[PATCH RFC 0/1] cpufreq/x86: Add P-state driver for sandy bridge.

[dirk.brandewie@gmail.com]

From: Dirk Brandewie <dirk.brandewie@gmail.com>

This driver provides a P state driver for Sandybridge and Ivybridge
processors.
 
Motivation:
The goal of this driver is to improve the power efficiency of
Sandybridge/Ivybridge based systems.  As the investigation into how to
achieve this goal progressed it became apparent (to me) that some of the
design assumptions of the cpufreq subsystem are no longer valid and
that a micro-architecure specific P state driver would be less complex
and potentially more effiecent.  As Intel continues to innovate in the
area of freqency/power control this will become more true IMHO.

General info:
The driver uses a PID controller to adjust the core frequency based on
the presented load. The driver exposes the tuning parameters for the
controller in the /sys/devices/system/cpu/cpufreq/snb directory.  The
controller code is being used in PI mode with the default tuning
parmeters.

Tuning parmeters:
   setpoint - load in percent on the core will attempt to maintain.	
   sample_rate_ms - rate at which the driver will sample the load on the core. 
   deadband  - percent ± around the setpoint the controller will
               consider zero error.
   p_gain_pct - Proportional gain in percent. 
   i_gain_pct - Integral gain in percent. 
   d_gain_pct - Derivative gain in percent

To use the driver as root run the following shell script:
   #!/bin/sh
   for file in /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor
   do 
      echo snb > $file
   done

Limitations:

ATM this driver will only run on SandyBridge systems testing on
Ivybridge systems is not complete.
 
Open Questions:

What is the correct way to integrate this driver into the system?  The
current implementation registers as a cpufreq frequency governor, this
was done to streamline testing using cpufreq to load/unload governors.

What tuning parameters should be exposed via sysfs (if any)?  ATM all
the PID parameters are exposed to enable tuning of the driver.


Performance information:

 --- Kernel build ---
The following is data collected for a bzImage kernel build.  The
commands used were:
	 make -j8 clean
	 sysctl -w vm.drop_caches=3
	 /usr/bin/time -f "%E %c" make -j8 bzImage

Time and context switches measured with /usr/bin/time -f "%E %c"

Energy measured with package energy status MSR described in section
14.7 in the Intel® 64 and IA-32 Architectures Software Developer’s
Manual Volume 3.
       http://download.intel.com/products/processor/manual/325384.pdf

Average watts calculated with energy/time in seconds

           time		ctx sw	energy	avg watts
perf      02:24.49	116721	6660	46.09
snb       02:27.03	114940	6591	44.83
ondemand  02:26.83	190948	6683	45.51

A graph of the power usage during the kernel build for each governor
is available here:
    http://git.fenrus.org/dirk/kernel.png

 --- Power benchmark ---
I used industry standard power bench suite to compare the performance and
ondemand governors against the Sandybridge governor.

  Governor    | ssj_ops/watt
 -----------------------------
  performance |   1855
  ondemand    |   1839
  snb         |   2016
 
A graph of the power usage for each governor is avavailable here:
    http://git.fenrus.org/dirk/power_benchmark.png

A graph showing the results of cpufreq-bench tool shipped with the
kernel Collected with
   cpufreq-bench -l 6000 -s 6000 -x 2000 -y 2000 -c 0 \
               -g {ondemand | snb} -n 40 -r 40 
is available here:
    http://git.fenrus.org/dirk/cpufreq-bench.png

Dirk Brandewie (1):
  cpufreq/x86: Add P-state driver for sandy bridge.

 drivers/cpufreq/Kconfig.x86   |    8 +
 drivers/cpufreq/Makefile      |    1 +
 drivers/cpufreq/cpufreq_snb.c |  727 +++++++++++++++++++++++++++++++++++++++++
 3 files changed, 736 insertions(+), 0 deletions(-)
 create mode 100644 drivers/cpufreq/cpufreq_snb.c

-- 
1.7.7.6