184 lines
6.8 KiB
Plaintext
184 lines
6.8 KiB
Plaintext
Real-Time group scheduling
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--------------------------
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CONTENTS
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========
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0. WARNING
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1. Overview
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1.1 The problem
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1.2 The solution
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2. The interface
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2.1 System-wide settings
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2.2 Default behaviour
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2.3 Basis for grouping tasks
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3. Future plans
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0. WARNING
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==========
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Fiddling with these settings can result in an unstable system, the knobs are
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root only and assumes root knows what he is doing.
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Most notable:
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* very small values in sched_rt_period_us can result in an unstable
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system when the period is smaller than either the available hrtimer
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resolution, or the time it takes to handle the budget refresh itself.
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* very small values in sched_rt_runtime_us can result in an unstable
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system when the runtime is so small the system has difficulty making
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forward progress (NOTE: the migration thread and kstopmachine both
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are real-time processes).
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1. Overview
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===========
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1.1 The problem
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---------------
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Realtime scheduling is all about determinism, a group has to be able to rely on
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the amount of bandwidth (eg. CPU time) being constant. In order to schedule
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multiple groups of realtime tasks, each group must be assigned a fixed portion
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of the CPU time available. Without a minimum guarantee a realtime group can
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obviously fall short. A fuzzy upper limit is of no use since it cannot be
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relied upon. Which leaves us with just the single fixed portion.
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1.2 The solution
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----------------
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CPU time is divided by means of specifying how much time can be spent running
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in a given period. We allocate this "run time" for each realtime group which
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the other realtime groups will not be permitted to use.
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Any time not allocated to a realtime group will be used to run normal priority
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tasks (SCHED_OTHER). Any allocated run time not used will also be picked up by
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SCHED_OTHER.
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Let's consider an example: a frame fixed realtime renderer must deliver 25
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frames a second, which yields a period of 0.04s per frame. Now say it will also
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have to play some music and respond to input, leaving it with around 80% CPU
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time dedicated for the graphics. We can then give this group a run time of 0.8
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* 0.04s = 0.032s.
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This way the graphics group will have a 0.04s period with a 0.032s run time
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limit. Now if the audio thread needs to refill the DMA buffer every 0.005s, but
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needs only about 3% CPU time to do so, it can do with a 0.03 * 0.005s =
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0.00015s. So this group can be scheduled with a period of 0.005s and a run time
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of 0.00015s.
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The remaining CPU time will be used for user input and other tasks. Because
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realtime tasks have explicitly allocated the CPU time they need to perform
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their tasks, buffer underruns in the graphics or audio can be eliminated.
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NOTE: the above example is not fully implemented yet. We still
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lack an EDF scheduler to make non-uniform periods usable.
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2. The Interface
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================
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2.1 System wide settings
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------------------------
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The system wide settings are configured under the /proc virtual file system:
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/proc/sys/kernel/sched_rt_period_us:
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The scheduling period that is equivalent to 100% CPU bandwidth
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/proc/sys/kernel/sched_rt_runtime_us:
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A global limit on how much time realtime scheduling may use. Even without
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CONFIG_RT_GROUP_SCHED enabled, this will limit time reserved to realtime
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processes. With CONFIG_RT_GROUP_SCHED it signifies the total bandwidth
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available to all realtime groups.
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* Time is specified in us because the interface is s32. This gives an
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operating range from 1us to about 35 minutes.
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* sched_rt_period_us takes values from 1 to INT_MAX.
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* sched_rt_runtime_us takes values from -1 to (INT_MAX - 1).
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* A run time of -1 specifies runtime == period, ie. no limit.
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2.2 Default behaviour
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---------------------
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The default values for sched_rt_period_us (1000000 or 1s) and
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sched_rt_runtime_us (950000 or 0.95s). This gives 0.05s to be used by
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SCHED_OTHER (non-RT tasks). These defaults were chosen so that a run-away
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realtime tasks will not lock up the machine but leave a little time to recover
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it. By setting runtime to -1 you'd get the old behaviour back.
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By default all bandwidth is assigned to the root group and new groups get the
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period from /proc/sys/kernel/sched_rt_period_us and a run time of 0. If you
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want to assign bandwidth to another group, reduce the root group's bandwidth
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and assign some or all of the difference to another group.
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Realtime group scheduling means you have to assign a portion of total CPU
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bandwidth to the group before it will accept realtime tasks. Therefore you will
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not be able to run realtime tasks as any user other than root until you have
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done that, even if the user has the rights to run processes with realtime
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priority!
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2.3 Basis for grouping tasks
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----------------------------
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Enabling CONFIG_RT_GROUP_SCHED lets you explicitly allocate real
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CPU bandwidth to task groups.
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This uses the cgroup virtual file system and "<cgroup>/cpu.rt_runtime_us"
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to control the CPU time reserved for each control group.
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For more information on working with control groups, you should read
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Documentation/cgroup-v1/cgroups.txt as well.
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Group settings are checked against the following limits in order to keep the
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configuration schedulable:
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\Sum_{i} runtime_{i} / global_period <= global_runtime / global_period
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For now, this can be simplified to just the following (but see Future plans):
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\Sum_{i} runtime_{i} <= global_runtime
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3. Future plans
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===============
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There is work in progress to make the scheduling period for each group
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("<cgroup>/cpu.rt_period_us") configurable as well.
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The constraint on the period is that a subgroup must have a smaller or
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equal period to its parent. But realistically its not very useful _yet_
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as its prone to starvation without deadline scheduling.
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Consider two sibling groups A and B; both have 50% bandwidth, but A's
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period is twice the length of B's.
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* group A: period=100000us, runtime=10000us
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- this runs for 0.01s once every 0.1s
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* group B: period= 50000us, runtime=10000us
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- this runs for 0.01s twice every 0.1s (or once every 0.05 sec).
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This means that currently a while (1) loop in A will run for the full period of
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B and can starve B's tasks (assuming they are of lower priority) for a whole
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period.
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The next project will be SCHED_EDF (Earliest Deadline First scheduling) to bring
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full deadline scheduling to the linux kernel. Deadline scheduling the above
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groups and treating end of the period as a deadline will ensure that they both
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get their allocated time.
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Implementing SCHED_EDF might take a while to complete. Priority Inheritance is
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the biggest challenge as the current linux PI infrastructure is geared towards
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the limited static priority levels 0-99. With deadline scheduling you need to
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do deadline inheritance (since priority is inversely proportional to the
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deadline delta (deadline - now)).
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This means the whole PI machinery will have to be reworked - and that is one of
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the most complex pieces of code we have.
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