admin-guide/pm/cpuidle.rst

1 .. SPDX-License-Identifier: GPL-2.0
5 .. |cpufreq| replace:: :doc:`CPU Performance Scaling <cpufreq>`
27 CPU idle time management is an energy-efficiency feature concerned about using
31 ------------
37 software as individual single-core processors.  In other words, a CPU is an
46 Second, if the processor is multi-core, each core in it is able to follow at
61 Finally, each core in a multi-core processor may be able to follow more than one
66 multiple individual single-core "processors", referred to as *hardware threads*
67 (or hyper-threads specifically on Intel hardware), that each can follow one
78 ---------
112 .. _idle-loop:
127 the platform or the processor architecture and organized in a one-dimensional
134 taken into account by the governor, the *target residency* and the (worst-case)
152 and exit it.  However, the CPU may be woken up by a non-timer event at any time
162 There are four ``CPUIdle`` governors available, ``menu``, `TEO <teo-gov_>`_,
165 tick can be `stopped by the idle loop <idle-cpus-and-tick_>`_.  Available
186 .. _idle-cpus-and-tick:
223 (non-tick) timer due to trigger within the tick range, stopping the tick clearly
225 reprogrammed in that case.  Second, if the governor is expecting a non-timer
247 loop altogether.  That can be done through the build-time configuration of it
255 generally regarded as more energy-efficient than the systems running kernels in
261 .. _menu-gov:
319 from the power management quality of service, or `PM QoS <cpu-pm-qos_>`_,
331 if it has not decided to `stop the scheduler tick <idle-cpus-and-tick_>`_.  That
340 .. _teo-gov:
347 <menu-gov_>`_: it always tries to find the deepest idle state suitable for the
361 Like in the ``menu`` governor `case <menu-gov_>`_, the first step is to obtain
369 state will "match" the observed (post-wakeup) idle duration if it "matches" the
384 "match" the observed (post-wakeup) idle duration if it does not "match" the
386 when the idle state corresponding to it "matches" the observed (post-wakeup)
401 <cpu-pm-qos_>`_ which is hit before reaching the deepest idle state with the
416 it has not decided to `stop the scheduler tick <idle-cpus-and-tick_>`_.  That
420 `case <menu-gov_>`_, the sleep length used in the previous computations may not
426 .. _idle-states-representation:
432 supported by the processor have to be represented as a one-dimensional array of
438 of it <idle-loop_>`_, must reflect the properties of the idle state at the
585 .. _cpu-pm-qos:
592 energy-efficiency features of the kernel to prevent performance from dropping
601 signed 32-bit integer) to it.  In turn, the resume latency constraint for a CPU
603 32-bit integer) to the :file:`power/pm_qos_resume_latency_us` file under
666 `disabled for individual CPUs <idle-states-representation_>`_, there are kernel
667 command line parameters affecting CPU idle time management.
677 however, so it is rather crude and not very energy-efficient.  For this reason,
706 P-states (see |cpufreq|) that require any number of CPUs in a package to be
707 idle, so it very well may hurt single-thread computations performance as well as
708 energy-efficiency.  Thus using it for performance reasons may not be a good idea
716 In addition to the architecture-level kernel command line options affecting CPU
717 idle time management, there are parameters affecting individual ``CPUIdle``
722 `Representation of Idle States <idle-states-representation_>`_), causes the