1. Scheduling Classes and Real-Time Scheduling in Linux
David Ferry, Chris Gill, Brian Kocoloski
CSE 422S - Operating Systems Organization
Washington University in St. Louis

2. Real-Time Applications
Have repeating functions that must execute within a precise period of time
Within the next 10 ms, do something, or else bad things could happen
Examples
Air traffic control systems
Video/audio communication systems
Web serving
Stock Trading
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3. CSE 422S – Operating Systems Organization
Real-Time Scheduling
Real-time tasks execute repeatedly (usually are periodic) under some time constraint
Task
Time
0ms
5ms
10ms
E.g., a task is released to execute every 5 msec,
and each invocation has a deadline of 5 msec
Separate priority range from the nice priorities for CFS:
Priorities are from 1 (low) to 99 (high), highest ones need root
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Real-Time OS Support
Why might supporting such applications be challenging, given our understanding of Linux’ CFS scheduler?
For example, if a user accessed the kernel: ”can you guarantee my task will run for 1 second at every 5 second interval?”
Challenges:
CFS uses proportional sharing – so the answer is highly dependent on other system activity
What if another process boosts its priority?
What if another process is starved?
vruntime is designed to achieve proportionality – not to give guarantees on deadlines or speak in terms of fixed time quantities
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5. CSE 422S – Operating Systems Organization
Real-Time OS Support
Goal is to achieve predictable execution:
Other sources of uncertainty (and solutions):
Interrupts (can mask some interrupts)
Migrations (can pin tasks to cores)
OS latency, jitter, and kernel non-preemptibility (real-time scheduling)
Ideal:
Real world:
Preempt
Migrate
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Kernel Preemption
Before the 2.6 kernel (2003), Linux did not have a pre-emptible kernel
If a process was executing in the kernel, it could hog the CPU indefinitely
User-level process in a system-call
Kernel threads
Major issue for interactive/latency-sensitive + real-time applications
Starting in 2.6, the kernel became fully1 preemptible
1 there are ways to selectively disable preemption, for example, during
interrupt handlers, or when taking special locks; more on this in a few weeks
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7. Three real-time scheduling classes
SCHED_FIFO
First-in, First-out scheduling
SCHED_RT
Round robin scheduling
SCHED_DEADLINE
Earliest deadline first
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8. CSE 422S – Operating Systems Organization
SCHED_FIFO
First-in, First-out scheduling
The first enqueued task of highest priority executes to completion
A task will only relinquish a processor when it completes, yields, or blocks
Only a higher priority SCHED_FIFO or SCHED_RR task can preempt a SCHED_FIFO task – all others will be starved as it runs
Task 1
Task 2
Task 3
Time
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SCHED_RR
Round-robin scheduling
Same as SCHED_FIFO but with timeslices
Among tasks of equal priority:
Rotate through all tasks
Each task gets a fixed time slice
Only a higher priority SCHED_FIFO or SCHED_RR task can preempt a SCHED_FIFO
Tasks of equal priority preempt each other after timeslice expiration
Task 1
Task 2
Task 3
Time
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SCHED_DEADLINE
Earliest Deadline First (EDF) scheduling
(only available with rt-preempt patch)
Whichever task has next deadline gets to run
Theory exists to analyze such systems
Linux implements bandwidth reservation to prevent deadline abuse
Task 3
Deadline: 12 Exec time: 2
Task 2
Deadline: 8 Exec time: 3
Task 1
Deadline: 5 Exec time: 4
Time 0
Task 1
Task 2
Task 3 5 8 12
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11. CSE 422S – Operating Systems Organization
Scheduling Classes
Multiple schedulers are implemented as different scheduling classes.
Normal:
SCHED_NORMAL: regular, interactive CFS tasks
SCHED_BATCH: low priority, non-interactive CFS tasks
SCHED_IDLE: very low priority tasks
Real-time:
SCHED_RR: round-robin
SCHED_FIFO: first-in, first-out
SCHED_DEADLINE: earliest deadline first (rt-preempt)
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12. CSE 422S – Operating Systems Organization
struct sched_class
A few major functions:
Other functions:
Task migration
Task state queries
Task yielding
Other utilities
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13. Current Scheduling Classes and Ordering
stop_sched_class //for halting processors
dl_sched_class // (rt-preempt patch only)
rt_sched_class // SCHED_RR, SCHED_FIFO
fair_sched_class // SCHED_OTHER
idle_sched_class
Declared in /kernel/sched/sched.h
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14. Example sched_class Definition
From:
/kernel/sched/rt.c
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15. How the Next Runnable Task is Picked
Function pick_next_task() from /kernel/sched/core.c
Have all scheduling classes try to pick a new task
The last class, SCHED_IDLE, should always return some idle task
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