Documentation/scheduler/sched-deadline.txt: Rewrite section 4 intro

[linux-block.git] / Documentation / scheduler / sched-deadline.txt

			  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
 earliest scheduling deadline is selected for execution). Notice that this
 guaranteed is respected if a proper "admission control" strategy (see Section
 "4. Bandwidth management") is used.

 Summing up, the CBS[2,3] algorithms assigns scheduling deadlines to tasks so
 that each task runs for at most its runtime every period, avoiding any
 interference between different tasks (bandwidth isolation), while the EDF[1]
 algorithm selects the task with the earliest scheduling deadline as the one
 to be executed next. Thanks to this feature, tasks that do not strictly comply
 with the "traditional" real-time task model (see Section 3) can effectively
 use the new policy.

 In more details, the CBS algorithm assigns scheduling deadlines to
 tasks in the following way:

  - 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 "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

                 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
    remaining runtime are re-initialised as

         scheduling deadline = current time + deadline
         remaining runtime = runtime

    otherwise, the scheduling deadline and the remaining runtime are
    left unchanged;

  - When a SCHED_DEADLINE task executes for an amount of time t, its
    remaining runtime is decreased as

         remaining runtime = remaining runtime - t

    (technically, the runtime is decreased at every tick, or when the
    task is descheduled / preempted);

  - 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 remaining runtime are
    updated as

         scheduling deadline = scheduling deadline + period
         remaining runtime = remaining 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]).
 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-
      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://disi.unitn.it/~abeni/tr-98-01.pdf

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
 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 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) <
 global_dl_utilization_cap).

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 -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:

   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.