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Diffstat (limited to 'Documentation/cpu-freq')
| -rw-r--r-- | Documentation/cpu-freq/intel-pstate.txt | 281 |
1 files changed, 0 insertions, 281 deletions
diff --git a/Documentation/cpu-freq/intel-pstate.txt b/Documentation/cpu-freq/intel-pstate.txt deleted file mode 100644 index 3fdcdfd968ba..000000000000 --- a/Documentation/cpu-freq/intel-pstate.txt +++ /dev/null @@ -1,281 +0,0 @@ -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, Refer to later section on placing "Per-CPU limits"). - - 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. - -There is one more sysfs attribute in /sys/devices/system/cpu/intel_pstate/ -that can be used for controlling the operation mode of the driver: - - status: Three settings are possible: - "off" - The driver is not in use at this time. - "active" - The driver works as a P-state governor (default). - "passive" - The driver works as a regular cpufreq one and collaborates - with the generic cpufreq governors (it sets P-states as - requested by those governors). - The current setting is returned by reads from this attribute. Writing one - of the above strings to it changes the operation mode as indicated by that - string, if possible. If HW-managed P-states (HWP) are enabled, it is not - possible to change the driver's operation mode and attempts to write to - this attribute will fail. - -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. - -Per-CPU limits - -The kernel command line option "intel_pstate=per_cpu_perf_limits" forces -the intel_pstate driver to use per-CPU performance limits. When it is set, -the sysfs control interface described above is subject to limitations. -- The following controls are not available for both read and write - /sys/devices/system/cpu/intel_pstate/max_perf_pct - /sys/devices/system/cpu/intel_pstate/min_perf_pct -- The following controls can be used to set performance limits, as far as the -architecture of the processor permits: - /sys/devices/system/cpu/cpu*/cpufreq/scaling_max_freq - /sys/devices/system/cpu/cpu*/cpufreq/scaling_min_freq - /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor -- User can still observe turbo percent and number of P-States from - /sys/devices/system/cpu/intel_pstate/turbo_pct - /sys/devices/system/cpu/intel_pstate/num_pstates -- User can read write system wide turbo status - /sys/devices/system/cpu/no_turbo - -Support of energy performance hints -It is possible to provide hints to the HWP algorithms in the processor -to be more performance centric to more energy centric. When the driver -is using HWP, two additional cpufreq sysfs attributes are presented for -each logical CPU. -These attributes are: - - energy_performance_available_preferences - - energy_performance_preference - -To get list of supported hints: -$ cat energy_performance_available_preferences - default performance balance_performance balance_power power - -The current preference can be read or changed via cpufreq sysfs -attribute "energy_performance_preference". Reading from this attribute -will display current effective setting. User can write any of the valid -preference string to this attribute. User can always restore to power-on -default by writing "default". - -Since threads can migrate to different CPUs, this is possible that the -new CPU may have different energy performance preference than the previous -one. To avoid such issues, either threads can be pinned to specific CPUs -or set the same energy performance preference value to all CPUs. - -Tuning Intel P-State driver - -When the performance can be tuned using PID (Proportional Integral -Derivative) controller, debugfs files are provided for adjusting performance. -They are presented under: -/sys/kernel/debug/pstate_snb/ - -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 |