282 строки
12 KiB
Plaintext
282 строки
12 KiB
Plaintext
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.
|