==========================
Real-Time group scheduling
==========================

.. CONTENTS

   0. WARNING
   1. Overview
     1.1 The problem
     1.2 The solution
   2. The interface
     2.1 System-wide settings
     2.2 Default behaviour
     2.3 Basis for grouping tasks
   3. Future plans


0. WARNING
==========

 Fiddling with these settings can result in an unstable system, the knobs are
 root only and assumes root knows what he is doing.

Most notable:

 * very small values in sched_rt_period_us can result in an unstable
   system when the period is smaller than either the available hrtimer
   resolution, or the time it takes to handle the budget refresh itself.

 * very small values in sched_rt_runtime_us can result in an unstable
   system when the runtime is so small the system has difficulty making
   forward progress (NOTE: the migration thread and kstopmachine both
   are real-time processes).

1. Overview
===========


1.1 The problem
---------------

Real-time scheduling is all about determinism, a group has to be able to rely on
the amount of bandwidth (eg. CPU time) being constant. In order to schedule
multiple groups of real-time tasks, each group must be assigned a fixed portion
of the CPU time available.  Without a minimum guarantee a real-time group can
obviously fall short. A fuzzy upper limit is of no use since it cannot be
relied upon. Which leaves us with just the single fixed portion.

1.2 The solution
----------------

CPU time is divided by means of specifying how much time can be spent running
in a given period. We allocate this "run time" for each real-time group which
the other real-time groups will not be permitted to use.

Any time not allocated to a real-time group will be used to run normal priority
tasks (SCHED_OTHER). Any allocated run time not used will also be picked up by
SCHED_OTHER.

Let's consider an example: a frame fixed real-time renderer must deliver 25
frames a second, which yields a period of 0.04s per frame. Now say it will also
have to play some music and respond to input, leaving it with around 80% CPU
time dedicated for the graphics. We can then give this group a run time of 0.8
* 0.04s = 0.032s.

This way the graphics group will have a 0.04s period with a 0.032s run time
limit. Now if the audio thread needs to refill the DMA buffer every 0.005s, but
needs only about 3% CPU time to do so, it can do with a 0.03 * 0.005s =
0.00015s. So this group can be scheduled with a period of 0.005s and a run time
of 0.00015s.

The remaining CPU time will be used for user input and other tasks. Because
real-time tasks have explicitly allocated the CPU time they need to perform
their tasks, buffer underruns in the graphics or audio can be eliminated.

NOTE: the above example is not fully implemented yet. We still
lack an EDF scheduler to make non-uniform periods usable.


2. The Interface
================


2.1 System wide settings
------------------------

The system wide settings are configured under the /proc virtual file system:

/proc/sys/kernel/sched_rt_period_us:
  The scheduling period that is equivalent to 100% CPU bandwidth.

/proc/sys/kernel/sched_rt_runtime_us:
  A global limit on how much time real-time scheduling may use. This is always
  less or equal to the period_us, as it denotes the time allocated from the
  period_us for the real-time tasks. Without CONFIG_RT_GROUP_SCHED enabled,
  this only serves for admission control of deadline tasks. With
  CONFIG_RT_GROUP_SCHED=y it also signifies the total bandwidth available to
  all real-time groups.

  * Time is specified in us because the interface is s32. This gives an
    operating range from 1us to about 35 minutes.
  * sched_rt_period_us takes values from 1 to INT_MAX.
  * sched_rt_runtime_us takes values from -1 to sched_rt_period_us.
  * A run time of -1 specifies runtime == period, ie. no limit.


2.2 Default behaviour
---------------------

The default values for sched_rt_period_us (1000000 or 1s) and
sched_rt_runtime_us (950000 or 0.95s).  This gives 0.05s to be used by
SCHED_OTHER (non-RT tasks). These defaults were chosen so that a run-away
real-time tasks will not lock up the machine but leave a little time to recover
it.  By setting runtime to -1 you'd get the old behaviour back.

By default all bandwidth is assigned to the root group and new groups get the
period from /proc/sys/kernel/sched_rt_period_us and a run time of 0. If you
want to assign bandwidth to another group, reduce the root group's bandwidth
and assign some or all of the difference to another group.

Real-time group scheduling means you have to assign a portion of total CPU
bandwidth to the group before it will accept real-time tasks. Therefore you will
not be able to run real-time tasks as any user other than root until you have
done that, even if the user has the rights to run processes with real-time
priority!


2.3 Basis for grouping tasks
----------------------------

Enabling CONFIG_RT_GROUP_SCHED lets you explicitly allocate real
CPU bandwidth to task groups.

This uses the cgroup virtual file system and "<cgroup>/cpu.rt_runtime_us"
to control the CPU time reserved for each control group.

For more information on working with control groups, you should read
Documentation/admin-guide/cgroup-v1/cgroups.rst as well.

Group settings are checked against the following limits in order to keep the
configuration schedulable:

   \Sum_{i} runtime_{i} / global_period <= global_runtime / global_period

For now, this can be simplified to just the following (but see Future plans):

   \Sum_{i} runtime_{i} <= global_runtime


3. Future plans
===============

There is work in progress to make the scheduling period for each group
("<cgroup>/cpu.rt_period_us") configurable as well.

The constraint on the period is that a subgroup must have a smaller or
equal period to its parent. But realistically its not very useful _yet_
as its prone to starvation without deadline scheduling.

Consider two sibling groups A and B; both have 50% bandwidth, but A's
period is twice the length of B's.

* group A: period=100000us, runtime=50000us

	- this runs for 0.05s once every 0.1s

* group B: period= 50000us, runtime=25000us

	- this runs for 0.025s twice every 0.1s (or once every 0.05 sec).

This means that currently a while (1) loop in A will run for the full period of
B and can starve B's tasks (assuming they are of lower priority) for a whole
period.

The next project will be SCHED_EDF (Earliest Deadline First scheduling) to bring
full deadline scheduling to the linux kernel. Deadline scheduling the above
groups and treating end of the period as a deadline will ensure that they both
get their allocated time.

Implementing SCHED_EDF might take a while to complete. Priority Inheritance is
the biggest challenge as the current linux PI infrastructure is geared towards
the limited static priority levels 0-99. With deadline scheduling you need to
do deadline inheritance (since priority is inversely proportional to the
deadline delta (deadline - now)).

This means the whole PI machinery will have to be reworked - and that is one of
the most complex pieces of code we have.
