -deadline tasks will receive at least 5% of the CPU time,
>>> + and that -deadline tasks will receive their runtime with a guaranteed
>>> + worst-case delay respect to the "deadline" parameter. If "deadline" = "period"
>>> + and the cpuset mechanism is used to implement partitioned scheduling (see
>>> + Section 5), then this simple setting of the bandwidth management is able to
>>> + deterministically guarantee that -deadline tasks will receive their runtime
>>> + in a period.
>>
>> The whole 950000 / 1000000, is at least 50 *consecutive* ms given to non
>> rt/dl tasks every second, or is this more finegrained now?
>>
>> If the 50ms can be given in a single go, then I don't think you can
>> guarantee that deadline-tasks will receive their runtime in a period - a
>> period can be <50ms, no?
> Uhmm... Maybe there is something I am missing in how the SCHED_DEADLINE admission
> control is implemented, but I do not know about any "50 consecutive ms to non dl
> tasks" rule. I agree that if there is such a rule then deadline tasks are screwed.
> Juri?
> 
> 

In SCHED_DEADLINE we use those values only at admission control time (when
the user calls sched_setattr()). Then, at runtime, we use tasks' parameters
to perform scheduling. So there is no consecutive 50ms time for !SCHED_DEADLINE
tasks.

We could probably clarify this aspect in the previous patch with something
like this:

[snip]
+ The interface used to control the fraction of CPU bandwidth that can be
+ allocated to -deadline tasks is similar to the one already used for -rt
+ tasks with real-time group scheduling (a.k.a. RT-throttling - see
+ Documentation/scheduler/sched-rt-group.txt), and is based on readable/
+ writable control files located in procfs (for system wide settings).
+ Notice that per-group settings (controlled through cgroupfs) are still not
+ defined for -deadline tasks, because more discussion is needed in order to
+ figure out how we want to manage SCHED_DEADLINE bandwidth at the task group
+ level.
+
+ A main difference between deadline bandwidth management and RT-throttling
  is that -deadline tasks have bandwidth on their own (while -rt ones don't!),
- and thus we don't need an higher level throttling mechanism to enforce the
- desired bandwidth.
+ and thus we don't need a higher level throttling mechanism to enforce the
---->
+ desired bandwidth. In other words, this means that interface parameters are
+ only used at admission control time (i.e., when the user calls
+ sched_setattr()). Scheduling is then performed considering actual tasks'
+ parameters, so that CPU bandwidth is allocated to SCHED_DEADLINE tasks
+ respecting their needs in terms of granularity. Therefore, using this simple
<---
+ interface we can put a cap on total utilization of -deadline tasks (i.e.,
+ \Sum (runtime_i / period_i) < some_desired_value).
[snip]

What you think?

Thanks,

- Juri

>>> + Finally, notice that in order not to jeopardize this admission control a
>>> + -deadline task cannot fork.
>>
>> s/this/the
>> (there aren't any other admission controls in the kernel)
> Ok; this will go in my updated patch
> 
> 
> 
> 			Thanks,
> 				Luca
> 

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

[Juri Lelli]

Hi Henrik,

On 02/09/14 22:10, Henrik Austad wrote:
> On Thu, Aug 28, 2014 at 11:00:26AM +0100, Juri Lelli wrote:
>> 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"
> 
> I'm tempted to say "earliest" (being part of the algorithm's name and all 
> ;)
> 
>>
>> 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
> 
> s/first/next/
> 
> Also, next sentence does not make much sense, I would drop the also;
> 
> "Thanks to this feature, tasks that do not strictly comply with the ..."
> 
>>   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
> 
> Apart from the minor nitpick, it looks good!
> 

Great! Nitpicks fixed.

Thanks,

- Juri

Re: [PATCH v3 2/4] Documentation/scheduler/sched-deadline.txt: Rewrite section 4 intro

[Juri Lelli]

Hi Henrik,

On 02/09/14 22:14, Henrik Austad wrote:
> On Thu, Aug 28, 2014 at 11:00:27AM +0100, Juri Lelli wrote:
>> Section 4 intro was still describing the old interface. Rewrite it.
>>
>> Signed-off-by: Juri Lelli <juri.lelli@arm.com>
>> Signed-off-by: Luca Abeni <luca.abeni@unitn.it>
>> 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 | 51 +++++++++++++++---------------
>>  1 file changed, 25 insertions(+), 26 deletions(-)
>>
>> diff --git a/Documentation/scheduler/sched-deadline.txt b/Documentation/scheduler/sched-deadline.txt
>> index dce6d63..0aff2d5 100644
>> --- a/Documentation/scheduler/sched-deadline.txt
>> +++ b/Documentation/scheduler/sched-deadline.txt
>> @@ -165,39 +165,38 @@ CONTENTS
>>  
>>   In order for the -deadline scheduling to be effective and useful, it is
>>   important to have some method to keep the allocation of the available CPU
>> - bandwidth to the tasks under control.
>> - This is usually called "admission control" and if it is not performed at all,
>> - no guarantee can be given on the actual scheduling of the -deadline tasks.
>> -
>> - Since when RT-throttling has been introduced each task group has a bandwidth
>> - associated, calculated as a certain amount of runtime over a period.
>> - Moreover, to make it possible to manipulate such bandwidth, readable/writable
>> - controls have been added to both procfs (for system wide settings) and cgroupfs
>> - (for per-group settings).
>> - Therefore, the same interface is being used for controlling the bandwidth
>> - distrubution to -deadline tasks.
>> -
>> - However, more discussion is needed in order to figure out how we want to manage
>> - SCHED_DEADLINE bandwidth at the task group level. Therefore, SCHED_DEADLINE
>> - uses (for now) a less sophisticated, but actually very sensible, mechanism to
>> - ensure that a certain utilization cap is not overcome per each root_domain.
>> -
>> - Another main difference between deadline bandwidth management and RT-throttling
>> + bandwidth to the tasks under control. This is usually called "admission
>> + control" and if it is not performed at all, no guarantee can be given on
>> + the actual scheduling of the -deadline tasks.
>> +
>> + The interface used to control the fraction of CPU bandwidth that can be
>> + allocated to -deadline tasks is similar to the one already used for -rt
>> + tasks with real-time group scheduling (a.k.a. RT-throttling - see
>> + Documentation/scheduler/sched-rt-group.txt), and is based on readable/
>> + writable control files located in procfs (for system wide settings).
>> + Notice that per-group settings (controlled through cgroupfs) are still not
>> + defined for -deadline tasks, because more discussion is needed in order to
>> + figure out how we want to manage SCHED_DEADLINE bandwidth at the task group
>> + level.
>> +
>> + A main difference between deadline bandwidth management and RT-throttling
>>   is that -deadline tasks have bandwidth on their own (while -rt ones don't!),
>> - and thus we don't need an higher level throttling mechanism to enforce the
>> - desired bandwidth.
>> + and thus we don't need a higher level throttling mechanism to enforce the
>> + desired bandwidth. Therefore, using this simple interface we can put a cap
>> + on total utilization of -deadline tasks (i.e., \Sum (runtime_i / period_i) <
>> + some_desired_value).
> 
> s/some_desired_value/global_dl_utilization_cap/  perhaps?
> 

Ok, fixed.

>>  4.1 System wide settings
>>  ------------------------
>>  
>>   The system wide settings are configured under the /proc virtual file system.
>>  
>> - For now the -rt knobs are used for dl admission control and the -deadline
>> - runtime is accounted against the -rt runtime. We realise that this isn't
>> - entirely desirable; however, it is better to have a small interface for now,
>> - and be able to change it easily later. The ideal situation (see 5.) is to run
>> - -rt tasks from a -deadline server; in which case the -rt bandwidth is a direct
>> - subset of dl_bw.
>> + For now the -rt knobs are used for -deadline admission control and the
>> + -deadline runtime is accounted against the -rt runtime. We realise that this
>> + isn't entirely desirable; however, it is better to have a small interface for
>> + now, and be able to change it easily later. The ideal situation (see 5.) is to
>> + run -rt tasks from a -deadline server; in which case the -rt bandwidth is a
>> + direct subset of dl_bw.
> 
> Isn't forking forbidden for _dl tasks? Or did I miss a memo somewhere?
> 

Yes, forking is forbidden. But, that's mainly because it is not entirely
clear how bandwidth has to be split between parent and child(ren). When
we'll have -rt tasks running from a -deadline server this problem will
disappear, as children will share bandwidth with other tasks in the same
server.

>>   This means that, for a root_domain comprising M CPUs, -deadline tasks
>>   can be created while the sum of their bandwidths stays below:
>> -- 
>> 2.0.4
>>
>>
> 
> All in all, looks pretty good to me!
> 

Great!

Thanks,

- Juri

Re: [PATCH v3 3/4] Documentation/scheduler/sched-deadline.txt: improve and clarify AC bits

[Juri Lelli]

Hi,

On 03/09/14 10:18, Juri Lelli wrote:
> Hi,
> 
> On 03/09/14 07:49, Luca Abeni wrote:
>> Hi,
>>
>> On 09/02/2014 11:45 PM, Henrik Austad wrote:
>> [...]
>>>> + On multiprocessor systems with global EDF scheduling (non partitioned
>>>> + systems), a sufficient test for schedulability can not be based on the
>>>> + utilisations (it can be shown that task sets with utilisations slightly
>>>> + larger than 1 can miss deadlines regardless of the number of CPUs M).
>>>> + However, as previously stated, enforcing that the total utilisation is smaller
>>>> + than M is enough to guarantee that non real-time tasks are not starved and
>>>> + that the tardiness of real-time tasks has an upper bound.
>>>
>>> I'd _really_ appreciate a link to a paper where all of this is presented
>>> and proved!
>> Well, my original plan was to add the bibliography in the next round of patches...
>> Is this ok?
>>
>> [...]
>>>> + As already stated in Section 3, a necessary condition to be respected to
>>>> + correctly schedule a set of real-time tasks is that the total utilisation
>>>> + is smaller than M. When talking about -deadline tasks, this requires to
>>>> + impose that the sum of the ratio between runtime and period for all tasks
>>>> + is smaller than M.
>>>
>>> "This requires to impose that .." uhm, what? Drop 'to impose'.
>> Ok. I'll send an updated patch to Juri in few days
>>
>>
>>>> [...] Notice that the ratio runtime/period is equivalent to
>>>> + the utilisation of a "traditional" real-time task, and is also often
>>>> + referred to as "bandwidth".
>>>> + The interface used to control the CPU bandwidth that can be allocated
>>>> + to -deadline tasks is similar to the one already used for -rt
>>>>    tasks with real-time group scheduling (a.k.a. RT-throttling - see
>>>>    Documentation/scheduler/sched-rt-group.txt), and is based on readable/
>>>>    writable control files located in procfs (for system wide settings).
>>>> @@ -232,8 +285,16 @@ CONTENTS
>>>>    950000. With rt_period equal to 1000000, by default, it means that -deadline
>>>>    tasks can use at most 95%, multiplied by the number of CPUs that compose the
>>>>    root_domain, for each root_domain.
>>>> -
>>>> - A -deadline task cannot fork.
>>>> + This means that non -deadline tasks will receive at least 5% of the CPU time,
>>>> + and that -deadline tasks will receive their runtime with a guaranteed
>>>> + worst-case delay respect to the "deadline" parameter. If "deadline" = "period"
>>>> + and the cpuset mechanism is used to implement partitioned scheduling (see
>>>> + Section 5), then this simple setting of the bandwidth management is able to
>>>> + deterministically guarantee that -deadline tasks will receive their runtime
>>>> + in a period.
>>>
>>> The whole 950000 / 1000000, is at least 50 *consecutive* ms given to non
>>> rt/dl tasks every second, or is this more finegrained now?
>>>
>>> If the 50ms can be given in a single go, then I don't think you can
>>> guarantee that deadline-tasks will receive their runtime in a period - a
>>> period can be <50ms, no?
>> Uhmm... Maybe there is something I am missing in how the SCHED_DEADLINE admission
>> control is implemented, but I do not know about any "50 consecutive ms to non dl
>> tasks" rule. I agree that if there is such a rule then deadline tasks are screwed.
>> Juri?
>>
>>
> 
> In SCHED_DEADLINE we use those values only at admission control time (when
> the user calls sched_setattr()). Then, at runtime, we use tasks' parameters
> to perform scheduling. So there is no consecutive 50ms time for !SCHED_DEADLINE
> tasks.
> 
> We could probably clarify this aspect in the previous patch with something
> like this:
> 
> [snip]
> + The interface used to control the fraction of CPU bandwidth that can be
> + allocated to -deadline tasks is similar to the one already used for -rt
> + tasks with real-time group scheduling (a.k.a. RT-throttling - see
> + Documentation/scheduler/sched-rt-group.txt), and is based on readable/
> + writable control files located in procfs (for system wide settings).
> + Notice that per-group settings (controlled through cgroupfs) are still not
> + defined for -deadline tasks, because more discussion is needed in order to
> + figure out how we want to manage SCHED_DEADLINE bandwidth at the task group
> + level.
> +
> + A main difference between deadline bandwidth management and RT-throttling
>   is that -deadline tasks have bandwidth on their own (while -rt ones don't!),
> - and thus we don't need an higher level throttling mechanism to enforce the
> - desired bandwidth.
> + and thus we don't need a higher level throttling mechanism to enforce the
> ---->
> + desired bandwidth. In other words, this means that interface parameters are
> + only used at admission control time (i.e., when the user calls
> + sched_setattr()). Scheduling is then performed considering actual tasks'
> + parameters, so that CPU bandwidth is allocated to SCHED_DEADLINE tasks
> + respecting their needs in terms of granularity. Therefore, using this simple
> <---
> + interface we can put a cap on total utilization of -deadline tasks (i.e.,
> + \Sum (runtime_i / period_i) < some_desired_value).
> [snip]
> 
> What you think?
> 

As agreed on IRC I fixed this, and the other typos.

Thanks,

- Juri

>>>> + Finally, notice that in order not to jeopardize this admission control a
>>>> + -deadline task cannot fork.
>>>
>>> s/this/the
>>> (there aren't any other admission controls in the kernel)
>> Ok; this will go in my updated patch
>>
>>
>>
>> 			Thanks,
>> 				Luca
>>
> 
> 

Re: [PATCH v3 4/4] Documentation/scheduler/sched-deadline.txt: add tests suite appendix

[Juri Lelli]

Hi Henrik,

On 02/09/14 22:53, Henrik Austad wrote:
> On Thu, Aug 28, 2014 at 11:00:29AM +0100, Juri Lelli wrote:
>> Add an appendix briefly describing tools that can be used to test SCHED_DEADLINE
>> (and the scheduler in general). Links to where source code of the tools is hosted
>> are also provided.
>>
>> 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 | 52 ++++++++++++++++++++++++++++++
>>  1 file changed, 52 insertions(+)
>>
>> diff --git a/Documentation/scheduler/sched-deadline.txt b/Documentation/scheduler/sched-deadline.txt
>> index 641395e..2f5b174 100644
>> --- a/Documentation/scheduler/sched-deadline.txt
>> +++ b/Documentation/scheduler/sched-deadline.txt
>> @@ -15,6 +15,7 @@ CONTENTS
>>   5. Tasks CPU affinity
>>     5.1 SCHED_DEADLINE and cpusets HOWTO
>>   6. Future plans
>> + A. Test suite
>>  
>>  
>>  0. WARNING
>> @@ -341,3 +342,54 @@ CONTENTS
>>   throttling patches [https://lkml.org/lkml/2010/2/23/239] but we still are in
>>   the preliminary phases of the merge and we really seek feedback that would
>>   help us decide on the direction it should take.
>> +
>> +Appendix A. Test suite
>> +======================
>> +
>> + The SCHED_DEADLINE policy can be easily tested using two applications that
>> + are part of a wider Linux Scheduler validation suite. The suite is
>> + available as a GitHub repository: https://github.com/scheduler-tools.
>> +
>> + The first testing application is called rt-app and can be used to
>> + start multiple threads with specific parameters. rt-app supports
>> + SCHED_{OTHER,FIFO,RR,DEADLINE} scheduling policies and their related
>> + parameters (e.g., niceness, priority, runtime/deadline/period). rt-app
>> + is a valuable tool, as it can be used to synthetically recreate certain
>> + workloads (maybe mimicking real use-cases) and evaluate how the scheduler
>> + behaves under such workloads. In this way, results are easily reproducible.
>> + rt-app is available at: https://github.com/scheduler-tools/rt-app.
>> +
>> + Thread parameters can be specified from the command line, with something like
>> + this:
>> +
>> +  # rt-app -t 100000:10000:d -t 150000:20000:f:10 -D5
>> +
>> + The above creates two threads. The first one, scheduled by SCHED_DEADLINE,
>> + executes for 10ms every 100ms and the second one, scheduled at RT priority 10
>> + with SCHED_FIFO, executes for 20ms every 150ms. The configuration runs
>> + for 5 seconds.
> 
> I'd prefer
> 
> The above creates 2 threads, T1 and T2. T1 is scheduled by SCHED_DEADLINE 
> with a 100ms period and 10ms WCET. T2 is handled by SCHED_FIFO priority 10, 
> 150ms period and 20ms WCET. The test will run for a total of 5 seconds.
> 
> One can expect the eager reader to have at least a grasp of the terminology 
> at this stage, using WCET should be acceptable.
> 

Not sure that naming the two threads T1 and T2 won't induce the reader to look
for such names in the system when the test is running. And the actual comms are
subject to change in future versions of rt-app. Also, giving the distinction
between tasks WCET (and BCET in this case) and the runtime you choose to
schedule them, I'd rather remain more general. So, what about this instead?

The above creates 2 threads. The first one, scheduled by SCHED_DEADLINE,
executes for 10ms every 100ms. The second one, scheduled at SCHED_FIFO
priority 10, executes for 20ms every 150ms. The test will run for a total
of 5 seconds.

>> +
>> + More interestingly, configurations can be described with a json file that
>> + can be passed as input to rt-app with something like this:
>> +
>> +  # rt-app my_config.json
>> +
>> + The parameters that can be specified with the second method are a superset
>> + of the command line options. Please refer to rt-app documentation for more
>> + details.
> 
> This can be found that this url: ....
> 

There is not a url yet (apart from the github repo). But, I can add:

(doc/*.json)

>> +
>> + The second testing application is a modification of schedtool, called
>> + schedtool-dl, which can be used to setup SCHED_DEADLINE parameters for a
>> + certain pid/application. schedtool-dl is available at:
>> + https://github.com/scheduler-tools/schedtool-dl.git.
>> +
>> + The usage is straightforward:
>> +
>> +  # schedtool -E -t 10000000:100000000 -e ./my_cpuhog_app
>> +
>> + With this, my_cpuhog_app is put to run inside a SCHED_DEADLINE reservation
>> + of 10ms every 100ms (note that parameters are expressed in microseconds).
>> + You can also use schedtool to create a reservation for an already running
>> + application, given that you know its pid:
>> +
>> +  # schedtool -E -t 10000000:100000000 my_app_pid
>> -- 
>> 2.0.4
> 
> Would it make sense to add an appendix B with a minimal SCHED_DEADLINE 
> main() ? I for one like to invent my own wheels.
> 

Ok, that could look like what follows (in a separate patch). What you
think?

Thanks,

- Juri

Appendix B. Minimal main()
==========================

We provide in what follows a simple (ugly) self-contained code snippet
showing how SCHED_DEADLINE reservations can be created by a real-time
application developer.

#define _GNU_SOURCE
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <linux/unistd.h>
#include <linux/kernel.h>
#include <linux/types.h>
#include <sys/syscall.h>
#include <pthread.h>

#define gettid() syscall(__NR_gettid)

#define SCHED_DEADLINE	6

/* XXX use the proper syscall numbers */
#ifdef __x86_64__
#define __NR_sched_setattr		314
#define __NR_sched_getattr		315
#endif

#ifdef __i386__
#define __NR_sched_setattr		351
#define __NR_sched_getattr		352
#endif

#ifdef __arm__
#define __NR_sched_setattr		380
#define __NR_sched_getattr		381
#endif

static volatile int done;

struct sched_attr {
	__u32 size;
	
	__u32 sched_policy;
	__u64 sched_flags;
	
	/* SCHED_NORMAL, SCHED_BATCH */
	__s32 sched_nice;
	
	/* SCHED_FIFO, SCHED_RR */
	__u32 sched_priority;
	
	/* SCHED_DEADLINE (nsec) */
	__u64 sched_runtime;
	__u64 sched_deadline;
	__u64 sched_period;
};

int sched_setattr(pid_t pid,
		  const struct sched_attr *attr,
		  unsigned int flags)
{
	return syscall(__NR_sched_setattr, pid, attr, flags);
}

int sched_getattr(pid_t pid,
		  struct sched_attr *attr,
		  unsigned int size,
		  unsigned int flags)
{
	return syscall(__NR_sched_getattr, pid, attr, size, flags);
}

void *run_deadline(void *data)
{
	struct sched_attr attr;
	int x = 0;
	int ret;
	unsigned int flags = 0;

	printf("deadline thread started [%ld]\n", gettid());

	attr.size = sizeof(attr);
	attr.sched_flags = 0;
	attr.sched_nice = 0;
	attr.sched_priority = 0;

	/* This creates a 10ms/30ms reservation */
	attr.sched_policy = SCHED_DEADLINE;
	attr.sched_runtime = 10 * 1000 * 1000;
	attr.sched_period = attr.sched_deadline = 30 * 1000 * 1000;

	ret = sched_setattr(0, &attr, flags);
	if (ret < 0) {
		done = 0;
		perror("sched_setattr");
		exit(-1);
	}

	while (!done) {
		x++;
	}

	printf("deadline thread dies [%ld]\n", gettid());
	return NULL;
}

int main (int argc, char **argv)
{
	pthread_t thread;

	printf("main thread [%ld]\n", gettid());

	pthread_create(&thread, NULL, run_deadline, NULL);

	sleep(10);

	done = 1;
	pthread_join(thread, NULL);

	printf("main dies [%ld]\n", gettid());
	return 0;
}