Documentation: cpufreq: intel_pstate: enhance documentation
This is an attempt to make documentation more user friendly. Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com> Reviewed-by: Doug Smythies <dsmythies@telus.net> Reviewed-by: Chen, Yu C <yu.c.chen@intel.com> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
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Srinivas Pandruvada
Rafael J. Wysocki
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@ -1,61 +1,131 @@
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Intel P-state driver
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Intel P-State driver
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--------------------
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This driver provides an interface to control the P state selection for
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SandyBridge+ Intel processors. The driver can operate two different
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modes based on the processor model, legacy mode and Hardware P state (HWP)
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mode.
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This driver provides an interface to control the P-State selection for the
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SandyBridge+ Intel processors.
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In legacy mode, the Intel P-state implements two internal governors,
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performance and powersave, that differ from the general cpufreq governors of
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the same name (the general cpufreq governors implement target(), whereas the
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internal Intel P-state governors implement setpolicy()). The internal
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performance governor sets the max_perf_pct and min_perf_pct to 100; that is,
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the governor selects the highest available P state to maximize the performance
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of the core. The internal powersave governor selects the appropriate P state
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based on the current load on the CPU.
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The following document explains P-States:
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http://events.linuxfoundation.org/sites/events/files/slides/LinuxConEurope_2015.pdf
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As stated in the document, P-State doesn’t exactly mean a frequency. However, for
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the sake of the relationship with cpufreq, P-State and frequency are used
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interchangeably.
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In HWP mode P state selection is implemented in the processor
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itself. The driver provides the interfaces between the cpufreq core and
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the processor to control P state selection based on user preferences
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and reporting frequency to the cpufreq core. In this mode the
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internal Intel P-state governor code is disabled.
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Understanding the cpufreq core governors and policies are important before
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discussing more details about the Intel P-State driver. Based on what callbacks
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a cpufreq driver provides to the cpufreq core, it can support two types of
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drivers:
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- with target_index() callback: In this mode, the drivers using cpufreq core
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simply provide the minimum and maximum frequency limits and an additional
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interface target_index() to set the current frequency. The cpufreq subsystem
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has a number of scaling governors ("performance", "powersave", "ondemand",
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etc.). Depending on which governor is in use, cpufreq core will call for
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transitions to a specific frequency using target_index() callback.
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- setpolicy() callback: In this mode, drivers do not provide target_index()
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callback, so cpufreq core can't request a transition to a specific frequency.
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The driver provides minimum and maximum frequency limits and callbacks to set a
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policy. The policy in cpufreq sysfs is referred to as the "scaling governor".
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The cpufreq core can request the driver to operate in any of the two policies:
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"performance: and "powersave". The driver decides which frequency to use based
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on the above policy selection considering minimum and maximum frequency limits.
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In addition to the interfaces provided by the cpufreq core for
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controlling frequency the driver provides sysfs files for
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controlling P state selection. These files have been added to
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/sys/devices/system/cpu/intel_pstate/
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The Intel P-State driver falls under the latter category, which implements the
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setpolicy() callback. This driver decides what P-State to use based on the
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requested policy from the cpufreq core. If the processor is capable of
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selecting its next P-State internally, then the driver will offload this
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responsibility to the processor (aka HWP: Hardware P-States). If not, the
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driver implements algorithms to select the next P-State.
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max_perf_pct: limits the maximum P state that will be requested by
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the driver stated as a percentage of the available performance. The
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available (P states) performance may be reduced by the no_turbo
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Since these policies are implemented in the driver, they are not same as the
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cpufreq scaling governors implementation, even if they have the same name in
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the cpufreq sysfs (scaling_governors). For example the "performance" policy is
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similar to cpufreq’s "performance" governor, but "powersave" is completely
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different than the cpufreq "powersave" governor. The strategy here is similar
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to cpufreq "ondemand", where the requested P-State is related to the system load.
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Sysfs Interface
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In addition to the frequency-controlling interfaces provided by the cpufreq
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core, the driver provides its own sysfs files to control the P-State selection.
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These files have been added to /sys/devices/system/cpu/intel_pstate/.
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Any changes made to these files are applicable to all CPUs (even in a
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multi-package system).
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max_perf_pct: Limits the maximum P-State that will be requested by
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the driver. It states it as a percentage of the available performance. The
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available (P-State) performance may be reduced by the no_turbo
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setting described below.
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min_perf_pct: limits the minimum P state that will be requested by
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the driver stated as a percentage of the max (non-turbo)
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min_perf_pct: Limits the minimum P-State that will be requested by
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the driver. It states it as a percentage of the max (non-turbo)
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performance level.
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no_turbo: limits the driver to selecting P states below the turbo
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no_turbo: Limits the driver to selecting P-State below the turbo
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frequency range.
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turbo_pct: displays the percentage of the total performance that
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turbo_pct: Displays the percentage of the total performance that
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is supported by hardware that is in the turbo range. This number
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is independent of whether turbo has been disabled or not.
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num_pstates: displays the number of pstates that are supported
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num_pstates: Displays the number of P-States that are supported
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by hardware. This number is independent of whether turbo has
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been disabled or not.
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For example, if a system has these parameters:
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Max 1 core turbo ratio: 0x21 (Max 1 core ratio is the maximum P-State)
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Max non turbo ratio: 0x17
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Minimum ratio : 0x08 (Here the ratio is called max efficiency ratio)
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Sysfs will show :
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max_perf_pct:100, which corresponds to 1 core ratio
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min_perf_pct:24, max_efficiency_ratio / max 1 Core ratio
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no_turbo:0, turbo is not disabled
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num_pstates:26 = (max 1 Core ratio - Max Efficiency Ratio + 1)
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turbo_pct:39 = (max 1 core ratio - max non turbo ratio) / num_pstates
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Refer to "Intel® 64 and IA-32 Architectures Software Developer’s Manual
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Volume 3: System Programming Guide" to understand ratios.
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cpufreq sysfs for Intel P-State
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Since this driver registers with cpufreq, cpufreq sysfs is also presented.
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There are some important differences, which need to be considered.
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scaling_cur_freq: This displays the real frequency which was used during
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the last sample period instead of what is requested. Some other cpufreq driver,
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like acpi-cpufreq, displays what is requested (Some changes are on the
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way to fix this for acpi-cpufreq driver). The same is true for frequencies
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displayed at /proc/cpuinfo.
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scaling_governor: This displays current active policy. Since each CPU has a
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cpufreq sysfs, it is possible to set a scaling governor to each CPU. But this
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is not possible with Intel P-States, as there is one common policy for all
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CPUs. Here, the last requested policy will be applicable to all CPUs. It is
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suggested that one use the cpupower utility to change policy to all CPUs at the
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same time.
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scaling_setspeed: This attribute can never be used with Intel P-State.
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scaling_max_freq/scaling_min_freq: This interface can be used similarly to
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the max_perf_pct/min_perf_pct of Intel P-State sysfs. However since frequencies
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are converted to nearest possible P-State, this is prone to rounding errors.
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This method is not preferred to limit performance.
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affected_cpus: Not used
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related_cpus: Not used
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For contemporary Intel processors, the frequency is controlled by the
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processor itself and the P-states exposed to software are related to
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processor itself and the P-State exposed to software is related to
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performance levels. The idea that frequency can be set to a single
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frequency is fiction for Intel Core processors. Even if the scaling
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driver selects a single P state the actual frequency the processor
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frequency is fictional for Intel Core processors. Even if the scaling
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driver selects a single P-State, the actual frequency the processor
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will run at is selected by the processor itself.
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For legacy mode debugfs files have also been added to allow tuning of
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the internal governor algorythm. These files are located at
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/sys/kernel/debug/pstate_snb/ These files are NOT present in HWP mode.
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Tuning Intel P-State driver
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When HWP mode is not used, debugfs files have also been added to allow the
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tuning of the internal governor algorithm. These files are located at
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/sys/kernel/debug/pstate_snb/. The algorithm uses a PID (Proportional
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Integral Derivative) controller. The PID tunable parameters are:
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deadband
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d_gain_pct
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@ -63,3 +133,90 @@ the internal governor algorythm. These files are located at
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p_gain_pct
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sample_rate_ms
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setpoint
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To adjust these parameters, some understanding of driver implementation is
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necessary. There are some tweeks described here, but be very careful. Adjusting
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them requires expert level understanding of power and performance relationship.
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These limits are only useful when the "powersave" policy is active.
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-To make the system more responsive to load changes, sample_rate_ms can
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be adjusted (current default is 10ms).
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-To make the system use higher performance, even if the load is lower, setpoint
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can be adjusted to a lower number. This will also lead to faster ramp up time
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to reach the maximum P-State.
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If there are no derivative and integral coefficients, The next P-State will be
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equal to:
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current P-State - ((setpoint - current cpu load) * p_gain_pct)
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For example, if the current PID parameters are (Which are defaults for the core
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processors like SandyBridge):
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deadband = 0
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d_gain_pct = 0
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i_gain_pct = 0
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p_gain_pct = 20
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sample_rate_ms = 10
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setpoint = 97
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If the current P-State = 0x08 and current load = 100, this will result in the
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next P-State = 0x08 - ((97 - 100) * 0.2) = 8.6 (rounded to 9). Here the P-State
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goes up by only 1. If during next sample interval the current load doesn't
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change and still 100, then P-State goes up by one again. This process will
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continue as long as the load is more than the setpoint until the maximum P-State
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is reached.
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For the same load at setpoint = 60, this will result in the next P-State
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= 0x08 - ((60 - 100) * 0.2) = 16
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So by changing the setpoint from 97 to 60, there is an increase of the
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next P-State from 9 to 16. So this will make processor execute at higher
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P-State for the same CPU load. If the load continues to be more than the
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setpoint during next sample intervals, then P-State will go up again till the
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maximum P-State is reached. But the ramp up time to reach the maximum P-State
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will be much faster when the setpoint is 60 compared to 97.
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Debugging Intel P-State driver
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Event tracing
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To debug P-State transition, the Linux event tracing interface can be used.
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There are two specific events, which can be enabled (Provided the kernel
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configs related to event tracing are enabled).
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# cd /sys/kernel/debug/tracing/
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# echo 1 > events/power/pstate_sample/enable
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# echo 1 > events/power/cpu_frequency/enable
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# cat trace
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gnome-terminal--4510 [001] ..s. 1177.680733: pstate_sample: core_busy=107
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scaled=94 from=26 to=26 mperf=1143818 aperf=1230607 tsc=29838618
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freq=2474476
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cat-5235 [002] ..s. 1177.681723: cpu_frequency: state=2900000 cpu_id=2
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Using ftrace
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If function level tracing is required, the Linux ftrace interface can be used.
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For example if we want to check how often a function to set a P-State is
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called, we can set ftrace filter to intel_pstate_set_pstate.
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# cd /sys/kernel/debug/tracing/
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# cat available_filter_functions | grep -i pstate
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intel_pstate_set_pstate
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intel_pstate_cpu_init
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...
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# echo intel_pstate_set_pstate > set_ftrace_filter
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# echo function > current_tracer
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# cat trace | head -15
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# tracer: function
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#
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# entries-in-buffer/entries-written: 80/80 #P:4
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#
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# _-----=> irqs-off
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# / _----=> need-resched
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# | / _---=> hardirq/softirq
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# || / _--=> preempt-depth
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# ||| / delay
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# TASK-PID CPU# |||| TIMESTAMP FUNCTION
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# | | | |||| | |
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Xorg-3129 [000] ..s. 2537.644844: intel_pstate_set_pstate <-intel_pstate_timer_func
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gnome-terminal--4510 [002] ..s. 2537.649844: intel_pstate_set_pstate <-intel_pstate_timer_func
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gnome-shell-3409 [001] ..s. 2537.650850: intel_pstate_set_pstate <-intel_pstate_timer_func
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<idle>-0 [000] ..s. 2537.654843: intel_pstate_set_pstate <-intel_pstate_timer_func
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