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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
----------------
-
-Realtime 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 realtime tasks, each group must be assigned a fixed portion
-of the CPU time available. Without a minimum guarantee a realtime 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 realtime group which
-the other realtime groups will not be permitted to use.
-
-Any time not allocated to a realtime 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 realtime 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
-realtime 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 realtime scheduling may use. Even without
- CONFIG_RT_GROUP_SCHED enabled, this will limit time reserved to realtime
- processes. With CONFIG_RT_GROUP_SCHED it signifies the total bandwidth
- available to all realtime 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 (INT_MAX - 1).
- * 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
-realtime 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.
-
-Realtime group scheduling means you have to assign a portion of total CPU
-bandwidth to the group before it will accept realtime tasks. Therefore you will
-not be able to run realtime tasks as any user other than root until you have
-done that, even if the user has the rights to run processes with realtime
-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/cgroups/cgroups.txt 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=10000us
- - this runs for 0.01s once every 0.1s
-
-* group B: period= 50000us, runtime=10000us
- - this runs for 0.01s 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.