Deadline Miss Detection with SCHED_DEADLINE
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The document presents an overview of a real-time scheduling system called sched_deadline, focusing on deadline miss detection and budget management in Linux control systems. It discusses the evaluation and implementation of the earliest deadline first scheduling algorithm along with the advantages and limitations of different scheduling techniques. Key enhancements for deadline miss detection and budget management, including the need for high-resolution timers, are emphasized in the context of real-time applications.
![Structure of sched_param2
struct sched_param2 {
int sched_priority;
unsigned int sched_flags;
u64 sched_runtime;
u64 sched_deadline;
u64 sched_period;
17Embedded Linux Conference 2013
u64 sched_period;
u64 __unused[12];
};](https://image.slidesharecdn.com/elc2013-kobayashi-170303135156/85/Deadline-Miss-Detection-with-SCHED_DEADLINE-17-320.jpg)
Deadline Miss Detection with SCHED_DEADLINE
- 1. Deadline Miss Detection with SCHED_DEADLINE Copyright 2013, Toshiba Corporation. Yoshitake Kobayashi Advanced Software Technology Group Corporate Software Engineering Center TOSHIBA CORPORATION
- 2. Resources The latest version slide is available on elinux wiki http://elinux.org/ELC_2013_Presentations Related source code is able to download from GitHub https://github.com/ystk/sched-deadline/tree/dlmiss-detection-dev 2Embedded Linux Conference 2013
- 3. Outline Motivation Deadline scheduler SCHED_DEADLINE and its evaluation Budget management Deadline miss detection Conclusion 3Embedded Linux Conference 2013 Conclusion
- 4. Motivation We would like to use Linux on control systems Real-time is one of the most critical topic Problem statement Need to evaluate deeply to meet the deadline CPU resource used too much by higher priority tasks 4Embedded Linux Conference 2013 CPU resource used too much by higher priority tasks EDF scheduler
- 5. Definition of deadline Wakeup time: The time of an event occurred (Ex. Timer interrupt) and target task’s state changed to runnable. Event response time: Interrupt latency Deadline: The time for a target task must finish Process time Event response time 5Embedded Linux Conference 2013 Wakeup time Deadline Period Event response time Wakeup time
- 6. Earliest Deadline Fisrt scheduling （EDF） The earliest deadline task has the highest priority Task’s priority is dynamically changed and managed SCHED_FIFO is static priority management Theoretically the total CPU usage by all tasks is up to 100% Includes the kernel overhead 6Embedded Linux Conference 2013 Includes the kernel overhead If usage of CPU by all tasks is less than 100%, all tasks meet the deadline Reference http://en.wikipedia.org/wiki/Earliest_deadline_first_schedu ling
- 7. An example of EDF Scheduling Task1: budget 1ms period 8ms Task2: budget 2ms period 5ms Task3: budget 4ms period 10ms T1 20ms0ms CPU usage＝＝＝＝0.925% ＜＜＜＜ 100% 7Embedded Linux Conference 2013 T1 T2 T3
- 8. Rate-Monotonic Scheduling （RMS） One of the popular scheduling algorithm for RTOS Assumptions for task behavior NO resource sharing such as hardware, a queue, or any kind of semaphore Deterministic deadlines are exactly equal to periods Static priorities (the task with the highest static priority that is runnable immediately preempts all other tasks) Static priorities assigned according to the rate monotonic conventions (tasks with shorter periods/deadlines are given higher priorities) Context switch times and other thread operations are free and have no impact on the model CPU utilization 8Embedded Linux Conference 2013 CPU utilization n: number of periodic tasks，Ti: Release period, Ci: Computation time CPU utilization depends on the combination of periodic tasks and it is possible to meet the deadline even the CPU utilization is around 80% Reference http://en.wikipedia.org/wiki/Earliest_deadline_first_scheduling 69.02ln)12(/ 0 ≈→−≤= ∞= = ∑ n n i n ii nTCU
- 9. Compared with the RMS scheduling Task1: budget 1ms period 4ms Task2: budget 2ms period 6ms Task3: budget 3ms period 8ms T1 T2 20ms0ms RMS CPU usage＝＝＝＝0.958% 9Embedded Linux Conference 2013 T3 deadline miss T1 T2 T3 20ms0ms EDF
- 10. Comparison of deadline algorithms Advantage Disadvantage RMS Easier to implement Evaluation for scheduling possibility is required to meet the deadline 10Embedded Linux Conference 2013 deadline EDF No evaluation for scheduling possibility is required to meet the deadline Difficult to implement
- 11. What SCHED_DEADLINE is? http://www.evidence.eu.com/sched_deadline.html Implements the EDF scheduling algorithm Posted to LKML by Dario Faggioli and Juri Lelli Latest version is V7 (2013/2/12) Our work based on V2 and V4. Key features of SCHED_DEADLINE 11Embedded Linux Conference 2013 Key features of SCHED_DEADLINE Temporal isolation The temporal behavior of each task (i.e., its ability to meet its deadlines) is not affected by the behavior of any other task in the system Each task is characterized by the following aspects: Budget: sched_runtime Period: sched_period, equal to its deadline
- 12. Build SCHED_DEADLINE enabled kernel Get the source code from the following places git clone git://github.com/jlelli/sched-deadline.git (for V4 to V7) git clone git://gitorious.org/rt-deadline (for V2) Kernel configuration CONFIG_EXPERIMENTAL = y 12Embedded Linux Conference 2013 CONFIG_EXPERIMENTAL = y CONFIG_CGROUPS = y CONFIG_CGROUP_SCHED = n CONFIG_HIGH_RES_TIMERS = y CONFIG_PREEMPT = y CONFIG_PREEMPT_RT = y CONFIG_HZ_1000 = y
- 13. Overview of SCHED_DEADLINE EDF task User sysctl_sched_dl_runtime sysctl_sched_dl_period procfs Set parameters for SCHED_DEADLINE sched_pram2 Set deadline and budget System call 13Embedded Linux Conference 2013 Kernel task_struct sched_calss dl_sched_class enqueue_task_dl dequeue_task_dl sched_dl_entity task_tick_dl set_curr_task_dl ・・・ ・・・ ・・・ ・・・ rb_node dl_runtime dl_deadline dl_timer ・・・・・・ ・・
- 14. Overview of SCHED_DEADLINE EDF task User sysctl_sched_dl_runtime sysctl_sched_dl_period procfs Set parameters for SCHED_DEADLINE sched_pram2 Set deadline and budget System call 14Embedded Linux Conference 2013 Kernel task_struct sched_calss dl_sched_class enqueue_task_dl dequeue_task_dl sched_dl_entity task_tick_dl set_curr_task_dl ・・・ ・・・ ・・・ ・・・ rb_node dl_runtime dl_deadline dl_timer ・・・・・・ ・・
- 15. Setting CPU utilization for EDF tasks Parameters can be setted via procfs CPU utilization for rt(SCHED_FIFO and SCHED_RR) and dl(SCHED_DEADLINE) should be under 100% Parameters for EDF scheduler /proc/sys/kernel/sched_dl_period_us /proc/sys/kernel/sched_dl_runtime_us When a task requires more than above limit, the task 15Embedded Linux Conference 2013 When a task requires more than above limit, the task cannot submit to run An example setting (rt: 50%, dl:50%）））） # echo 500000 > /proc/sys/kernel/sched_rt_runtime_us (500ms) # echo 100000 > /proc/sys/kernel/sched_dl_period_us (100ms) # echo 50000 > /proc/sys/kernel/sched_dl_runtime_us (50ms)
- 16. Overview of SCHED_DEADLINE EDF task User sysctl_sched_dl_runtime sysctl_sched_dl_period procfs Set parameters for SCHED_DEADLINE sched_pram2 Set deadline and runtime System call 16Embedded Linux Conference 2013 Kernel task_struct sched_calss dl_sched_class enqueue_task_dl dequeue_task_dl sched_dl_entity task_tick_dl set_curr_task_dl ・・・ ・・・ ・・・ ・・・ rb_node dl_runtime dl_deadline dl_timer ・・・・・・ ・・
- 17. Structure of sched_param2 struct sched_param2 { int sched_priority; unsigned int sched_flags; u64 sched_runtime; u64 sched_deadline; u64 sched_period; 17Embedded Linux Conference 2013 u64 sched_period; u64 __unused[12]; };
- 18. Run a EDF task Schedtool # schedtool -E -t 10000:100000 -a 0 -e ./yes Options -E： a task runs on SCHED_DEADLINE -t： <execution time> and <period> in micro seconds -a： Affinity mask -e： command 18Embedded Linux Conference 2013 System call sched_setscheduler2()
- 19. Budget management for EDF tasks EDF task User sysctl_sched_dl_runtime sysctl_sched_dl_period procfs Set parameters for SCHED_DEADLINE sched_prama2 Set deadline and runtime System call 19Embedded Linux Conference 2013 Kernel task_struct sched_calss dl_sched_class enqueue_task_dl dequeue_task_dl sched_dl_entity task_tick_dl set_curr_task_dl ・・・ ・・・ ・・・ ・・・ rb_node dl_runtime dl_deadline dl_timer ・・・・・・ ・・
- 20. Budget management for EDF tasks Each task on SCHED_DEADLINE has budget which allows it to use CPU Budget management Refill budget : dl_timer ( high resolution timer ) Use budget : task_tick_dl ( tick based ) task_tick_dl 20Embedded Linux Conference 2013 Wakeup time Period Wakeup time dl_timer Task task_tick_dl dl_timer
- 21. Evaluation (Period:100ms, Budget: 50ms) 21Embedded Linux Conference 2013
- 22. Evaluation Task T1: budget 1ms period 4ms Task T2: budget 2ms period 6ms Task T3: budget 3ms period 8ms 22Embedded Linux Conference 2013 T1 T2 T3 20ms0ms Compare with page 6
- 23. Evaluation (Period: 1ms, Budget: 0.5ms) 23Embedded Linux Conference 2013
- 24. Budget management for EDF tasks Each task on SCHED_DEADLINE has budget which allows it to use CPU Budget management Refill budget : dl_timer ( high resolution timer ) Use budget : task_tick_dl ( tick based ) An Issue Difficult to keep task’s budget if the budget has micro seconds granularity 1.5ms 1.5ms 24Embedded Linux Conference 2013 Wakeup Period Task execution Time dl_timer 1.5ms task_tick_dl 1ms Wakeup dl_timer 1.5ms 1ms Step1: Refill budget Step2: Use budget Period 2ms
- 25. Overview When a task’s budget is less than 1ms, set HRTICK for the rest of budget Step1: Refill budget 3.2ms 1ms 2ms 3ms Support for micro seconds granularity 25Embedded Linux Conference 2013 Wakeup Period Task execution Set HRTICK here Step2: Use budget Time 1ms 2ms 3ms Wakeup
- 26. Evaluation (Cycle: 1ms, Budget: 0.5ms) 26Embedded Linux Conference 2013
- 27. Advantage and Disadvantage Advantage Easy to support high resolution budget Disadvantage Increase overhead 27Embedded Linux Conference 2013
- 28. Summary of scheduling granularity improvement An Enhancement for budget management Support fine grained budget such as 100 micro seconds HRTICK is needed to support fine grained budget 28Embedded Linux Conference 2013
- 29. DEMO “Big Buck Bunny” http://www.bigbuckbunny.org/ (c) copyright 2008, Blender Foundation License: Creative Commons Attribution 3.0 Play the movie about 3 times faster than normal speed Requirement for a frame decoding 29Embedded Linux Conference 2013 Requirement for a frame decoding Period: 10ms CPU budget: 3.6ms When a frame decode is not finished in a period, the player shows “DEADLINE MISS” on screen and add 3ms penalty
- 30. Let’s play the movie ModifiedOriginal 30Embedded Linux Conference 2013 Note: Both movies are running on same spec x86 based PC
- 31. Requirement for deadline miss detection Current behavior Have a counter to count the number of deadline misses Not able to control tasks that missed the deadline Wakeup time Period （（（（ Deadline ) Task Period （（（（ Deadline ) DetectDetect 31Embedded Linux Conference 2013 Requirement Detect the deadline miss at the begging of period Detect Here Wakeup time Period （（（（ Deadline ) Task Period （（（（ Deadline ) Need to detect Here Detect Here
- 32. Current limitation of SCHED_DEADLINE SCHED_DEADLINE is able to count the number of deadline misses SCHED_DEADLINE is not able to control tasks that missed the deadline 32Embedded Linux Conference 2013
- 33. Possible approaches to detect deadline misses Userland Monitor process or thread Run at the end or begging of each period A pair of periodic timer and signal handler Have to cancel if the process meet the deadline or Have to catch the signal to check the deadline miss status 33Embedded Linux Conference 2013 Have to catch the signal to check the deadline miss status … Kernel This presentation takes this approach
- 34. Overview of SCHED_DEADLINE EDF task User sysctl_sched_dl_runtime sysctl_sched_dl_period procfs Set parameters for SCHED_DEADLINE sched_pram2 Set deadline and runtime System call sched_flags 34Embedded Linux Conference 2013 Kernel task_struct sched_calss dl_sched_class enqueue_task_dl dequeue_task_dl sched_dl_entity task_tick_dl set_curr_task_dl ・・・ ・・・ ・・・ ・・・ rb_node dl_runtime dl_deadline dl_timer ・・・・・・ ・・ flags
- 35. Overview of deadline miss detection Set parameters to sched_param2.sched_flags Parameters for sched_flags Enable or disable the deadline miss detection SCHED_DL_DMISS_DETECT_DISABLE SCHED_DL_DMISS_DETECT_ENABLE Behaviors 35Embedded Linux Conference 2013 Behaviors next slide..
- 36. Behaviors ( sched_param2.sched_flags ) Nothing to do SCHED_DL_DMISS_ACT_NONE When the SCHED_DL_DMISS_DETECT_ENABLE flag is set, the kernel will count the number of deadline misses on /proc/sys/kernel/sched_dl_dmiss_count Send a signal To the process itself SCHED_DL_DMISS_ACT_SEND_SIG_OWN Send a SIGXCPU signal SCHED_DL_DMISS_ACT_PROCESS_END Send a SIGKILL signal SCHED_DL_DMISS_ACT_PROCESS_STOP 36Embedded Linux Conference 2013 SCHED_DL_DMISS_ACT_PROCESS_STOP Send a SIGSTOP signal To the other task Set the PID to /proc/sys/kernel/sched_dl_dmiss_sig_pid SCHED_DL_DMISS_ACT_SEND_SIG_OTHER_WITH_RUN Send a signal to the specified process Deadline missed process continues to run SCHED_DL_DMISS_ACT_SEND_SIG_OTHER_WITH_STOP Send a signal to the specified process Deadline missed process is stoped
- 37. Evaluation Behavior of process Period: 10ms Budget: 5ms Most of case the process is able to call sched_yield() between 4.5ms to 4.8ms The process missed the deadline at the last period in the log 37Embedded Linux Conference 2013 OK DL miss here
- 38. Conclusion SCHED_DEADLINE is useful for real time systems This presentation enhanced SCHED_DEADLINE For budget management Support fine grained budget such as 100 micro seconds HRTICK is needed to support fine grained budget For deadline miss detection 38Embedded Linux Conference 2013 For deadline miss detection Add a function to control deadline missed tasks All source code are available at the following URL https://github.com/ystk/sched-deadline/tree/dlmiss-detection-dev
- 39. Thank you 39Embedded Linux Conference 2013 Thank you