Documentation/cpu-freq/intel-pstate.txt - linux-imx - Git at Google

 Intel P-State driver
 --------------------

 This driver provides an interface to control the P-State selection for the
 SandyBridge+ Intel processors.

 The following document explains P-States:
 http://events.linuxfoundation.org/sites/events/files/slides/LinuxConEurope_2015.pdf
 As stated in the document, P-State doesn’t exactly mean a frequency. However, for
 the sake of the relationship with cpufreq, P-State and frequency are used
 interchangeably.

 Understanding the cpufreq core governors and policies are important before
 discussing more details about the Intel P-State driver. Based on what callbacks
 a cpufreq driver provides to the cpufreq core, it can support two types of
 drivers:
 - with target_index() callback: In this mode, the drivers using cpufreq core
 simply provide the minimum and maximum frequency limits and an additional
 interface target_index() to set the current frequency. The cpufreq subsystem
 has a number of scaling governors ("performance", "powersave", "ondemand",
 etc.). Depending on which governor is in use, cpufreq core will call for
 transitions to a specific frequency using target_index() callback.
 - setpolicy() callback: In this mode, drivers do not provide target_index()
 callback, so cpufreq core can't request a transition to a specific frequency.
 The driver provides minimum and maximum frequency limits and callbacks to set a
 policy. The policy in cpufreq sysfs is referred to as the "scaling governor".
 The cpufreq core can request the driver to operate in any of the two policies:
 "performance" and "powersave". The driver decides which frequency to use based
 on the above policy selection considering minimum and maximum frequency limits.

 The Intel P-State driver falls under the latter category, which implements the
 setpolicy() callback. This driver decides what P-State to use based on the
 requested policy from the cpufreq core. If the processor is capable of
 selecting its next P-State internally, then the driver will offload this
 responsibility to the processor (aka HWP: Hardware P-States). If not, the
 driver implements algorithms to select the next P-State.

 Since these policies are implemented in the driver, they are not same as the
 cpufreq scaling governors implementation, even if they have the same name in
 the cpufreq sysfs (scaling_governors). For example the "performance" policy is
 similar to cpufreq’s "performance" governor, but "powersave" is completely
 different than the cpufreq "powersave" governor. The strategy here is similar
 to cpufreq "ondemand", where the requested P-State is related to the system load.

 Sysfs Interface

 In addition to the frequency-controlling interfaces provided by the cpufreq
 core, the driver provides its own sysfs files to control the P-State selection.
 These files have been added to /sys/devices/system/cpu/intel_pstate/.
 Any changes made to these files are applicable to all CPUs (even in a
 multi-package system).

       max_perf_pct: Limits the maximum P-State that will be requested by
       the driver. It states it as a percentage of the available performance. The
       available (P-State) performance may be reduced by the no_turbo
       setting described below.

       min_perf_pct: Limits the minimum P-State that will be requested by
       the driver. It states it as a percentage of the max (non-turbo)
       performance level.

       no_turbo: Limits the driver to selecting P-State below the turbo
       frequency range.

       turbo_pct: Displays the percentage of the total performance that
       is supported by hardware that is in the turbo range. This number
       is independent of whether turbo has been disabled or not.

       num_pstates: Displays the number of P-States that are supported
       by hardware. This number is independent of whether turbo has
       been disabled or not.

 For example, if a system has these parameters:
 	Max 1 core turbo ratio: 0x21 (Max 1 core ratio is the maximum P-State)
 	Max non turbo ratio: 0x17
 	Minimum ratio : 0x08 (Here the ratio is called max efficiency ratio)

 Sysfs will show :
 	max_perf_pct:100, which corresponds to 1 core ratio
 	min_perf_pct:24, max_efficiency_ratio / max 1 Core ratio
 	no_turbo:0, turbo is not disabled
 	num_pstates:26 = (max 1 Core ratio - Max Efficiency Ratio + 1)
 	turbo_pct:39 = (max 1 core ratio - max non turbo ratio) / num_pstates

 Refer to "Intel® 64 and IA-32 Architectures Software Developer’s Manual
 Volume 3: System Programming Guide" to understand ratios.

 cpufreq sysfs for Intel P-State

 Since this driver registers with cpufreq, cpufreq sysfs is also presented.
 There are some important differences, which need to be considered.

 scaling_cur_freq: This displays the real frequency which was used during
 the last sample period instead of what is requested. Some other cpufreq driver,
 like acpi-cpufreq, displays what is requested (Some changes are on the
 way to fix this for acpi-cpufreq driver). The same is true for frequencies
 displayed at /proc/cpuinfo.

 scaling_governor: This displays current active policy. Since each CPU has a
 cpufreq sysfs, it is possible to set a scaling governor to each CPU. But this
 is not possible with Intel P-States, as there is one common policy for all
 CPUs. Here, the last requested policy will be applicable to all CPUs. It is
 suggested that one use the cpupower utility to change policy to all CPUs at the
 same time.

 scaling_setspeed: This attribute can never be used with Intel P-State.

 scaling_max_freq/scaling_min_freq: This interface can be used similarly to
 the max_perf_pct/min_perf_pct of Intel P-State sysfs. However since frequencies
 are converted to nearest possible P-State, this is prone to rounding errors.
 This method is not preferred to limit performance.

 affected_cpus: Not used
 related_cpus: Not used

 For contemporary Intel processors, the frequency is controlled by the
 processor itself and the P-State exposed to software is related to
 performance levels.  The idea that frequency can be set to a single
 frequency is fictional for Intel Core processors. Even if the scaling
 driver selects a single P-State, the actual frequency the processor
 will run at is selected by the processor itself.

 Tuning Intel P-State driver

 When HWP mode is not used, debugfs files have also been added to allow the
 tuning of the internal governor algorithm. These files are located at
 /sys/kernel/debug/pstate_snb/. The algorithm uses a PID (Proportional
 Integral Derivative) controller. The PID tunable parameters are:

       deadband
       d_gain_pct
       i_gain_pct
       p_gain_pct
       sample_rate_ms
       setpoint

 To adjust these parameters, some understanding of driver implementation is
 necessary. There are some tweeks described here, but be very careful. Adjusting
 them requires expert level understanding of power and performance relationship.
 These limits are only useful when the "powersave" policy is active.

 -To make the system more responsive to load changes, sample_rate_ms can
 be adjusted  (current default is 10ms).
 -To make the system use higher performance, even if the load is lower, setpoint
 can be adjusted to a lower number. This will also lead to faster ramp up time
 to reach the maximum P-State.
 If there are no derivative and integral coefficients, The next P-State will be
 equal to:
 	current P-State - ((setpoint - current cpu load) * p_gain_pct)

 For example, if the current PID parameters are (Which are defaults for the core
 processors like SandyBridge):
       deadband = 0
       d_gain_pct = 0
       i_gain_pct = 0
       p_gain_pct = 20
       sample_rate_ms = 10
       setpoint = 97

 If the current P-State = 0x08 and current load = 100, this will result in the
 next P-State = 0x08 - ((97 - 100) * 0.2) = 8.6 (rounded to 9). Here the P-State
 goes up by only 1. If during next sample interval the current load doesn't
 change and still 100, then P-State goes up by one again. This process will
 continue as long as the load is more than the setpoint until the maximum P-State
 is reached.

 For the same load at setpoint = 60, this will result in the next P-State
 = 0x08 - ((60 - 100) * 0.2) = 16
 So by changing the setpoint from 97 to 60, there is an increase of the
 next P-State from 9 to 16. So this will make processor execute at higher
 P-State for the same CPU load. If the load continues to be more than the
 setpoint during next sample intervals, then P-State will go up again till the
 maximum P-State is reached. But the ramp up time to reach the maximum P-State
 will be much faster when the setpoint is 60 compared to 97.

 Debugging Intel P-State driver

 Event tracing
 To debug P-State transition, the Linux event tracing interface can be used.
 There are two specific events, which can be enabled (Provided the kernel
 configs related to event tracing are enabled).

 # cd /sys/kernel/debug/tracing/
 # echo 1 > events/power/pstate_sample/enable
 # echo 1 > events/power/cpu_frequency/enable
 # cat trace
 gnome-terminal--4510  [001] ..s.  1177.680733: pstate_sample: core_busy=107
 	scaled=94 from=26 to=26 mperf=1143818 aperf=1230607 tsc=29838618
 		freq=2474476
 cat-5235  [002] ..s.  1177.681723: cpu_frequency: state=2900000 cpu_id=2


 Using ftrace

 If function level tracing is required, the Linux ftrace interface can be used.
 For example if we want to check how often a function to set a P-State is
 called, we can set ftrace filter to intel_pstate_set_pstate.

 # cd /sys/kernel/debug/tracing/
 # cat available_filter_functions | grep -i pstate
 intel_pstate_set_pstate
 intel_pstate_cpu_init
 ...

 # echo intel_pstate_set_pstate > set_ftrace_filter
 # echo function > current_tracer
 # cat trace | head -15
 # tracer: function
 #
 # entries-in-buffer/entries-written: 80/80   #P:4
 #
 #                              _-----=> irqs-off
 #                             / _----=> need-resched
 #                            | / _---=> hardirq/softirq
 #                            || / _--=> preempt-depth
 #                            ||| /     delay
 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
 #              | |       |   ||||       |         |
             Xorg-3129  [000] ..s.  2537.644844: intel_pstate_set_pstate <-intel_pstate_timer_func
  gnome-terminal--4510  [002] ..s.  2537.649844: intel_pstate_set_pstate <-intel_pstate_timer_func
      gnome-shell-3409  [001] ..s.  2537.650850: intel_pstate_set_pstate <-intel_pstate_timer_func
           <idle>-0     [000] ..s.  2537.654843: intel_pstate_set_pstate <-intel_pstate_timer_func
	Intel P-State driver
	--------------------

	This driver provides an interface to control the P-State selection for the
	SandyBridge+ Intel processors.

	The following document explains P-States:
	http://events.linuxfoundation.org/sites/events/files/slides/LinuxConEurope_2015.pdf
	As stated in the document, P-State doesn’t exactly mean a frequency. However, for
	the sake of the relationship with cpufreq, P-State and frequency are used
	interchangeably.

	Understanding the cpufreq core governors and policies are important before
	discussing more details about the Intel P-State driver. Based on what callbacks
	a cpufreq driver provides to the cpufreq core, it can support two types of
	drivers:
	- with target_index() callback: In this mode, the drivers using cpufreq core
	simply provide the minimum and maximum frequency limits and an additional
	interface target_index() to set the current frequency. The cpufreq subsystem
	has a number of scaling governors ("performance", "powersave", "ondemand",
	etc.). Depending on which governor is in use, cpufreq core will call for
	transitions to a specific frequency using target_index() callback.
	- setpolicy() callback: In this mode, drivers do not provide target_index()
	callback, so cpufreq core can't request a transition to a specific frequency.
	The driver provides minimum and maximum frequency limits and callbacks to set a
	policy. The policy in cpufreq sysfs is referred to as the "scaling governor".
	The cpufreq core can request the driver to operate in any of the two policies:
	"performance" and "powersave". The driver decides which frequency to use based
	on the above policy selection considering minimum and maximum frequency limits.

	The Intel P-State driver falls under the latter category, which implements the
	setpolicy() callback. This driver decides what P-State to use based on the
	requested policy from the cpufreq core. If the processor is capable of
	selecting its next P-State internally, then the driver will offload this
	responsibility to the processor (aka HWP: Hardware P-States). If not, the
	driver implements algorithms to select the next P-State.

	Since these policies are implemented in the driver, they are not same as the
	cpufreq scaling governors implementation, even if they have the same name in
	the cpufreq sysfs (scaling_governors). For example the "performance" policy is
	similar to cpufreq’s "performance" governor, but "powersave" is completely
	different than the cpufreq "powersave" governor. The strategy here is similar
	to cpufreq "ondemand", where the requested P-State is related to the system load.

	Sysfs Interface

	In addition to the frequency-controlling interfaces provided by the cpufreq
	core, the driver provides its own sysfs files to control the P-State selection.
	These files have been added to /sys/devices/system/cpu/intel_pstate/.
	Any changes made to these files are applicable to all CPUs (even in a
	multi-package system).

	max_perf_pct: Limits the maximum P-State that will be requested by
	the driver. It states it as a percentage of the available performance. The
	available (P-State) performance may be reduced by the no_turbo
	setting described below.

	min_perf_pct: Limits the minimum P-State that will be requested by
	the driver. It states it as a percentage of the max (non-turbo)
	performance level.

	no_turbo: Limits the driver to selecting P-State below the turbo
	frequency range.

	turbo_pct: Displays the percentage of the total performance that
	is supported by hardware that is in the turbo range. This number
	is independent of whether turbo has been disabled or not.

	num_pstates: Displays the number of P-States that are supported
	by hardware. This number is independent of whether turbo has
	been disabled or not.

	For example, if a system has these parameters:
	Max 1 core turbo ratio: 0x21 (Max 1 core ratio is the maximum P-State)
	Max non turbo ratio: 0x17
	Minimum ratio : 0x08 (Here the ratio is called max efficiency ratio)

	Sysfs will show :
	max_perf_pct:100, which corresponds to 1 core ratio
	min_perf_pct:24, max_efficiency_ratio / max 1 Core ratio
	no_turbo:0, turbo is not disabled
	num_pstates:26 = (max 1 Core ratio - Max Efficiency Ratio + 1)
	turbo_pct:39 = (max 1 core ratio - max non turbo ratio) / num_pstates

	Refer to "Intel® 64 and IA-32 Architectures Software Developer’s Manual
	Volume 3: System Programming Guide" to understand ratios.

	cpufreq sysfs for Intel P-State

	Since this driver registers with cpufreq, cpufreq sysfs is also presented.
	There are some important differences, which need to be considered.

	scaling_cur_freq: This displays the real frequency which was used during
	the last sample period instead of what is requested. Some other cpufreq driver,
	like acpi-cpufreq, displays what is requested (Some changes are on the
	way to fix this for acpi-cpufreq driver). The same is true for frequencies
	displayed at /proc/cpuinfo.

	scaling_governor: This displays current active policy. Since each CPU has a
	cpufreq sysfs, it is possible to set a scaling governor to each CPU. But this
	is not possible with Intel P-States, as there is one common policy for all
	CPUs. Here, the last requested policy will be applicable to all CPUs. It is
	suggested that one use the cpupower utility to change policy to all CPUs at the
	same time.

	scaling_setspeed: This attribute can never be used with Intel P-State.

	scaling_max_freq/scaling_min_freq: This interface can be used similarly to
	the max_perf_pct/min_perf_pct of Intel P-State sysfs. However since frequencies
	are converted to nearest possible P-State, this is prone to rounding errors.
	This method is not preferred to limit performance.

	affected_cpus: Not used
	related_cpus: Not used

	For contemporary Intel processors, the frequency is controlled by the
	processor itself and the P-State exposed to software is related to
	performance levels. The idea that frequency can be set to a single
	frequency is fictional for Intel Core processors. Even if the scaling
	driver selects a single P-State, the actual frequency the processor
	will run at is selected by the processor itself.

	Tuning Intel P-State driver

	When HWP mode is not used, debugfs files have also been added to allow the
	tuning of the internal governor algorithm. These files are located at
	/sys/kernel/debug/pstate_snb/. The algorithm uses a PID (Proportional
	Integral Derivative) controller. The PID tunable parameters are:

	deadband
	d_gain_pct
	i_gain_pct
	p_gain_pct
	sample_rate_ms
	setpoint

	To adjust these parameters, some understanding of driver implementation is
	necessary. There are some tweeks described here, but be very careful. Adjusting
	them requires expert level understanding of power and performance relationship.
	These limits are only useful when the "powersave" policy is active.

	-To make the system more responsive to load changes, sample_rate_ms can
	be adjusted (current default is 10ms).
	-To make the system use higher performance, even if the load is lower, setpoint
	can be adjusted to a lower number. This will also lead to faster ramp up time
	to reach the maximum P-State.
	If there are no derivative and integral coefficients, The next P-State will be
	equal to:
	current P-State - ((setpoint - current cpu load) * p_gain_pct)

	For example, if the current PID parameters are (Which are defaults for the core
	processors like SandyBridge):
	deadband = 0
	d_gain_pct = 0
	i_gain_pct = 0
	p_gain_pct = 20
	sample_rate_ms = 10
	setpoint = 97

	If the current P-State = 0x08 and current load = 100, this will result in the
	next P-State = 0x08 - ((97 - 100) * 0.2) = 8.6 (rounded to 9). Here the P-State
	goes up by only 1. If during next sample interval the current load doesn't
	change and still 100, then P-State goes up by one again. This process will
	continue as long as the load is more than the setpoint until the maximum P-State
	is reached.

	For the same load at setpoint = 60, this will result in the next P-State
	= 0x08 - ((60 - 100) * 0.2) = 16
	So by changing the setpoint from 97 to 60, there is an increase of the
	next P-State from 9 to 16. So this will make processor execute at higher
	P-State for the same CPU load. If the load continues to be more than the
	setpoint during next sample intervals, then P-State will go up again till the
	maximum P-State is reached. But the ramp up time to reach the maximum P-State
	will be much faster when the setpoint is 60 compared to 97.

	Debugging Intel P-State driver

	Event tracing
	To debug P-State transition, the Linux event tracing interface can be used.
	There are two specific events, which can be enabled (Provided the kernel
	configs related to event tracing are enabled).

	# cd /sys/kernel/debug/tracing/
	# echo 1 > events/power/pstate_sample/enable
	# echo 1 > events/power/cpu_frequency/enable
	# cat trace
	gnome-terminal--4510 [001] ..s. 1177.680733: pstate_sample: core_busy=107
	scaled=94 from=26 to=26 mperf=1143818 aperf=1230607 tsc=29838618
	freq=2474476
	cat-5235 [002] ..s. 1177.681723: cpu_frequency: state=2900000 cpu_id=2


	Using ftrace

	If function level tracing is required, the Linux ftrace interface can be used.
	For example if we want to check how often a function to set a P-State is
	called, we can set ftrace filter to intel_pstate_set_pstate.

	# cd /sys/kernel/debug/tracing/
	# cat available_filter_functions \| grep -i pstate
	intel_pstate_set_pstate
	intel_pstate_cpu_init
	...

	# echo intel_pstate_set_pstate > set_ftrace_filter
	# echo function > current_tracer
	# cat trace \| head -15
	# tracer: function
	#
	# entries-in-buffer/entries-written: 80/80 #P:4
	#
	# _-----=> irqs-off
	# / _----=> need-resched
	# \| / _---=> hardirq/softirq
	# \|\| / _--=> preempt-depth
	# \|\|\| / delay
	# TASK-PID CPU# \|\|\|\| TIMESTAMP FUNCTION
	# \| \| \| \|\|\|\| \| \|
	Xorg-3129 [000] ..s. 2537.644844: intel_pstate_set_pstate <-intel_pstate_timer_func
	gnome-terminal--4510 [002] ..s. 2537.649844: intel_pstate_set_pstate <-intel_pstate_timer_func
	gnome-shell-3409 [001] ..s. 2537.650850: intel_pstate_set_pstate <-intel_pstate_timer_func
	<idle>-0 [000] ..s. 2537.654843: intel_pstate_set_pstate <-intel_pstate_timer_func