The document discusses the ktimer subsystem in the F9 Microkernel. It provides an overview of operating system kernel time subsystems and concepts like system time, ticks, and tickless kernels. It then describes the features of the F9 Microkernel ktimer, including supporting time events, being tickless, and using a two-stage interrupt model. The ktimer aims for efficiency and flexibility while providing time services to the microkernel.

1. Your Logo Here
Viller Hsiao <villerhsiao@gmail.com>
F9 Microkernel ktimer

2. Rights to Copy
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copyright © 2014 Viller Hsiao
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Your fair use and other rights are in no way affected by the above.
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3. F9 Microkernel [1]
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New open source implementation built from scratch, which deploys
modern kernel techniques, derived from L4 microkernel designs, to
deep embedded devices.
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BSD License
Characteristics of F9 microkernel
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Efficiency: performance + power consumption
–
Tickless
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Security: memory protection + isolated execution
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Flexible development environment
Repository
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https://github.com/f9micro
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4. Agenda
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Concept about operating system kernel
time subsystem
F9 Microkernel ktimer introduction
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5. LOGO
Concept about Operating System Kernel
Time Subsystem
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6. OS Kernel Time Subsystem
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Current Time
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System time, UTC/GMP time, … etc
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Alarm, the time event
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Process timeslice
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Timeout
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Time delay
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7. System Time and Tick
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From Wikipedia [2],
“System time represents a computer system's notion of
the passing of time.“
“System time is measured by a system clock, which is
typically implemented as a simple count of the number
of ticks that have transpired since some arbitrary
starting date, called the epoch.”
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8. How Is Tick Implemented?
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Hardware timer device
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Assert interrupt after a programmable inteval
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Handling tick stuff in Timeout Interrupt Service
Routine (ISR)
HW
Timer
CPU
Adjust system time
Handle timeout event
….
Timeout interrupt
Control
Read Counts
Setup timeout value
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9. How Is Tick Implemented? (Cont.)
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Type 1: Incremental timer
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Example: Cortex-A9 MP Global Timer
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10. How Is Tick Implemented? (Cont.)
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Incremental timer functionality
1
Current Value
+
0x12abc
=
Incremental value
0x36000
Compare Value
+
0x36000
Y
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Y
Auto
Increament?
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Interrupt

11. How Is Tick Implemented? (Cont.)
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Type 2: Counting down timer
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Example: Cortex-M4 SysTick
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12. How Is Tick Implemented? (Cont.)
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Counting down timer functionality
0
=
1
-
Reload value
=
0x36000
Current Value
0x12abc
Y
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Y
Auto
Load?
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Interrupt

13. How Is Tick Implemented? (Cont.)
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Prescaler
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Tweak HW tick period
Timer Module
clk
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Prescaler
val
clk/val
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Counting Part
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14. How Is Tick Implemented? (cont.)
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Timeout ISR
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Increase system ticks
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Execute handler of timeout event
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Re-schedule if required
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15. Simple Graph of CPU Operating States
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INT
Processes
Threads
Tasks
ISR
Softirq
CTX
INT
CTX
CTX
Idle thread
CTX
INT
INT
T
sleep
T
Deep sleep
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INT : interrupt
CTX: context switch
T : after a while

16. Time Diagram of Legacy Ticks
event1
event2
event3
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event4
HW
Timer
interrupt
CPU
activities
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17. Where Is the Problem?
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Context switch overhead
INT
Processes
Threads
Tasks
ISR
Softirq
CTX
INT
CTX
CTX
Idle thread
T
sleep
T
Deep sleep
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CTX
INT
INT

18. Where Is the Problem?
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Unecessary power consumed
INT
Processes
Threads
Tasks
ISR
Softirq
CTX
INT
CTX
CTX
Idle thread
T
sleep
T
Deep sleep
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CTX
INT
INT

19. Where Is the Problem?
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Tick resolution
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Performance, power v.s. timer precision
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20. Time Diagram of Legacy Ticks
event1
event2
event3
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event4
HW
Timer
interrupt
CPU
activities
CPU waken up for timekeeping only
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21. Solution: Tickless
event1
event2
event3
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event4
Timer
interrupt
New
CPU
activities
Previous
CPU
activities
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22. Drawback of Tickless
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Tickless is not free [3],
“It increases the number of instructions executed on
the path to and from the idle loop.”
“On many architectures, dyntick-idle mode also
increases the number of expensive clockreprogramming operations”
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Systems with aggressive real-time response
constraints often run periodic tick
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23. Example: Linux Time Subsystem
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Device level
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clocksource
–
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jiffies
clockevent
High resolution kernel timer
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hrtimers
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24. Example: Linux Time Subsystem (Cont.)
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Tickless
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(Nearly) Full tickless operation after 3.10
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dyntick-idle
–
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adaptive-tick
–
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CONFIG_NO_HZ_IDLE
CONFIG_NO_HZ_FULL
periodic
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CONFIG_HZ_PERIODIC
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Consumes battery power 2-3 times as fast as dyntick-idle one. [3]
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25. LOGO
F9 Microkernel ktimer Introduction
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26. Example Hardware Device
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STM32F4Discovery board
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Cortex-M4 EVB
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Use Cortex-M4 SysTick as the tick HW
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24-bit system timer
Counting down
Please refer to [4] for more detail
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27. ktimer Features
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System time
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Time event
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Tickless
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28. ktimer Configurations
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CONFIG_KTIMER_HEARTBEAT
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CONFIG_KTIMER_MINTICKS
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HW cycles per ktimer tick
Minimum ktimer ticks unit for time event
CONFIG_KTIMER_TICKLESS
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Enable tickless
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29. ktimer Event Utility
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ktimer_event_create()
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Arguments
–
–
Timeout handler
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Timeout ticks
Pointer to arguments for timeout handler
Return Non-zero to reschedule event again
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Freed automatically if not be re-scheduled
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30. ktimer Anotomy
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System time
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ktime_now
ktimer event list
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event_queue
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Managed by ktable
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31. ktimer Anotomy (Cont.)
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ktable
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Basic kernel library in F9 Microkernel
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Fast objects pool management
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To allocate/free objects of pre-defined size and
numbers easiler
ktable
ktable_free()
ktable_init()
Used
Unused
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32. ktimer Anotomy (Cont.)
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Life cycle of time event
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One-shot
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Continuous
How to control life cycle in ktimer?
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The return value of handler is treated as next
timeout delta
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33. ktimer Anotomy (Cont.)
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HW Timer ISR
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__ktimer_handler()
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Increase tick
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Arrange Softirq for handling timer events
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34. ktimer Anotomy (Cont.)
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Two-stage interrupt handling
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ISR
–
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IRQ context
Softirq
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Thread context
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Real time preemptive characteristic.
Can be scheduled like any other threads in the system
[5].
Handled in kernel thread
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35. ktimer Anotomy (Cont.)
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HW Timer ISR
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__ktimer_handler()
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Increase tick
–
Arrange Softirq for handling timer events
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36. ktimer Anotomy (Cont.)
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HW timer softirq handler
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ktimer_event_handler()
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Find timeout event and executing its handler
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Re-insert event if required
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37. ktimer Anotomy (Cont.)
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Tickless
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Enter tickless right before going to CPU idle
state
Set interval of next timer interrupt as delta of
next event
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Or KTIMER_MAXTICKS
Adjust system time after waked up
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38. ktimer Anotomy (Cont.)
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Tickless Compensation
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SysTick frequency distortion when enter/exit standby
mode
tickless
active
idle
handle
other
interrupt
Timer
interrupt
idle
active
CPU
Activity
systick
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39. ktimer Anotomy (Cont.)
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Tickless Compensation
tickless
active
idle
handle
other
interrupt
Timer
interrupt
idle
active
CPU
Activity
systick
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40. ktimer Anotomy (Cont.)
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Tickless Compensation
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Pre-defined value
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CONFIG_KTIMER_TICKLESS_COMPENSATION
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CONFIG_KTIMER_TICKLESS_INT_COMPENSATION
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Compensation value before entering idle
Compensation value when CPU is waked up by non-timer
interrupt.
But how much to set these value to?
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41. ktimer Anotomy (Cont.)
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Tickless Compensation
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But how much to set these value to?
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Get average compensation from general pupose
timer
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(total diff / count of entering tickless)
SYSTICK
HW timer
TIM2
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42. ktimer Anotomy (Cont.)
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Tickless Compensation
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How to calibrate the compensation value?
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CONFIG_KTIMER_TICKLESS_VERIFY
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CONFIG_KDB
Set
compensation
To 0
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Get
average
Compensation
from KDB
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Re-define
Compensation
value

43. ktimer Anotomy (Cont.)
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KDB
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Kernel debugger
## KDB ##
commands:
K: print kernel tables
e: dump ktimer events
n: show timer (now)
s: show softirqs
t: dump threads
m: dump memory pools
a: dump address spaces
p: show sampling
v: show tickless scheduling stat
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## KDB ##
-------NOW-----Now is 70993
Ktimer T=1297 D=0
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44. Reference:
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[1]
Jim Huang (Dec 9, 2013),
“F9: A Secure and Efficient Microkernel Built for Deep Embedded Systems”
[2]
Wikipedia, “System time”
[3]
P. E. McKenney (May 14, 2013), “NO_HZ: Reducing Scheduling-Clock Ticks”
[4]
ST Mcroelectronics,
STM32F3xxx and STM32F4xxx Cortex-M4 programming manual
[5]
J. Altenberg, “Using the Realtime Preemption Patch on ARM CPUs”
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45. LOGO
THE END
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