1.. SPDX-License-Identifier: GPL-2.0 2.. include:: <isonum.txt> 3 4============================================== 5``intel_idle`` CPU Idle Time Management Driver 6============================================== 7 8:Copyright: |copy| 2020 Intel Corporation 9 10:Author: Rafael J. Wysocki <rafael.j.wysocki@intel.com> 11 12 13General Information 14=================== 15 16``intel_idle`` is a part of the 17:doc:`CPU idle time management subsystem <cpuidle>` in the Linux kernel 18(``CPUIdle``). It is the default CPU idle time management driver for the 19Nehalem and later generations of Intel processors, but the level of support for 20a particular processor model in it depends on whether or not it recognizes that 21processor model and may also depend on information coming from the platform 22firmware. [To understand ``intel_idle`` it is necessary to know how ``CPUIdle`` 23works in general, so this is the time to get familiar with :doc:`cpuidle` if you 24have not done that yet.] 25 26``intel_idle`` uses the ``MWAIT`` instruction to inform the processor that the 27logical CPU executing it is idle and so it may be possible to put some of the 28processor's functional blocks into low-power states. That instruction takes two 29arguments (passed in the ``EAX`` and ``ECX`` registers of the target CPU), the 30first of which, referred to as a *hint*, can be used by the processor to 31determine what can be done (for details refer to Intel Software Developer’s 32Manual [1]_). Accordingly, ``intel_idle`` refuses to work with processors in 33which the support for the ``MWAIT`` instruction has been disabled (for example, 34via the platform firmware configuration menu) or which do not support that 35instruction at all. 36 37``intel_idle`` is not modular, so it cannot be unloaded, which means that the 38only way to pass early-configuration-time parameters to it is via the kernel 39command line. 40 41 42.. _intel-idle-enumeration-of-states: 43 44Enumeration of Idle States 45========================== 46 47Each ``MWAIT`` hint value is interpreted by the processor as a license to 48reconfigure itself in a certain way in order to save energy. The processor 49configurations (with reduced power draw) resulting from that are referred to 50as C-states (in the ACPI terminology) or idle states. The list of meaningful 51``MWAIT`` hint values and idle states (i.e. low-power configurations of the 52processor) corresponding to them depends on the processor model and it may also 53depend on the configuration of the platform. 54 55In order to create a list of available idle states required by the ``CPUIdle`` 56subsystem (see :ref:`idle-states-representation` in :doc:`cpuidle`), 57``intel_idle`` can use two sources of information: static tables of idle states 58for different processor models included in the driver itself and the ACPI tables 59of the system. The former are always used if the processor model at hand is 60recognized by ``intel_idle`` and the latter are used if that is required for 61the given processor model (which is the case for all server processor models 62recognized by ``intel_idle``) or if the processor model is not recognized. 63[There is a module parameter that can be used to make the driver use the ACPI 64tables with any processor model recognized by it; see 65`below <intel-idle-parameters_>`_.] 66 67If the ACPI tables are going to be used for building the list of available idle 68states, ``intel_idle`` first looks for a ``_CST`` object under one of the ACPI 69objects corresponding to the CPUs in the system (refer to the ACPI specification 70[2]_ for the description of ``_CST`` and its output package). Because the 71``CPUIdle`` subsystem expects that the list of idle states supplied by the 72driver will be suitable for all of the CPUs handled by it and ``intel_idle`` is 73registered as the ``CPUIdle`` driver for all of the CPUs in the system, the 74driver looks for the first ``_CST`` object returning at least one valid idle 75state description and such that all of the idle states included in its return 76package are of the FFH (Functional Fixed Hardware) type, which means that the 77``MWAIT`` instruction is expected to be used to tell the processor that it can 78enter one of them. The return package of that ``_CST`` is then assumed to be 79applicable to all of the other CPUs in the system and the idle state 80descriptions extracted from it are stored in a preliminary list of idle states 81coming from the ACPI tables. [This step is skipped if ``intel_idle`` is 82configured to ignore the ACPI tables; see `below <intel-idle-parameters_>`_.] 83 84Next, the first (index 0) entry in the list of available idle states is 85initialized to represent a "polling idle state" (a pseudo-idle state in which 86the target CPU continuously fetches and executes instructions), and the 87subsequent (real) idle state entries are populated as follows. 88 89If the processor model at hand is recognized by ``intel_idle``, there is a 90(static) table of idle state descriptions for it in the driver. In that case, 91the "internal" table is the primary source of information on idle states and the 92information from it is copied to the final list of available idle states. If 93using the ACPI tables for the enumeration of idle states is not required 94(depending on the processor model), all of the listed idle state are enabled by 95default (so all of them will be taken into consideration by ``CPUIdle`` 96governors during CPU idle state selection). Otherwise, some of the listed idle 97states may not be enabled by default if there are no matching entries in the 98preliminary list of idle states coming from the ACPI tables. In that case user 99space still can enable them later (on a per-CPU basis) with the help of 100the ``disable`` idle state attribute in ``sysfs`` (see 101:ref:`idle-states-representation` in :doc:`cpuidle`). This basically means that 102the idle states "known" to the driver may not be enabled by default if they have 103not been exposed by the platform firmware (through the ACPI tables). 104 105If the given processor model is not recognized by ``intel_idle``, but it 106supports ``MWAIT``, the preliminary list of idle states coming from the ACPI 107tables is used for building the final list that will be supplied to the 108``CPUIdle`` core during driver registration. For each idle state in that list, 109the description, ``MWAIT`` hint and exit latency are copied to the corresponding 110entry in the final list of idle states. The name of the idle state represented 111by it (to be returned by the ``name`` idle state attribute in ``sysfs``) is 112"CX_ACPI", where X is the index of that idle state in the final list (note that 113the minimum value of X is 1, because 0 is reserved for the "polling" state), and 114its target residency is based on the exit latency value. Specifically, for 115C1-type idle states the exit latency value is also used as the target residency 116(for compatibility with the majority of the "internal" tables of idle states for 117various processor models recognized by ``intel_idle``) and for the other idle 118state types (C2 and C3) the target residency value is 3 times the exit latency 119(again, that is because it reflects the target residency to exit latency ratio 120in the majority of cases for the processor models recognized by ``intel_idle``). 121All of the idle states in the final list are enabled by default in this case. 122 123 124.. _intel-idle-initialization: 125 126Initialization 127============== 128 129The initialization of ``intel_idle`` starts with checking if the kernel command 130line options forbid the use of the ``MWAIT`` instruction. If that is the case, 131an error code is returned right away. 132 133The next step is to check whether or not the processor model is known to the 134driver, which determines the idle states enumeration method (see 135`above <intel-idle-enumeration-of-states_>`_), and whether or not the processor 136supports ``MWAIT`` (the initialization fails if that is not the case). Then, 137the ``MWAIT`` support in the processor is enumerated through ``CPUID`` and the 138driver initialization fails if the level of support is not as expected (for 139example, if the total number of ``MWAIT`` substates returned is 0). 140 141Next, if the driver is not configured to ignore the ACPI tables (see 142`below <intel-idle-parameters_>`_), the idle states information provided by the 143platform firmware is extracted from them. 144 145Then, ``CPUIdle`` device objects are allocated for all CPUs and the list of 146available idle states is created as explained 147`above <intel-idle-enumeration-of-states_>`_. 148 149Finally, ``intel_idle`` is registered with the help of cpuidle_register_driver() 150as the ``CPUIdle`` driver for all CPUs in the system and a CPU online callback 151for configuring individual CPUs is registered via cpuhp_setup_state(), which 152(among other things) causes the callback routine to be invoked for all of the 153CPUs present in the system at that time (each CPU executes its own instance of 154the callback routine). That routine registers a ``CPUIdle`` device for the CPU 155running it (which enables the ``CPUIdle`` subsystem to operate that CPU) and 156optionally performs some CPU-specific initialization actions that may be 157required for the given processor model. 158 159 160.. _intel-idle-parameters: 161 162Kernel Command Line Options and Module Parameters 163================================================= 164 165The *x86* architecture support code recognizes three kernel command line 166options related to CPU idle time management: ``idle=poll``, ``idle=halt``, 167and ``idle=nomwait``. If any of them is present in the kernel command line, the 168``MWAIT`` instruction is not allowed to be used, so the initialization of 169``intel_idle`` will fail. 170 171Apart from that there are four module parameters recognized by ``intel_idle`` 172itself that can be set via the kernel command line (they cannot be updated via 173sysfs, so that is the only way to change their values). 174 175The ``max_cstate`` parameter value is the maximum idle state index in the list 176of idle states supplied to the ``CPUIdle`` core during the registration of the 177driver. It is also the maximum number of regular (non-polling) idle states that 178can be used by ``intel_idle``, so the enumeration of idle states is terminated 179after finding that number of usable idle states (the other idle states that 180potentially might have been used if ``max_cstate`` had been greater are not 181taken into consideration at all). Setting ``max_cstate`` can prevent 182``intel_idle`` from exposing idle states that are regarded as "too deep" for 183some reason to the ``CPUIdle`` core, but it does so by making them effectively 184invisible until the system is shut down and started again which may not always 185be desirable. In practice, it is only really necessary to do that if the idle 186states in question cannot be enabled during system startup, because in the 187working state of the system the CPU power management quality of service (PM 188QoS) feature can be used to prevent ``CPUIdle`` from touching those idle states 189even if they have been enumerated (see :ref:`cpu-pm-qos` in :doc:`cpuidle`). 190Setting ``max_cstate`` to 0 causes the ``intel_idle`` initialization to fail. 191 192The ``no_acpi`` and ``use_acpi`` module parameters (recognized by ``intel_idle`` 193if the kernel has been configured with ACPI support) can be set to make the 194driver ignore the system's ACPI tables entirely or use them for all of the 195recognized processor models, respectively (they both are unset by default and 196``use_acpi`` has no effect if ``no_acpi`` is set). 197 198The value of the ``states_off`` module parameter (0 by default) represents a 199list of idle states to be disabled by default in the form of a bitmask. 200 201Namely, the positions of the bits that are set in the ``states_off`` value are 202the indices of idle states to be disabled by default (as reflected by the names 203of the corresponding idle state directories in ``sysfs``, :file:`state0`, 204:file:`state1` ... :file:`state<i>` ..., where ``<i>`` is the index of the given 205idle state; see :ref:`idle-states-representation` in :doc:`cpuidle`). 206 207For example, if ``states_off`` is equal to 3, the driver will disable idle 208states 0 and 1 by default, and if it is equal to 8, idle state 3 will be 209disabled by default and so on (bit positions beyond the maximum idle state index 210are ignored). 211 212The idle states disabled this way can be enabled (on a per-CPU basis) from user 213space via ``sysfs``. 214 215 216.. _intel-idle-core-and-package-idle-states: 217 218Core and Package Levels of Idle States 219====================================== 220 221Typically, in a processor supporting the ``MWAIT`` instruction there are (at 222least) two levels of idle states (or C-states). One level, referred to as 223"core C-states", covers individual cores in the processor, whereas the other 224level, referred to as "package C-states", covers the entire processor package 225and it may also involve other components of the system (GPUs, memory 226controllers, I/O hubs etc.). 227 228Some of the ``MWAIT`` hint values allow the processor to use core C-states only 229(most importantly, that is the case for the ``MWAIT`` hint value corresponding 230to the ``C1`` idle state), but the majority of them give it a license to put 231the target core (i.e. the core containing the logical CPU executing ``MWAIT`` 232with the given hint value) into a specific core C-state and then (if possible) 233to enter a specific package C-state at the deeper level. For example, the 234``MWAIT`` hint value representing the ``C3`` idle state allows the processor to 235put the target core into the low-power state referred to as "core ``C3``" (or 236``CC3``), which happens if all of the logical CPUs (SMT siblings) in that core 237have executed ``MWAIT`` with the ``C3`` hint value (or with a hint value 238representing a deeper idle state), and in addition to that (in the majority of 239cases) it gives the processor a license to put the entire package (possibly 240including some non-CPU components such as a GPU or a memory controller) into the 241low-power state referred to as "package ``C3``" (or ``PC3``), which happens if 242all of the cores have gone into the ``CC3`` state and (possibly) some additional 243conditions are satisfied (for instance, if the GPU is covered by ``PC3``, it may 244be required to be in a certain GPU-specific low-power state for ``PC3`` to be 245reachable). 246 247As a rule, there is no simple way to make the processor use core C-states only 248if the conditions for entering the corresponding package C-states are met, so 249the logical CPU executing ``MWAIT`` with a hint value that is not core-level 250only (like for ``C1``) must always assume that this may cause the processor to 251enter a package C-state. [That is why the exit latency and target residency 252values corresponding to the majority of ``MWAIT`` hint values in the "internal" 253tables of idle states in ``intel_idle`` reflect the properties of package 254C-states.] If using package C-states is not desirable at all, either 255:ref:`PM QoS <cpu-pm-qos>` or the ``max_cstate`` module parameter of 256``intel_idle`` described `above <intel-idle-parameters_>`_ must be used to 257restrict the range of permissible idle states to the ones with core-level only 258``MWAIT`` hint values (like ``C1``). 259 260 261References 262========== 263 264.. [1] *Intel® 64 and IA-32 Architectures Software Developer’s Manual Volume 2B*, 265 https://www.intel.com/content/www/us/en/architecture-and-technology/64-ia-32-architectures-software-developer-vol-2b-manual.html 266 267.. [2] *Advanced Configuration and Power Interface (ACPI) Specification*, 268 https://uefi.org/specifications 269