1. Overview of the Lab 2 Assignment: Multicore Real-Time Tasks
Chris Gill and Son Dinh
CSE 522S – Advanced Operating Systems
Washington University in St. Louis
St. Louis, MO 63143

2. CSE 522S – Advanced Operating Systems
Lab Objectives
Kernel module releases multiple end-to-end tasks periodically on different cores
Timer releases first sub-task
Entire task must finish before next release
Subtasks may span multiple cores
Each subtask has its own release time, sub-deadline, execution time, and priority
Each sub-task runs in its own kthread
Completion releases downstream subtask
Release guards prevent early execution
You will calibrate subtasks’ execution times
Number of iterations of a specific loop
Measured in isolation first
Also monitored at run-time
Core 0
Core 1
Core 2
Core 3
CSE 522S – Advanced Operating Systems

3. “Data Driven” Module Configuration
Each task (and subtask) is described by configuration data
A custom header file contains config data
General module functions read that data, configure threads, timers, counters, etc.
Module init and exit functions
Init allocates, exit deallocates
Again use dynamic data structures
Don’t assume fixed sizes/numbers for things that may vary per program inputs
Module can be loaded in “calibrate” mode
If loaded in calibrate mode, runs one subtask at a time on its core, finds number of iterations closest to given execution time, prints results to system log file
Otherwise, will run all tasks at once, using the given numbers of iterations for each subtask that’s found in the config info
struct rt_subtask_spec {
unsigned int task_id;
unsigned int subtask_id;
unsigned int st_core;
unsigned int st_exectime; … };
struct rt_subtask_spec*
configuration = 0;
int define_config(void);
int setup_subtasks(void);
CSE 522S – Advanced Operating Systems

4. Task Release Mechanisms
Timer expires, wakes up thread for first subtask(s), reschedules next timer wakeup
Keeps a “wheel” of subtask release times
Uses absolute time to avoid release jitter
Thread for first sub-task has no dependence
First subtask is released when timer expires
T 2,1
T 1,1
T 3,1 4 5 8 10 12 15
CSE 522S – Advanced Operating Systems

5. Subtask Release Mechanisms
When subtask completes, updates a counter
Also notifies a condition variable
When a subtask wakes up it checks counter
If not updated, blocks on the condition variable
This handles both early and late completion
Not awoken until release time, doesn’t start until predecessor has recorded its completion
T 2,2
T 1,2
T 3,2 4 5 8 10 12 15
CSE 522S – Advanced Operating Systems

6. Monitoring and Evaluation
Release time is when a job of a task is ready
Start time is when a job is first scheduled
Release guards should prevent start before release
Execution time is sum of scheduled intervals
Record subtask execution times, compare to what was specified in the configuration data
Missed deadline if task runs past next release
Τ1,1,1
Τ2,1,1
Τ1,1,1
Time
released
resumed
preempted
completed
CSE 522S – Advanced Operating Systems

7. CSE 522S – Advanced Operating Systems
Extra Credit Options
Extend the calibration mode so it calculates subtasks’ sub-deadlines, priorities, etc., too
Outputs that calculated data to the system log, in a nicely structured format
Write a helper program that can extract that calculated system log file, and use it to generate .h file that the module can include to run correctly
Add a mode to run using SCHED_DEADLINE
Vary parameters to see their effects on behavior
Compare results to standard SCHED_FIFO behavior
CSE 522S – Advanced Operating Systems

8. CSE 522S – Advanced Operating Systems
Lab Write-up
As you work, please record your observations in detail
It’s a good idea to read the entire assignment, and to plan and design a bit before starting to work on it
It’s also a good idea to develop and test incrementally
First, design and implement calibration mode
Then, design and implement run mode
Then design and implement extra credit options, if you’d like
Make sure Kernelshark, etc. traces validate your assumptions
Design and run and document results of validation experiments
Write a cohesive report that analyzes, integrates, and offers explanations for what you observed
Document the thinking behind your design
Attribute sources for any code or design ideas you obtained from elsewhere (caveat – only use what you understand, and know to be correct)
Explain not only what you did and what you saw, but also why you did that, and why you think what you saw happened
CSE 522S – Advanced Operating Systems