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Diffstat (limited to 'Documentation/cpu-freq/intel-pstate.txt')
| -rw-r--r-- | Documentation/cpu-freq/intel-pstate.txt | 241 |
1 files changed, 199 insertions, 42 deletions
diff --git a/Documentation/cpu-freq/intel-pstate.txt b/Documentation/cpu-freq/intel-pstate.txt index be8d4006b..f7b12c071 100644 --- a/Documentation/cpu-freq/intel-pstate.txt +++ b/Documentation/cpu-freq/intel-pstate.txt @@ -1,61 +1,131 @@ -Intel P-state driver +Intel P-State driver -------------------- -This driver provides an interface to control the P state selection for -SandyBridge+ Intel processors. The driver can operate two different -modes based on the processor model, legacy mode and Hardware P state (HWP) -mode. - -In legacy mode, the Intel P-state implements two internal governors, -performance and powersave, that differ from the general cpufreq governors of -the same name (the general cpufreq governors implement target(), whereas the -internal Intel P-state governors implement setpolicy()). The internal -performance governor sets the max_perf_pct and min_perf_pct to 100; that is, -the governor selects the highest available P state to maximize the performance -of the core. The internal powersave governor selects the appropriate P state -based on the current load on the CPU. - -In HWP mode P state selection is implemented in the processor -itself. The driver provides the interfaces between the cpufreq core and -the processor to control P state selection based on user preferences -and reporting frequency to the cpufreq core. In this mode the -internal Intel P-state governor code is disabled. - -In addition to the interfaces provided by the cpufreq core for -controlling frequency the driver provides sysfs files for -controlling P state selection. These files have been added to -/sys/devices/system/cpu/intel_pstate/ - - max_perf_pct: limits the maximum P state that will be requested by - the driver stated as a percentage of the available performance. The - available (P states) performance may be reduced by the no_turbo +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 stated as a percentage of the max (non-turbo) + 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 states below the turbo + 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 + 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 pstates that are supported - by hardware. This number is independent of whether turbo has + 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-states exposed to software are related to +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 fiction for Intel Core processors. Even if the scaling -driver selects a single P state the actual frequency the processor +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. -For legacy mode debugfs files have also been added to allow tuning of -the internal governor algorythm. These files are located at -/sys/kernel/debug/pstate_snb/ These files are NOT present in HWP mode. +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 @@ -63,3 +133,90 @@ the internal governor algorythm. These files are located at 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 |