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Diffstat (limited to 'Documentation/admin-guide/pm/intel_pstate.rst')
| -rw-r--r-- | Documentation/admin-guide/pm/intel_pstate.rst | 61 |
1 files changed, 8 insertions, 53 deletions
diff --git a/Documentation/admin-guide/pm/intel_pstate.rst b/Documentation/admin-guide/pm/intel_pstate.rst index 1d6249825efc..d2b6fda3d67b 100644 --- a/Documentation/admin-guide/pm/intel_pstate.rst +++ b/Documentation/admin-guide/pm/intel_pstate.rst @@ -167,35 +167,17 @@ is set. ``powersave`` ............. -Without HWP, this P-state selection algorithm generally depends on the -processor model and/or the system profile setting in the ACPI tables and there -are two variants of it. - -One of them is used with processors from the Atom line and (regardless of the -processor model) on platforms with the system profile in the ACPI tables set to -"mobile" (laptops mostly), "tablet", "appliance PC", "desktop", or -"workstation". It is also used with processors supporting the HWP feature if -that feature has not been enabled (that is, with the ``intel_pstate=no_hwp`` -argument in the kernel command line). It is similar to the algorithm +Without HWP, this P-state selection algorithm is similar to the algorithm implemented by the generic ``schedutil`` scaling governor except that the utilization metric used by it is based on numbers coming from feedback registers of the CPU. It generally selects P-states proportional to the -current CPU utilization, so it is referred to as the "proportional" algorithm. - -The second variant of the ``powersave`` P-state selection algorithm, used in all -of the other cases (generally, on processors from the Core line, so it is -referred to as the "Core" algorithm), is based on the values read from the APERF -and MPERF feedback registers and the previously requested target P-state. -It does not really take CPU utilization into account explicitly, but as a rule -it causes the CPU P-state to ramp up very quickly in response to increased -utilization which is generally desirable in server environments. - -Regardless of the variant, this algorithm is run by the driver's utilization -update callback for the given CPU when it is invoked by the CPU scheduler, but -not more often than every 10 ms (that can be tweaked via ``debugfs`` in `this -particular case <Tuning Interface in debugfs_>`_). Like in the ``performance`` -case, the hardware configuration is not touched if the new P-state turns out to -be the same as the current one. +current CPU utilization. + +This algorithm is run by the driver's utilization update callback for the +given CPU when it is invoked by the CPU scheduler, but not more often than +every 10 ms. Like in the ``performance`` case, the hardware configuration +is not touched if the new P-state turns out to be the same as the current +one. This is the default P-state selection algorithm if the :c:macro:`CONFIG_CPU_FREQ_DEFAULT_GOV_PERFORMANCE` kernel configuration option @@ -720,34 +702,7 @@ P-state is called, the ``ftrace`` filter can be set to to 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 -Tuning Interface in ``debugfs`` -------------------------------- - -The ``powersave`` algorithm provided by ``intel_pstate`` for `the Core line of -processors in the active mode <powersave_>`_ is based on a `PID controller`_ -whose parameters were chosen to address a number of different use cases at the -same time. However, it still is possible to fine-tune it to a specific workload -and the ``debugfs`` interface under ``/sys/kernel/debug/pstate_snb/`` is -provided for this purpose. [Note that the ``pstate_snb`` directory will be -present only if the specific P-state selection algorithm matching the interface -in it actually is in use.] - -The following files present in that directory can be used to modify the PID -controller parameters at run time: - -| ``deadband`` -| ``d_gain_pct`` -| ``i_gain_pct`` -| ``p_gain_pct`` -| ``sample_rate_ms`` -| ``setpoint`` - -Note, however, that achieving desirable results this way generally requires -expert-level understanding of the power vs performance tradeoff, so extra care -is recommended when attempting to do that. - .. _LCEU2015: http://events.linuxfoundation.org/sites/events/files/slides/LinuxConEurope_2015.pdf .. _SDM: http://www.intel.com/content/www/us/en/architecture-and-technology/64-ia-32-architectures-software-developer-system-programming-manual-325384.html .. _ACPI specification: http://www.uefi.org/sites/default/files/resources/ACPI_6_1.pdf -.. _PID controller: https://en.wikipedia.org/wiki/PID_controller |