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==========================
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. Even without CONFIG_RT_GROUP_SCHED enabled,
this will limit time reserved to real-time processes. With
CONFIG_RT_GROUP_SCHED=y it 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.
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