[v3 PATCH 0/4]: CPUIDLE/POWER: Introducing cpuidle infrastructure to POWER
Arun R Bharadwaj
arun at linux.vnet.ibm.com
Thu Aug 27 21:49:08 EST 2009
Changes from previous iteration:
* Remove the EXPORT_SYMBOL(pm_idle) from
arch/powerpc/platform/pseries/processor_idle.c and introduce a
generic cpuidle_pm_idle in cpuidle.c which was earlier assuming pm_idle
to be the default idle routine. (As suggested by Peter and Ben).
* Move the cpu_idle_wait function from arch/powerpc/platforms/pseries/setup.c
to arch/powerpc/kernel/idle.c which would prevent breaking the build of
other platforms. (As suggested by Ben).
"Cpuidle" is a CPU Power Management infrastrusture which helps manage
idle CPUs in a clean and efficient manner. The architecture can register
its driver (in this case, pseries_idle driver) so that it subscribes for
cpuidle feature. Cpuidle has a set of governors (ladder and menu),
which will decide the best idle state to be chosen for the current situation,
based on heuristics, and calculates the expected residency time
for the current idle state. So based on this, the cpu is put into
the right idle state.
Currently, cpuidle infrasture is exploited by ACPI to choose between
the available ACPI C-states. This patch-set is aimed at enabling
cpuidle for powerpc and provides a sample implementation for pseries.
Currently, in the pseries_dedicated_idle_sleep(), the processor would
poll for a time period, which is called the snooze, and only then it
is ceded, which would put the processor in nap state. Cpuidle aims at
separating this into 2 different idle states. Based on the expected
residency time predicted by the cpuidle governor, the idle state is
chosen directly. So, choosing to enter the nap state directly based on
the decision made by cpuidle would avoid unnecessary snoozing before
This patch-set tries to achieve the above objective by introducing a
pseries processor idle driver called pseries_idle_driver in
arch/powerpc/platform/pseries/processor_idle.c, which implements the
idle loop which would replace the pseries_dedicated_idle_sleep()
when cpuidle is enabled.
The following experiment was conducted on a completely idle JS22 blade,
to prove that using cpuidle infrastructure, the amount of nap time increases.
Nap and snooze times were sampled for all the cpus.
For a window of 1000 samples, When cpuidle was enabled,
the total nap time was of the order of a few seconds (5-10s), whereas
the total snooze time was of the order of a few milliseconds(10-30 ms).
When cpuidle infrastructure was disabled and the regular
pseries_dedicated_idle_sleep() idle loop was used, the snooze time itself
was of the order of hundreds of milliseconds. (100 - 500 ms).
This is clearly due to unnecessary snoozing before napping even on a
completely idle system.
The previous post in this area can be found at
Patches included in this set:
PATCH 1/4 - Enable cpuidle for pSeries.
PATCH 2/4 - Introduce architecture independent cpuidle_pm_idle in
PATCH 3/4 - Register for cpuidle_pm_idle in drivers/acpi/processor_idle.c
PATCH 4/4 - Implement Pseries Processor Idle idle module
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