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 24Documentation/admin-guide/pm/cpuidle.rst if you have 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 57Documentation/admin-guide/pm/cpuidle.rst), 58``intel_idle`` can use two sources of information: static tables of idle states 59for different processor models included in the driver itself and the ACPI tables 60of the system. The former are always used if the processor model at hand is 61recognized by ``intel_idle`` and the latter are used if that is required for 62the given processor model (which is the case for all server processor models 63recognized by ``intel_idle``) or if the processor model is not recognized. 64[There is a module parameter that can be used to make the driver use the ACPI 65tables with any processor model recognized by it; see 66`below <intel-idle-parameters_>`_.] 67 68If the ACPI tables are going to be used for building the list of available idle 69states, ``intel_idle`` first looks for a ``_CST`` object under one of the ACPI 70objects corresponding to the CPUs in the system (refer to the ACPI specification 71[2]_ for the description of ``_CST`` and its output package). Because the 72``CPUIdle`` subsystem expects that the list of idle states supplied by the 73driver will be suitable for all of the CPUs handled by it and ``intel_idle`` is 74registered as the ``CPUIdle`` driver for all of the CPUs in the system, the 75driver looks for the first ``_CST`` object returning at least one valid idle 76state description and such that all of the idle states included in its return 77package are of the FFH (Functional Fixed Hardware) type, which means that the 78``MWAIT`` instruction is expected to be used to tell the processor that it can 79enter one of them. The return package of that ``_CST`` is then assumed to be 80applicable to all of the other CPUs in the system and the idle state 81descriptions extracted from it are stored in a preliminary list of idle states 82coming from the ACPI tables. [This step is skipped if ``intel_idle`` is 83configured to ignore the ACPI tables; see `below <intel-idle-parameters_>`_.] 84 85Next, the first (index 0) entry in the list of available idle states is 86initialized to represent a "polling idle state" (a pseudo-idle state in which 87the target CPU continuously fetches and executes instructions), and the 88subsequent (real) idle state entries are populated as follows. 89 90If the processor model at hand is recognized by ``intel_idle``, there is a 91(static) table of idle state descriptions for it in the driver. In that case, 92the "internal" table is the primary source of information on idle states and the 93information from it is copied to the final list of available idle states. If 94using the ACPI tables for the enumeration of idle states is not required 95(depending on the processor model), all of the listed idle state are enabled by 96default (so all of them will be taken into consideration by ``CPUIdle`` 97governors during CPU idle state selection). Otherwise, some of the listed idle 98states may not be enabled by default if there are no matching entries in the 99preliminary list of idle states coming from the ACPI tables. In that case user 100space still can enable them later (on a per-CPU basis) with the help of 101the ``disable`` idle state attribute in ``sysfs`` (see 102:ref:`idle-states-representation` in 103Documentation/admin-guide/pm/cpuidle.rst). This basically means that 104the idle states "known" to the driver may not be enabled by default if they have 105not been exposed by the platform firmware (through the ACPI tables). 106 107If the given processor model is not recognized by ``intel_idle``, but it 108supports ``MWAIT``, the preliminary list of idle states coming from the ACPI 109tables is used for building the final list that will be supplied to the 110``CPUIdle`` core during driver registration. For each idle state in that list, 111the description, ``MWAIT`` hint and exit latency are copied to the corresponding 112entry in the final list of idle states. The name of the idle state represented 113by it (to be returned by the ``name`` idle state attribute in ``sysfs``) is 114"CX_ACPI", where X is the index of that idle state in the final list (note that 115the minimum value of X is 1, because 0 is reserved for the "polling" state), and 116its target residency is based on the exit latency value. Specifically, for 117C1-type idle states the exit latency value is also used as the target residency 118(for compatibility with the majority of the "internal" tables of idle states for 119various processor models recognized by ``intel_idle``) and for the other idle 120state types (C2 and C3) the target residency value is 3 times the exit latency 121(again, that is because it reflects the target residency to exit latency ratio 122in the majority of cases for the processor models recognized by ``intel_idle``). 123All of the idle states in the final list are enabled by default in this case. 124 125 126.. _intel-idle-initialization: 127 128Initialization 129============== 130 131The initialization of ``intel_idle`` starts with checking if the kernel command 132line options forbid the use of the ``MWAIT`` instruction. If that is the case, 133an error code is returned right away. 134 135The next step is to check whether or not the processor model is known to the 136driver, which determines the idle states enumeration method (see 137`above <intel-idle-enumeration-of-states_>`_), and whether or not the processor 138supports ``MWAIT`` (the initialization fails if that is not the case). Then, 139the ``MWAIT`` support in the processor is enumerated through ``CPUID`` and the 140driver initialization fails if the level of support is not as expected (for 141example, if the total number of ``MWAIT`` substates returned is 0). 142 143Next, if the driver is not configured to ignore the ACPI tables (see 144`below <intel-idle-parameters_>`_), the idle states information provided by the 145platform firmware is extracted from them. 146 147Then, ``CPUIdle`` device objects are allocated for all CPUs and the list of 148available idle states is created as explained 149`above <intel-idle-enumeration-of-states_>`_. 150 151Finally, ``intel_idle`` is registered with the help of cpuidle_register_driver() 152as the ``CPUIdle`` driver for all CPUs in the system and a CPU online callback 153for configuring individual CPUs is registered via cpuhp_setup_state(), which 154(among other things) causes the callback routine to be invoked for all of the 155CPUs present in the system at that time (each CPU executes its own instance of 156the callback routine). That routine registers a ``CPUIdle`` device for the CPU 157running it (which enables the ``CPUIdle`` subsystem to operate that CPU) and 158optionally performs some CPU-specific initialization actions that may be 159required for the given processor model. 160 161 162.. _intel-idle-parameters: 163 164Kernel Command Line Options and Module Parameters 165================================================= 166 167The *x86* architecture support code recognizes three kernel command line 168options related to CPU idle time management: ``idle=poll``, ``idle=halt``, 169and ``idle=nomwait``. If any of them is present in the kernel command line, the 170``MWAIT`` instruction is not allowed to be used, so the initialization of 171``intel_idle`` will fail. 172 173Apart from that there are four module parameters recognized by ``intel_idle`` 174itself that can be set via the kernel command line (they cannot be updated via 175sysfs, so that is the only way to change their values). 176 177The ``max_cstate`` parameter value is the maximum idle state index in the list 178of idle states supplied to the ``CPUIdle`` core during the registration of the 179driver. It is also the maximum number of regular (non-polling) idle states that 180can be used by ``intel_idle``, so the enumeration of idle states is terminated 181after finding that number of usable idle states (the other idle states that 182potentially might have been used if ``max_cstate`` had been greater are not 183taken into consideration at all). Setting ``max_cstate`` can prevent 184``intel_idle`` from exposing idle states that are regarded as "too deep" for 185some reason to the ``CPUIdle`` core, but it does so by making them effectively 186invisible until the system is shut down and started again which may not always 187be desirable. In practice, it is only really necessary to do that if the idle 188states in question cannot be enabled during system startup, because in the 189working state of the system the CPU power management quality of service (PM 190QoS) feature can be used to prevent ``CPUIdle`` from touching those idle states 191even if they have been enumerated (see :ref:`cpu-pm-qos` in 192Documentation/admin-guide/pm/cpuidle.rst). 193Setting ``max_cstate`` to 0 causes the ``intel_idle`` initialization to fail. 194 195The ``no_acpi`` and ``use_acpi`` module parameters (recognized by ``intel_idle`` 196if the kernel has been configured with ACPI support) can be set to make the 197driver ignore the system's ACPI tables entirely or use them for all of the 198recognized processor models, respectively (they both are unset by default and 199``use_acpi`` has no effect if ``no_acpi`` is set). 200 201The value of the ``states_off`` module parameter (0 by default) represents a 202list of idle states to be disabled by default in the form of a bitmask. 203 204Namely, the positions of the bits that are set in the ``states_off`` value are 205the indices of idle states to be disabled by default (as reflected by the names 206of the corresponding idle state directories in ``sysfs``, :file:`state0`, 207:file:`state1` ... :file:`state<i>` ..., where ``<i>`` is the index of the given 208idle state; see :ref:`idle-states-representation` in 209Documentation/admin-guide/pm/cpuidle.rst). 210 211For example, if ``states_off`` is equal to 3, the driver will disable idle 212states 0 and 1 by default, and if it is equal to 8, idle state 3 will be 213disabled by default and so on (bit positions beyond the maximum idle state index 214are ignored). 215 216The idle states disabled this way can be enabled (on a per-CPU basis) from user 217space via ``sysfs``. 218 219 220.. _intel-idle-core-and-package-idle-states: 221 222Core and Package Levels of Idle States 223====================================== 224 225Typically, in a processor supporting the ``MWAIT`` instruction there are (at 226least) two levels of idle states (or C-states). One level, referred to as 227"core C-states", covers individual cores in the processor, whereas the other 228level, referred to as "package C-states", covers the entire processor package 229and it may also involve other components of the system (GPUs, memory 230controllers, I/O hubs etc.). 231 232Some of the ``MWAIT`` hint values allow the processor to use core C-states only 233(most importantly, that is the case for the ``MWAIT`` hint value corresponding 234to the ``C1`` idle state), but the majority of them give it a license to put 235the target core (i.e. the core containing the logical CPU executing ``MWAIT`` 236with the given hint value) into a specific core C-state and then (if possible) 237to enter a specific package C-state at the deeper level. For example, the 238``MWAIT`` hint value representing the ``C3`` idle state allows the processor to 239put the target core into the low-power state referred to as "core ``C3``" (or 240``CC3``), which happens if all of the logical CPUs (SMT siblings) in that core 241have executed ``MWAIT`` with the ``C3`` hint value (or with a hint value 242representing a deeper idle state), and in addition to that (in the majority of 243cases) it gives the processor a license to put the entire package (possibly 244including some non-CPU components such as a GPU or a memory controller) into the 245low-power state referred to as "package ``C3``" (or ``PC3``), which happens if 246all of the cores have gone into the ``CC3`` state and (possibly) some additional 247conditions are satisfied (for instance, if the GPU is covered by ``PC3``, it may 248be required to be in a certain GPU-specific low-power state for ``PC3`` to be 249reachable). 250 251As a rule, there is no simple way to make the processor use core C-states only 252if the conditions for entering the corresponding package C-states are met, so 253the logical CPU executing ``MWAIT`` with a hint value that is not core-level 254only (like for ``C1``) must always assume that this may cause the processor to 255enter a package C-state. [That is why the exit latency and target residency 256values corresponding to the majority of ``MWAIT`` hint values in the "internal" 257tables of idle states in ``intel_idle`` reflect the properties of package 258C-states.] If using package C-states is not desirable at all, either 259:ref:`PM QoS <cpu-pm-qos>` or the ``max_cstate`` module parameter of 260``intel_idle`` described `above <intel-idle-parameters_>`_ must be used to 261restrict the range of permissible idle states to the ones with core-level only 262``MWAIT`` hint values (like ``C1``). 263 264 265References 266========== 267 268.. [1] *Intel® 64 and IA-32 Architectures Software Developer’s Manual Volume 2B*, 269 https://www.intel.com/content/www/us/en/architecture-and-technology/64-ia-32-architectures-software-developer-vol-2b-manual.html 270 271.. [2] *Advanced Configuration and Power Interface (ACPI) Specification*, 272 https://uefi.org/specifications 273