ACPI: create Processor Aggregator Device driver

ACPI 4.0 created the logical "processor aggregator device" as
a mechinism for platforms to ask the OS to force otherwise busy
processors to enter (power saving) idle.

The intent is to lower power consumption to ride-out
transient electrical and thermal emergencies,
rather than powering off the server.

On platforms that can save more power/performance via P-states,
the platform will first exhaust P-states before forcing idle.
However, the relative benefit of P-states vs. idle states
is platform dependent, and thus this driver need not know
or care about it.

This driver does not use the kernel's CPU hot-plug mechanism
because after the transient emergency is over, the system must
be returned to its normal state, and hotplug would permanently
break both cpusets and binding.

So to force idle, the driver creates a power saving thread.
The scheduler will migrate the thread to the preferred CPU.
The thread has max priority and has SCHED_RR policy,
so it can occupy one CPU.  To save power, the thread will
invoke the deep C-state entry instructions.

To avoid starvation, the thread will sleep 5% of the time
time for every second (current RT scheduler has threshold
to avoid starvation, but if other CPUs are idle,
the CPU can borrow CPU timer from other,
which makes the mechanism not work here)

Vaidyanathan Srinivasan has proposed scheduler enhancements
to allow injecting idle time into the system.  This driver doesn't
depend on those enhancements, but could cut over to them
when they are available.

Peter Z. does not favor upstreaming this driver until
the those scheduler enhancements are in place.  However,
we favor upstreaming this driver now because it is useful
now, and can be enhanced over time.
Signed-off-by: Shaohua Li <shaohua.li@intel.com>
NACKed-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>
Signed-off-by: Len Brown <len.brown@intel.com>

MAINTAINERS

@@ -288,6 +288,14 @@ L:	linux-pci@vger.kernel.org
 S:	Supported
 F:	drivers/pci/hotplug/acpi*
 
+ACPI PROCESSOR AGGREGATOR DRIVER
+P:	Shaohua Li
+M:	shaohua.li@intel.com
+L:	linux-acpi@vger.kernel.org
+W:	http://www.lesswatts.org/projects/acpi/
+S:	Supported
+F:	drivers/acpi/acpi_pad.c
+
 ACPI THERMAL DRIVER
 P:	Zhang Rui
 M:	rui.zhang@intel.com

drivers/acpi/Kconfig

@@ -196,6 +196,17 @@ config ACPI_HOTPLUG_CPU
 	select ACPI_CONTAINER
 	default y
 
+config ACPI_PROCESSOR_AGGREGATOR
+	tristate "Processor Aggregator"
+	depends on ACPI_PROCESSOR
+	depends on EXPERIMENTAL
+	help
+	  ACPI 4.0 defines processor Aggregator, which enables OS to perform
+	  specfic processor configuration and control that applies to all
+	  processors in the platform. Currently only logical processor idling
+	  is defined, which is to reduce power consumption. This driver
+	  support the new device.
+
 config ACPI_THERMAL
 	tristate "Thermal Zone"
 	depends on ACPI_PROCESSOR

drivers/acpi/Makefile

@@ -61,3 +61,5 @@ obj-$(CONFIG_ACPI_SBS)		+= sbs.o
 processor-y			:= processor_core.o processor_throttling.o
 processor-y			+= processor_idle.o processor_thermal.o
 processor-$(CONFIG_CPU_FREQ)	+= processor_perflib.o
+
+obj-$(CONFIG_ACPI_PROCESSOR_AGGREGATOR) += acpi_pad.o