cpufreq: interactive: New 'interactive' governor

This governor is designed for latency-sensitive workloads, such as
interactive user interfaces.  The interactive governor aims to be
significantly more responsive to ramp CPU quickly up when CPU-intensive
activity begins.

Existing governors sample CPU load at a particular rate, typically
every X ms.  This can lead to under-powering UI threads for the period of
time during which the user begins interacting with a previously-idle system
until the next sample period happens.

The 'interactive' governor uses a different approach. Instead of sampling
the CPU at a specified rate, the governor will check whether to scale the
CPU frequency up soon after coming out of idle.  When the CPU comes out of
idle, a timer is configured to fire within 1-2 ticks.  If the CPU is very
busy from exiting idle to when the timer fires then we assume the CPU is
underpowered and ramp to MAX speed.

If the CPU was not sufficiently busy to immediately ramp to MAX speed, then
the governor evaluates the CPU load since the last speed adjustment,
choosing the highest value between that longer-term load or the short-term
load since idle exit to determine the CPU speed to ramp to.

A realtime thread is used for scaling up, giving the remaining tasks the
CPU performance benefit, unlike existing governors which are more likely to
schedule rampup work to occur after your performance starved tasks have
completed.

The tuneables for this governor are:
/sys/devices/system/cpu/cpufreq/interactive/min_sample_time:
	The minimum amount of time to spend at the current frequency before
	ramping down. This is to ensure that the governor has seen enough
	historic CPU load data to determine the appropriate workload.
	Default is 80000 uS.
/sys/devices/system/cpu/cpufreq/interactive/go_maxspeed_load
	The CPU load at which to ramp to max speed.  Default is 85.

Change-Id: Ib2b362607c62f7c56d35f44a9ef3280f98c17585
Signed-off-by: Mike Chan <mike@android.com>
Signed-off-by: Todd Poynor <toddpoynor@google.com>
Bug: 3152864

1 files changed, 36 insertions, 0 deletions

diff --git a/Documentation/cpu-freq/governors.txt b/Documentation/cpu-freq/governors.txt
index e74d0a2eb1cf..16799cede680 100644
--- a/Documentation/cpu-freq/governors.txt
+++ b/Documentation/cpu-freq/governors.txt
@@ -28,6 +28,7 @@ Contents:
 2.3  Userspace
 2.4  Ondemand
 2.5  Conservative
+2.6  Interactive
 
 3.   The Governor Interface in the CPUfreq Core
 
@@ -193,6 +194,41 @@ governor but for the opposite direction.  For example when set to its
 default value of '20' it means that if the CPU usage needs to be below
 20% between samples to have the frequency decreased.
 
+
+2.6 Interactive
+---------------
+
+The CPUfreq governor "interactive" is designed for latency-sensitive,
+interactive workloads. This governor sets the CPU speed depending on
+usage, similar to "ondemand" and "conservative" governors.  However,
+the governor is more aggressive about scaling the CPU speed up in
+response to CPU-intensive activity.
+
+Sampling the CPU load every X ms can lead to under-powering the CPU
+for X ms, leading to dropped frames, stuttering UI, etc.  Instead of
+sampling the cpu at a specified rate, the interactive governor will
+check whether to scale the cpu frequency up soon after coming out of
+idle.  When the cpu comes out of idle, a timer is configured to fire
+within 1-2 ticks.  If the cpu is very busy between exiting idle and
+when the timer fires then we assume the cpu is underpowered and ramp
+to MAX speed.
+    
+If the cpu was not sufficiently busy to immediately ramp to MAX speed,
+then governor evaluates the cpu load since the last speed adjustment,
+choosing th highest value between that longer-term load or the
+short-term load since idle exit to determine the cpu speed to ramp to.
+
+The tuneable value for this governor are:
+
+min_sample_time: The minimum amount of time to spend at the current
+frequency before ramping down. This is to ensure that the governor has
+seen enough historic cpu load data to determine the appropriate
+workload.  Default is 80000 uS.
+
+go_maxspeed_load: The CPU load at which to ramp to max speed.  Default
+is 85.
+
+
 3. The Governor Interface in the CPUfreq Core
 =============================================