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Deadline Task Scheduling
------------------------
CONTENTS
========
0. WARNING
1. Overview
2. Scheduling algorithm
3. Scheduling Real-Time Tasks
4. Bandwidth management
4.1 System-wide settings
4.2 Task interface
4.3 Default behavior
5. Tasks CPU affinity
5.1 SCHED_DEADLINE and cpusets HOWTO
6. Future plans
0. WARNING
==========
Fiddling with these settings can result in an unpredictable or even unstable
system behavior. As for -rt (group) scheduling, it is assumed that root users
know what they're doing.
1. Overview
===========
The SCHED_DEADLINE policy contained inside the sched_dl scheduling class is
basically an implementation of the Earliest Deadline First (EDF) scheduling
algorithm, augmented with a mechanism (called Constant Bandwidth Server, CBS)
that makes it possible to isolate the behavior of tasks between each other.
2. Scheduling algorithm
==================
SCHED_DEADLINE uses three parameters, named "runtime", "period", and
"deadline" to schedule tasks. A SCHED_DEADLINE task is guaranteed to 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
smallest scheduling deadline is selected for execution). Notice that this
guaranteed is respected if a proper "admission control" strategy (see Section
"4. Bandwidth management") is used.
Summing up, the CBS[2,3] algorithms assigns scheduling deadlines to tasks so
that each task runs for at most its runtime every period, avoiding any
interference between different tasks (bandwidth isolation), while the EDF[1]
algorithm selects the task with the smallest scheduling deadline as the one
to be executed first. Thanks to this feature, also tasks that do not
strictly comply with the "traditional" real-time task model (see Section 3)
can effectively use the new policy.
In more details, the CBS algorithm assigns scheduling deadlines to
tasks in the following way:
- Each SCHED_DEADLINE task is characterised by the "runtime",
"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;
- When a SCHED_DEADLINE task wakes up (becomes ready for execution),
the scheduler checks if
current 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
scheduling deadline = current time + deadline
current runtime = runtime
otherwise, the scheduling deadline and the current runtime are
left unchanged;
- When a SCHED_DEADLINE task executes for an amount of time t, its
current runtime is decreased as
current runtime = current 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
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
updated as
scheduling deadline = scheduling deadline + period
current runtime = current runtime + runtime
3. Scheduling Real-Time Tasks
=============================
* BIG FAT WARNING ******************************************************
*
* This section contains a (not-thorough) summary on classical deadline
* scheduling theory, and how it applies to SCHED_DEADLINE.
* The reader can "safely" skip to Section 4 if only interested in seeing
* how the scheduling policy can be used. Anyway, we strongly recommend
* to come back here and continue reading (once the urge for testing is
* satisfied :P) to be sure of fully understanding all technical details.
************************************************************************
There are no limitations on what kind of task can exploit this new
scheduling discipline, even if it must be said that it is particularly
suited for periodic or sporadic real-time tasks that need guarantees on their
timing behavior, e.g., multimedia, streaming, control applications, etc.
A typical real-time task is composed of a repetition of computation phases
(task instances, or jobs) which are activated on a periodic or sporadic
fashion.
Each job J_j (where J_j is the j^th job of the task) is characterised by an
arrival time r_j (the time when the job starts), an amount of computation
time c_j needed to finish the job, and a job absolute deadline d_j, which
is the time within which the job should be finished. The maximum execution
time max_j{c_j} is called "Worst Case Execution Time" (WCET) for the task.
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.
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
must be scheduled by setting:
- runtime >= WCET
- deadline = D
- period <= P
IOW, if runtime >= WCET and if period is >= P, then the scheduling deadlines
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]).
References:
1 - C. L. Liu and J. W. Layland. Scheduling algorithms for multiprogram-
ming in a hard-real-time environment. Journal of the Association for
Computing Machinery, 20(1), 1973.
2 - L. Abeni , G. Buttazzo. Integrating Multimedia Applications in Hard
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
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.
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
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.
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.
This means that, for a root_domain comprising M CPUs, -deadline tasks
can be created while the sum of their bandwidths stays below:
M * (sched_rt_runtime_us / sched_rt_period_us)
It is also possible to disable this bandwidth management logic, and
be thus free of oversubscribing the system up to any arbitrary level.
This is done by writing -1 in /proc/sys/kernel/sched_rt_runtime_us.
4.2 Task interface
------------------
Specifying a periodic/sporadic task that executes for a given amount of
runtime at each instance, and that is scheduled according to the urgency of
its own timing constraints needs, in general, a way of declaring:
- a (maximum/typical) instance execution time,
- a minimum interval between consecutive instances,
- a time constraint by which each instance must be completed.
Therefore:
* a new struct sched_attr, containing all the necessary fields is
provided;
* the new scheduling related syscalls that manipulate it, i.e.,
sched_setattr() and sched_getattr() are implemented.
4.3 Default behavior
---------------------
The default value for SCHED_DEADLINE bandwidth is to have rt_runtime equal to
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.
5. Tasks CPU affinity
=====================
-deadline tasks cannot have an affinity mask smaller that the entire
root_domain they are created on. However, affinities can be specified
through the cpuset facility (Documentation/cgroups/cpusets.txt).
5.1 SCHED_DEADLINE and cpusets HOWTO
------------------------------------
An example of a simple configuration (pin a -deadline task to CPU0)
follows (rt-app is used to create a -deadline task).
mkdir /dev/cpuset
mount -t cgroup -o cpuset cpuset /dev/cpuset
cd /dev/cpuset
mkdir cpu0
echo 0 > cpu0/cpuset.cpus
echo 0 > cpu0/cpuset.mems
echo 1 > cpuset.cpu_exclusive
echo 0 > cpuset.sched_load_balance
echo 1 > cpu0/cpuset.cpu_exclusive
echo 1 > cpu0/cpuset.mem_exclusive
echo $$ > cpu0/tasks
rt-app -t 100000:10000:d:0 -D5 (it is now actually superfluous to specify
task affinity)
6. Future plans
===============
Still missing:
- refinements to deadline inheritance, especially regarding the possibility
of retaining bandwidth isolation among non-interacting tasks. This is
being studied from both theoretical and practical points of view, and
hopefully we should be able to produce some demonstrative code soon;
- (c)group based bandwidth management, and maybe scheduling;
- access control for non-root users (and related security concerns to
address), which is the best way to allow unprivileged use of the mechanisms
and how to prevent non-root users "cheat" the system?
As already discussed, we are planning also to merge this work with the EDF
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.
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