cpufreq: introduce hotplug governor

The "hotplug" governor scales CPU frequency based on load, similar to
"ondemand".  It scales up to the highest frequency when "up_threshold"
is crossed and scales down one frequency at a time when "down_threshold"
is crossed.  Unlike those governors, target frequencies are determined
by directly accessing the CPUfreq frequency table, instead of taking
some percentage of maximum available frequency.

The key difference in the "hotplug" governor is that it will disable
auxillary CPUs when the system is very idle, and enable them again once
the system becomes busy.  This is achieved by averaging load over
multiple sampling periods; if CPUs were online or offlined based on a
single sampling period then thrashing will occur.

Sysfs entries exist for "hotplug_in_sampling_periods" and for
"hotplug_out_sampling_periods" which determine how many consecutive
periods get averaged to determine if auxillery CPUs should be onlined or
offlined.  Defaults are 5 periods and 20 periods respectively.
Otherwise the standard sysfs entries you might find for "ondemand" and
"conservative" governors are there.

To use this governor it is assumed that your CPUfreq driver has
populated the CPUfreq table, CONFIG_NO_HZ is enabled and
CONFIG_HOTPLUG_CPU is enabled.

Changes in V2:
	Corrected default sampling periods
	Optimized load history array resizing
	Maintain load history when resizing array
	Add locking to dbs_check_cpu
	Switch from enable_nonboot_cpus to cpu_up
	Switch from disable_nonboot_cpus to down_cpu
	Fix some printks
	Coding style around for-loops

Signed-off-by: Mike Turquette <mturquette@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>

1 files changed, 33 insertions, 0 deletions

diff --git a/drivers/cpufreq/Kconfig b/drivers/cpufreq/Kconfig
index 9fb8485..07711d1 100644
--- a/drivers/cpufreq/Kconfig
+++ b/drivers/cpufreq/Kconfig
@@ -99,6 +99,19 @@ config CPU_FREQ_DEFAULT_GOV_CONSERVATIVE
 	  Be aware that not all cpufreq drivers support the conservative
 	  governor. If unsure have a look at the help section of the
 	  driver. Fallback governor will be the performance governor.
+
+config CPU_FREQ_DEFAULT_GOV_HOTPLUG
+	bool "hotplug"
+	select CPU_FREQ_GOV_HOTPLUG
+	select CPU_FREQ_GOV_PERFORMANCE
+	help
+	  Use the CPUFreq governor 'hotplug' as default. This allows you
+	  to get a full dynamic frequency capable system with CPU
+	  hotplug support by simply loading your cpufreq low-level
+	  hardware driver.  Be aware that not all cpufreq drivers
+	  support the hotplug governor. If unsure have a look at
+	  the help section of the driver. Fallback governor will be the
+	  performance governor.
 endchoice
 
 config CPU_FREQ_GOV_PERFORMANCE
@@ -184,5 +197,25 @@ depends on X86
 source "drivers/cpufreq/Kconfig.x86"
 endmenu
 
+config CPU_FREQ_GOV_HOTPLUG
+	tristate "'hotplug' cpufreq governor"
+	depends on CPU_FREQ && NO_HZ && HOTPLUG_CPU
+	help
+	  'hotplug' - this driver mimics the frequency scaling behavior
+	  in 'ondemand', but with several key differences.  First is
+	  that frequency transitions use the CPUFreq table directly,
+	  instead of incrementing in a percentage of the maximum
+	  available frequency.  Second 'hotplug' will offline auxillary
+	  CPUs when the system is idle, and online those CPUs once the
+	  system becomes busy again.  This last feature is needed for
+	  architectures which transition to low power states when only
+	  the "master" CPU is online, or for thermally constrained
+	  devices.
+
+	  If you don't have one of these architectures or devices, use
+	  'ondemand' instead.
+
+	  If in doubt, say N.
+
 endif
 endmenu