cpufreq: intel_pstate: Do not use PID-based P-state selection

All systems with a defined ACPI preferred profile that are not
"servers" have been using the load-based P-state selection algorithm
in intel_pstate since 4.12-rc1 (mobile systems and laptops have been
using it since 4.10-rc1) and no problems with it have been reported
to date.  In particular, no regressions with respect to the PID-based
P-state selection have been reported.  Also testing indicates that
the P-state selection algorithm based on CPU load is generally on par
with the PID-based algorithm performance-wise, and for some workloads
it turns out to be better than the other one, while being more
straightforward and easier to understand at the same time.

Moreover, the PID-based P-state selection algorithm in intel_pstate
is known to be unstable in some situation and generally problematic,
the issues with it are hard to address and it has become a
significant maintenance burden.

For these reasons, make intel_pstate use the "powersave" P-state
selection algorithm based on CPU load in the active mode on all
systems and drop the PID-based P-state selection code along with
all things related to it from the driver.  Also update the
documentation accordingly.
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

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