Name: CPUIdle: consolidation status and discussion about next steps Date: 11-07-2013 Speaker: Daniel Lezcano Video: http://youtu.be/6sSGRT2r3QA

1. June 2013
LCE-13 Engineering Review
PMWG WG

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CPUIdle: consolidation status and
discussion about next steps
Daniel Lezcano <daniel.lezcano@linaro.org>

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CPUIdle: status
Dynamic timer irq affinity
Timer broadcast consolidation
A simplified function to register the driver and device
Code fixes and cleanup

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Timer broadcast unoptimized behavior
CPU0
Broadcast timer local timer
Tick framework
CPU1
local timer
Enter broadcast mode
Setup timer
Interrupt
Handle broadcast interrupt
Send IPI timer
ZZZ Z Z
CPU0 is woken up to wake up CPU1
ZZZ Z Z
Dynamic timer irq affinity

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Dynamic timer irq affinity
Timer broadcast optimized behaviour
CPU0
Broadcast timer local timer
Tick framework
CPU1
local timer
Enter broadcast mode
Setup timer
Interrupt
ZZZ Z Z
CPU1 is directly wake up by the timer device
ZZZ Z Z
Set irq to CPU1

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Dynamic timer irq affinity
without dynamic timer irq affinity
with dynamic timer irq affinity
A specific test case 'usleep 10000' on CPU1

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Dynamic timer irq affinity
Results:
reduced by 40% the number of wake up on the system
reduced by 49% the number of wake up for CPU0
increased by factor two idle time for CPU0
increase by 16% package idle hits + 16% average package idle time

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Dynamic timer irq affinity
Pre-requisite :
timer device must support CPU affinity
Irq field in the clockdevice driver must be initialized
Dynamic irq affinity + timer / work queues migration = cpu
quiescent for a long period
In conjunction with the small task packing, we have a cluster
down for long periods
Add the CLOCK_EVT_FEAT_DYNIRQ flag to the clock
device driver if this feature is worth to add to the SoC

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Timer broadcast consolidation
CPUIdle framework CPUIdle backend driver
IDLE(cpu1) Enter idle state
Tick framework
Clockevents notify enter
Low level
sleep code
CPU0 CPU1
Setup broadcast timer
SMP cross call
ZZZ Z Z
Old behaviour : the timer broadcast is handled in the
backend driver
Clockevents notify exit
Interrupt

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Timer broadcast consolidation
New behaviour : the timer broadcast is handled by the
cpuidle framework with the idle state flag :
CPUIDLE_FLAG_TIMER_STOP
CPUIdle framework CPUIdle backend driver
IDLE(cpu1) Enter idle state
Tick framework
Clockevents notify enter
Low level
sleep code
CPU0 CPU1
Setup broadcast timer
SMP cross call
ZZZ Z Z
Clockevents notify exit
Interrupt

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Timer broadcast consolidation
Duplicated code across back end drivers removed
A flag to tell to use the timer broadcast for a specific state
CPUIdle backend drivers only cares about PM work
Idle state callbacks more and more simple and focused on SoC
specific function

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Simplified register function
Usually the initialization routines are always the same
1. Register the driver : cpuidle_register_driver
2. Register all the cpu devices : cpuidle_register_device
With the cpuidle_device statically declared per cpu in each
driver

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Simplified register function
Introduced a new generic register function

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Simplified register function
Results in a nice code consolidation

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Code fixes and cleanup
Multiple driver support simplified
Cpumask in the cpuidle_driver structure
cpuidle_register_driver / cpuidle_register could be used
Removed API cpuidle_register_driver_cpu
➔ Results in internal framework code simplified
Made governors optional at compile time
menu/tickless – ladder/periodic tick
Removed en_core_tk_irqen flag : all drivers have the same
behavior
Time keeping done and local irq enabled by the framework
Misc code cleanups, fixes and unification of the init functions

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Conclusion
Where are we ?
Very similar init / exit functions
Some PM functions can be now factored out
davinci – kirkwood - at91 DDR self refresh
Platform driver and device tree more and more used
Kirkwood, davinci
PM arch dependant code and driver more and more
separated
ux500 and at91 drivers will be moved to drivers/cpuidle soon

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Next steps
b.L cpuidle driver for TC2 is incoming (planned for 3.12)
A single ARM driver
With the code consolidation, we are close now but still depends on the
points below
Handle cpu hotplug in the cpuidle framework
Continue to split arch specific PM code from the driver
And move the drivers to the drivers/cpuidle directory
More use of the device tree
Latencies

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