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.. SPDX-License-Identifier: GPL-2.0
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.. include:: <isonum.txt>
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==============================================
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``intel_idle`` CPU Idle Time Management Driver
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==============================================
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:Copyright: |copy| 2020 Intel Corporation
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:Author: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
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General Information
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===================
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``intel_idle`` is a part of the
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:doc:`CPU idle time management subsystem <cpuidle>` in the Linux kernel
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(``CPUIdle``). It is the default CPU idle time management driver for the
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Nehalem and later generations of Intel processors, but the level of support for
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a particular processor model in it depends on whether or not it recognizes that
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processor model and may also depend on information coming from the platform
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firmware. [To understand ``intel_idle`` it is necessary to know how ``CPUIdle``
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works in general, so this is the time to get familiar with :doc:`cpuidle` if you
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have not done that yet.]
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``intel_idle`` uses the ``MWAIT`` instruction to inform the processor that the
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logical CPU executing it is idle and so it may be possible to put some of the
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processor's functional blocks into low-power states. That instruction takes two
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arguments (passed in the ``EAX`` and ``ECX`` registers of the target CPU), the
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first of which, referred to as a *hint*, can be used by the processor to
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determine what can be done (for details refer to Intel Software Developer’s
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Manual [1]_). Accordingly, ``intel_idle`` refuses to work with processors in
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which the support for the ``MWAIT`` instruction has been disabled (for example,
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via the platform firmware configuration menu) or which do not support that
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instruction at all.
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``intel_idle`` is not modular, so it cannot be unloaded, which means that the
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only way to pass early-configuration-time parameters to it is via the kernel
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command line.
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.. _intel-idle-enumeration-of-states:
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Enumeration of Idle States
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==========================
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Each ``MWAIT`` hint value is interpreted by the processor as a license to
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reconfigure itself in a certain way in order to save energy. The processor
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configurations (with reduced power draw) resulting from that are referred to
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as C-states (in the ACPI terminology) or idle states. The list of meaningful
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``MWAIT`` hint values and idle states (i.e. low-power configurations of the
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processor) corresponding to them depends on the processor model and it may also
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depend on the configuration of the platform.
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In order to create a list of available idle states required by the ``CPUIdle``
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subsystem (see :ref:`idle-states-representation` in :doc:`cpuidle`),
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``intel_idle`` can use two sources of information: static tables of idle states
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for different processor models included in the driver itself and the ACPI tables
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of the system. The former are always used if the processor model at hand is
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recognized by ``intel_idle`` and the latter are used if that is required for
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the given processor model (which is the case for all server processor models
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recognized by ``intel_idle``) or if the processor model is not recognized.
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[There is a module parameter that can be used to make the driver use the ACPI
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tables with any processor model recognized by it; see
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`below <intel-idle-parameters_>`_.]
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If the ACPI tables are going to be used for building the list of available idle
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states, ``intel_idle`` first looks for a ``_CST`` object under one of the ACPI
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objects corresponding to the CPUs in the system (refer to the ACPI specification
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[2]_ for the description of ``_CST`` and its output package). Because the
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``CPUIdle`` subsystem expects that the list of idle states supplied by the
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driver will be suitable for all of the CPUs handled by it and ``intel_idle`` is
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registered as the ``CPUIdle`` driver for all of the CPUs in the system, the
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driver looks for the first ``_CST`` object returning at least one valid idle
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state description and such that all of the idle states included in its return
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package are of the FFH (Functional Fixed Hardware) type, which means that the
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``MWAIT`` instruction is expected to be used to tell the processor that it can
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enter one of them. The return package of that ``_CST`` is then assumed to be
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applicable to all of the other CPUs in the system and the idle state
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descriptions extracted from it are stored in a preliminary list of idle states
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coming from the ACPI tables. [This step is skipped if ``intel_idle`` is
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configured to ignore the ACPI tables; see `below <intel-idle-parameters_>`_.]
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Next, the first (index 0) entry in the list of available idle states is
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initialized to represent a "polling idle state" (a pseudo-idle state in which
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the target CPU continuously fetches and executes instructions), and the
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subsequent (real) idle state entries are populated as follows.
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If the processor model at hand is recognized by ``intel_idle``, there is a
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(static) table of idle state descriptions for it in the driver. In that case,
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the "internal" table is the primary source of information on idle states and the
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information from it is copied to the final list of available idle states. If
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using the ACPI tables for the enumeration of idle states is not required
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(depending on the processor model), all of the listed idle state are enabled by
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default (so all of them will be taken into consideration by ``CPUIdle``
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governors during CPU idle state selection). Otherwise, some of the listed idle
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states may not be enabled by default if there are no matching entries in the
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preliminary list of idle states coming from the ACPI tables. In that case user
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space still can enable them later (on a per-CPU basis) with the help of
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the ``disable`` idle state attribute in ``sysfs`` (see
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:ref:`idle-states-representation` in :doc:`cpuidle`). This basically means that
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the idle states "known" to the driver may not be enabled by default if they have
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not been exposed by the platform firmware (through the ACPI tables).
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If the given processor model is not recognized by ``intel_idle``, but it
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supports ``MWAIT``, the preliminary list of idle states coming from the ACPI
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tables is used for building the final list that will be supplied to the
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``CPUIdle`` core during driver registration. For each idle state in that list,
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the description, ``MWAIT`` hint and exit latency are copied to the corresponding
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entry in the final list of idle states. The name of the idle state represented
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by it (to be returned by the ``name`` idle state attribute in ``sysfs``) is
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"CX_ACPI", where X is the index of that idle state in the final list (note that
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the minimum value of X is 1, because 0 is reserved for the "polling" state), and
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its target residency is based on the exit latency value. Specifically, for
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C1-type idle states the exit latency value is also used as the target residency
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(for compatibility with the majority of the "internal" tables of idle states for
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various processor models recognized by ``intel_idle``) and for the other idle
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state types (C2 and C3) the target residency value is 3 times the exit latency
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(again, that is because it reflects the target residency to exit latency ratio
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in the majority of cases for the processor models recognized by ``intel_idle``).
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All of the idle states in the final list are enabled by default in this case.
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.. _intel-idle-initialization:
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Initialization
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==============
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The initialization of ``intel_idle`` starts with checking if the kernel command
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line options forbid the use of the ``MWAIT`` instruction. If that is the case,
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an error code is returned right away.
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The next step is to check whether or not the processor model is known to the
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driver, which determines the idle states enumeration method (see
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`above <intel-idle-enumeration-of-states_>`_), and whether or not the processor
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supports ``MWAIT`` (the initialization fails if that is not the case). Then,
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the ``MWAIT`` support in the processor is enumerated through ``CPUID`` and the
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driver initialization fails if the level of support is not as expected (for
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example, if the total number of ``MWAIT`` substates returned is 0).
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Next, if the driver is not configured to ignore the ACPI tables (see
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`below <intel-idle-parameters_>`_), the idle states information provided by the
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platform firmware is extracted from them.
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Then, ``CPUIdle`` device objects are allocated for all CPUs and the list of
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available idle states is created as explained
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`above <intel-idle-enumeration-of-states_>`_.
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Finally, ``intel_idle`` is registered with the help of cpuidle_register_driver()
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as the ``CPUIdle`` driver for all CPUs in the system and a CPU online callback
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for configuring individual CPUs is registered via cpuhp_setup_state(), which
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(among other things) causes the callback routine to be invoked for all of the
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CPUs present in the system at that time (each CPU executes its own instance of
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the callback routine). That routine registers a ``CPUIdle`` device for the CPU
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running it (which enables the ``CPUIdle`` subsystem to operate that CPU) and
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optionally performs some CPU-specific initialization actions that may be
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required for the given processor model.
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.. _intel-idle-parameters:
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Kernel Command Line Options and Module Parameters
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=================================================
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The *x86* architecture support code recognizes three kernel command line
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options related to CPU idle time management: ``idle=poll``, ``idle=halt``,
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and ``idle=nomwait``. If any of them is present in the kernel command line, the
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``MWAIT`` instruction is not allowed to be used, so the initialization of
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``intel_idle`` will fail.
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Apart from that there are four module parameters recognized by ``intel_idle``
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itself that can be set via the kernel command line (they cannot be updated via
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sysfs, so that is the only way to change their values).
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The ``max_cstate`` parameter value is the maximum idle state index in the list
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of idle states supplied to the ``CPUIdle`` core during the registration of the
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driver. It is also the maximum number of regular (non-polling) idle states that
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can be used by ``intel_idle``, so the enumeration of idle states is terminated
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after finding that number of usable idle states (the other idle states that
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potentially might have been used if ``max_cstate`` had been greater are not
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taken into consideration at all). Setting ``max_cstate`` can prevent
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``intel_idle`` from exposing idle states that are regarded as "too deep" for
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some reason to the ``CPUIdle`` core, but it does so by making them effectively
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invisible until the system is shut down and started again which may not always
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be desirable. In practice, it is only really necessary to do that if the idle
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states in question cannot be enabled during system startup, because in the
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working state of the system the CPU power management quality of service (PM
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QoS) feature can be used to prevent ``CPUIdle`` from touching those idle states
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even if they have been enumerated (see :ref:`cpu-pm-qos` in :doc:`cpuidle`).
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Setting ``max_cstate`` to 0 causes the ``intel_idle`` initialization to fail.
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The ``no_acpi`` and ``use_acpi`` module parameters (recognized by ``intel_idle``
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if the kernel has been configured with ACPI support) can be set to make the
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driver ignore the system's ACPI tables entirely or use them for all of the
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recognized processor models, respectively (they both are unset by default and
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``use_acpi`` has no effect if ``no_acpi`` is set).
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The value of the ``states_off`` module parameter (0 by default) represents a
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list of idle states to be disabled by default in the form of a bitmask.
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Namely, the positions of the bits that are set in the ``states_off`` value are
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the indices of idle states to be disabled by default (as reflected by the names
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of the corresponding idle state directories in ``sysfs``, :file:`state0`,
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:file:`state1` ... :file:`state<i>` ..., where ``<i>`` is the index of the given
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idle state; see :ref:`idle-states-representation` in :doc:`cpuidle`).
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For example, if ``states_off`` is equal to 3, the driver will disable idle
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states 0 and 1 by default, and if it is equal to 8, idle state 3 will be
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disabled by default and so on (bit positions beyond the maximum idle state index
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are ignored).
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The idle states disabled this way can be enabled (on a per-CPU basis) from user
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space via ``sysfs``.
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.. _intel-idle-core-and-package-idle-states:
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Core and Package Levels of Idle States
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======================================
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Typically, in a processor supporting the ``MWAIT`` instruction there are (at
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least) two levels of idle states (or C-states). One level, referred to as
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"core C-states", covers individual cores in the processor, whereas the other
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level, referred to as "package C-states", covers the entire processor package
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and it may also involve other components of the system (GPUs, memory
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controllers, I/O hubs etc.).
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Some of the ``MWAIT`` hint values allow the processor to use core C-states only
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(most importantly, that is the case for the ``MWAIT`` hint value corresponding
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to the ``C1`` idle state), but the majority of them give it a license to put
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the target core (i.e. the core containing the logical CPU executing ``MWAIT``
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with the given hint value) into a specific core C-state and then (if possible)
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to enter a specific package C-state at the deeper level. For example, the
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``MWAIT`` hint value representing the ``C3`` idle state allows the processor to
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put the target core into the low-power state referred to as "core ``C3``" (or
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``CC3``), which happens if all of the logical CPUs (SMT siblings) in that core
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have executed ``MWAIT`` with the ``C3`` hint value (or with a hint value
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representing a deeper idle state), and in addition to that (in the majority of
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cases) it gives the processor a license to put the entire package (possibly
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including some non-CPU components such as a GPU or a memory controller) into the
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low-power state referred to as "package ``C3``" (or ``PC3``), which happens if
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all of the cores have gone into the ``CC3`` state and (possibly) some additional
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conditions are satisfied (for instance, if the GPU is covered by ``PC3``, it may
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be required to be in a certain GPU-specific low-power state for ``PC3`` to be
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reachable).
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As a rule, there is no simple way to make the processor use core C-states only
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if the conditions for entering the corresponding package C-states are met, so
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the logical CPU executing ``MWAIT`` with a hint value that is not core-level
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only (like for ``C1``) must always assume that this may cause the processor to
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enter a package C-state. [That is why the exit latency and target residency
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values corresponding to the majority of ``MWAIT`` hint values in the "internal"
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tables of idle states in ``intel_idle`` reflect the properties of package
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C-states.] If using package C-states is not desirable at all, either
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:ref:`PM QoS <cpu-pm-qos>` or the ``max_cstate`` module parameter of
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``intel_idle`` described `above <intel-idle-parameters_>`_ must be used to
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restrict the range of permissible idle states to the ones with core-level only
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``MWAIT`` hint values (like ``C1``).
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References
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==========
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.. [1] *Intel® 64 and IA-32 Architectures Software Developer’s Manual Volume 2B*,
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https://www.intel.com/content/www/us/en/architecture-and-technology/64-ia-32-architectures-software-developer-vol-2b-manual.html
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.. [2] *Advanced Configuration and Power Interface (ACPI) Specification*,
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https://uefi.org/specifications
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