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Power management updates for v4.14-rc1

Browse source 
- Drop the P-state selection algorithm based on a PID controller
    from intel_pstate and make it use the same P-state selection
    method (based on the CPU load) for all types of systems in the
    active mode (Rafael Wysocki, Srinivas Pandruvada).
 
  - Rework the cpufreq core and governors to make it possible to
    take cross-CPU utilization updates into account and modify the
    schedutil governor to actually do so (Viresh Kumar).
 
  - Clean up the handling of transition latency information in the
    cpufreq core and untangle it from the information on which drivers
    cannot do dynamic frequency switching (Viresh Kumar).
 
  - Add support for new SoCs (MT2701/MT7623 and MT7622) to the
    mediatek cpufreq driver and update its DT bindings (Sean Wang).
 
  - Modify the cpufreq dt-platdev driver to autimatically create
    cpufreq devices for the new (v2) Operating Performance Points
    (OPP) DT bindings and update its whitelist of supported systems
    (Viresh Kumar, Shubhrajyoti Datta, Marc Gonzalez, Khiem Nguyen,
    Finley Xiao).
 
  - Add support for Ux500 to the cpufreq-dt driver and drop the
    obsolete dbx500 cpufreq driver (Linus Walleij, Arnd Bergmann).
 
  - Add new SoC (R8A7795) support to the cpufreq rcar driver (Khiem
    Nguyen).
 
  - Fix and clean up assorted issues in the cpufreq drivers and core
    (Arvind Yadav, Christophe Jaillet, Colin Ian King, Gustavo Silva,
    Julia Lawall, Leonard Crestez, Rob Herring, Sudeep Holla).
 
  - Update the IO-wait boost handling in the schedutil governor to
    make it less aggressive (Joel Fernandes).
 
  - Rework system suspend diagnostics to make it print fewer messages
    to the kernel log by default, add a sysfs knob to allow more
    suspend-related messages to be printed and add Low Power S0 Idle
    constraints checks to the ACPI suspend-to-idle code (Rafael
    Wysocki, Srinivas Pandruvada).
 
  - Prefer suspend-to-idle over S3 on ACPI-based systems with the
    ACPI_FADT_LOW_POWER_S0 flag set and the Low Power Idle S0 _DSM
    interface present in the ACPI tables (Rafael Wysocki).
 
  - Update documentation related to system sleep and rename a number
    of items in the code to make it cleare that they are related to
    suspend-to-idle (Rafael Wysocki).
 
  - Export a variable allowing device drivers to check the target
    system sleep state from the core system suspend code (Florian
    Fainelli).
 
  - Clean up the cpuidle subsystem to handle the polling state on
    x86 in a more straightforward way and to use %pOF instead of
    full_name (Rafael Wysocki, Rob Herring).
 
  - Update the devfreq framework to fix and clean up a few minor
    issues (Chanwoo Choi, Rob Herring).
 
  - Extend diagnostics in the generic power domains (genpd) framework
    and clean it up slightly (Thara Gopinath, Rob Herring).
 
  - Fix and clean up a couple of issues in the operating performance
    points (OPP) framework (Viresh Kumar, Waldemar Rymarkiewicz).
 
  - Add support for RV1108 to the rockchip-io Adaptive Voltage Scaling
    (AVS) driver (David Wu).
 
  - Fix the usage of notifiers in CPU power management on some
    platforms (Alex Shi).
 
  - Update the pm-graph system suspend/hibernation and boot profiling
    utility (Todd Brandt).
 
  - Make it possible to run the cpupower utility without CPU0 (Prarit
    Bhargava).
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Merge tag 'pm-4.14-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/rafael/linux-pm

Pull power management updates from Rafael Wysocki:
 "This time (again) cpufreq gets the majority of changes which mostly
  are driver updates (including a major consolidation of intel_pstate),
  some schedutil governor modifications and core cleanups.

  There also are some changes in the system suspend area, mostly related
  to diagnostics and debug messages plus some renames of things related
  to suspend-to-idle. One major change here is that suspend-to-idle is
  now going to be preferred over S3 on systems where the ACPI tables
  indicate to do so and provide requsite support (the Low Power Idle S0
  _DSM in particular). The system sleep documentation and the tools
  related to it are updated too.

  The rest is a few cpuidle changes (nothing major), devfreq updates,
  generic power domains (genpd) framework updates and a few assorted
  modifications elsewhere.

  Specifics:

   - Drop the P-state selection algorithm based on a PID controller from
     intel_pstate and make it use the same P-state selection method
     (based on the CPU load) for all types of systems in the active mode
     (Rafael Wysocki, Srinivas Pandruvada).

   - Rework the cpufreq core and governors to make it possible to take
     cross-CPU utilization updates into account and modify the schedutil
     governor to actually do so (Viresh Kumar).

   - Clean up the handling of transition latency information in the
     cpufreq core and untangle it from the information on which drivers
     cannot do dynamic frequency switching (Viresh Kumar).

   - Add support for new SoCs (MT2701/MT7623 and MT7622) to the mediatek
     cpufreq driver and update its DT bindings (Sean Wang).

   - Modify the cpufreq dt-platdev driver to autimatically create
     cpufreq devices for the new (v2) Operating Performance Points (OPP)
     DT bindings and update its whitelist of supported systems (Viresh
     Kumar, Shubhrajyoti Datta, Marc Gonzalez, Khiem Nguyen, Finley
     Xiao).

   - Add support for Ux500 to the cpufreq-dt driver and drop the
     obsolete dbx500 cpufreq driver (Linus Walleij, Arnd Bergmann).

   - Add new SoC (R8A7795) support to the cpufreq rcar driver (Khiem
     Nguyen).

   - Fix and clean up assorted issues in the cpufreq drivers and core
     (Arvind Yadav, Christophe Jaillet, Colin Ian King, Gustavo Silva,
     Julia Lawall, Leonard Crestez, Rob Herring, Sudeep Holla).

   - Update the IO-wait boost handling in the schedutil governor to make
     it less aggressive (Joel Fernandes).

   - Rework system suspend diagnostics to make it print fewer messages
     to the kernel log by default, add a sysfs knob to allow more
     suspend-related messages to be printed and add Low Power S0 Idle
     constraints checks to the ACPI suspend-to-idle code (Rafael
     Wysocki, Srinivas Pandruvada).

   - Prefer suspend-to-idle over S3 on ACPI-based systems with the
     ACPI_FADT_LOW_POWER_S0 flag set and the Low Power Idle S0 _DSM
     interface present in the ACPI tables (Rafael Wysocki).

   - Update documentation related to system sleep and rename a number of
     items in the code to make it cleare that they are related to
     suspend-to-idle (Rafael Wysocki).

   - Export a variable allowing device drivers to check the target
     system sleep state from the core system suspend code (Florian
     Fainelli).

   - Clean up the cpuidle subsystem to handle the polling state on x86
     in a more straightforward way and to use %pOF instead of full_name
     (Rafael Wysocki, Rob Herring).

   - Update the devfreq framework to fix and clean up a few minor issues
     (Chanwoo Choi, Rob Herring).

   - Extend diagnostics in the generic power domains (genpd) framework
     and clean it up slightly (Thara Gopinath, Rob Herring).

   - Fix and clean up a couple of issues in the operating performance
     points (OPP) framework (Viresh Kumar, Waldemar Rymarkiewicz).

   - Add support for RV1108 to the rockchip-io Adaptive Voltage Scaling
     (AVS) driver (David Wu).

   - Fix the usage of notifiers in CPU power management on some
     platforms (Alex Shi).

   - Update the pm-graph system suspend/hibernation and boot profiling
     utility (Todd Brandt).

   - Make it possible to run the cpupower utility without CPU0 (Prarit
     Bhargava)"

* tag 'pm-4.14-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/rafael/linux-pm: (87 commits)
  cpuidle: Make drivers initialize polling state
  cpuidle: Move polling state initialization code to separate file
  cpuidle: Eliminate the CPUIDLE_DRIVER_STATE_START symbol
  cpufreq: imx6q: Fix imx6sx low frequency support
  cpufreq: speedstep-lib: make several arrays static, makes code smaller
  PM: docs: Delete the obsolete states.txt document
  PM: docs: Describe high-level PM strategies and sleep states
  PM / devfreq: Fix memory leak when fail to register device
  PM / devfreq: Add dependency on PM_OPP
  PM / devfreq: Move private devfreq_update_stats() into devfreq
  PM / devfreq: Convert to using %pOF instead of full_name
  PM / AVS: rockchip-io: add io selectors and supplies for RV1108
  cpufreq: ti: Fix 'of_node_put' being called twice in error handling path
  cpufreq: dt-platdev: Drop few entries from whitelist
  cpufreq: dt-platdev: Automatically create cpufreq device with OPP v2
  ARM: ux500: don't select CPUFREQ_DT
  cpuidle: Convert to using %pOF instead of full_name
  cpufreq: Convert to using %pOF instead of full_name
  PM / Domains: Convert to using %pOF instead of full_name
  cpufreq: Cap the default transition delay value to 10 ms
  ...
This commit is contained in:
Linus Torvalds 2017-09-05 12:19:08 -07:00
commit 439644096c
101 changed files with 2835 additions and 1746 deletions
Download patch file Download diff file Expand all files Collapse all files

8
Documentation/admin-guide/pm/cpufreq.rst
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@ -479,14 +479,6 @@ This governor exposes the following tunables:
# echo `$(($(cat cpuinfo_transition_latency) * 750 / 1000)) > ondemand/sampling_rate
``min_sampling_rate``
The minimum value of ``sampling_rate``.
Equal to 10000 (10 ms) if :c:macro:`CONFIG_NO_HZ_COMMON` and
:c:data:`tick_nohz_active` are both set or to 20 times the value of
:c:data:`jiffies` in microseconds otherwise.
``up_threshold``
If the estimated CPU load is above this value (in percent), the governor
will set the frequency to the maximum value allowed for the policy.

12
Documentation/admin-guide/pm/index.rst
View file

@ -5,12 +5,6 @@ Power Management
.. toctree::
:maxdepth: 2
cpufreq
intel_pstate
.. only:: subproject and html
Indices
=======
* :ref:`genindex`
strategies
system-wide
working-state

59
Documentation/admin-guide/pm/intel_pstate.rst
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@ -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.
current CPU utilization.
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.
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

245
Documentation/admin-guide/pm/sleep-states.rst Normal file
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@ -0,0 +1,245 @@
===================
System Sleep States
===================
::
Copyright (c) 2017 Intel Corp., Rafael J. Wysocki <[email protected]>
Sleep states are global low-power states of the entire system in which user
space code cannot be executed and the overall system activity is significantly
reduced.
Sleep States That Can Be Supported
==================================
Depending on its configuration and the capabilities of the platform it runs on,
the Linux kernel can support up to four system sleep states, includig
hibernation and up to three variants of system suspend. The sleep states that
can be supported by the kernel are listed below.
.. _s2idle:
Suspend-to-Idle
---------------
This is a generic, pure software, light-weight variant of system suspend (also
referred to as S2I or S2Idle). It allows more energy to be saved relative to
runtime idle by freezing user space, suspending the timekeeping and putting all
I/O devices into low-power states (possibly lower-power than available in the
working state), such that the processors can spend time in their deepest idle
states while the system is suspended.
The system is woken up from this state by in-band interrupts, so theoretically
any devices that can cause interrupts to be generated in the working state can
also be set up as wakeup devices for S2Idle.
This state can be used on platforms without support for :ref:`standby <standby>`
or :ref:`suspend-to-RAM <s2ram>`, or it can be used in addition to any of the
deeper system suspend variants to provide reduced resume latency. It is always
supported if the :c:macro:`CONFIG_SUSPEND` kernel configuration option is set.
.. _standby:
Standby
-------
This state, if supported, offers moderate, but real, energy savings, while
providing a relatively straightforward transition back to the working state. No
operating state is lost (the system core logic retains power), so the system can
go back to where it left off easily enough.
In addition to freezing user space, suspending the timekeeping and putting all
I/O devices into low-power states, which is done for :ref:`suspend-to-idle
<s2idle>` too, nonboot CPUs are taken offline and all low-level system functions
are suspended during transitions into this state. For this reason, it should
allow more energy to be saved relative to :ref:`suspend-to-idle <s2idle>`, but
the resume latency will generally be greater than for that state.
The set of devices that can wake up the system from this state usually is
reduced relative to :ref:`suspend-to-idle <s2idle>` and it may be necessary to
rely on the platform for setting up the wakeup functionality as appropriate.
This state is supported if the :c:macro:`CONFIG_SUSPEND` kernel configuration
option is set and the support for it is registered by the platform with the
core system suspend subsystem. On ACPI-based systems this state is mapped to
the S1 system state defined by ACPI.
.. _s2ram:
Suspend-to-RAM
--------------
This state (also referred to as STR or S2RAM), if supported, offers significant
energy savings as everything in the system is put into a low-power state, except
for memory, which should be placed into the self-refresh mode to retain its
contents. All of the steps carried out when entering :ref:`standby <standby>`
are also carried out during transitions to S2RAM. Additional operations may
take place depending on the platform capabilities. In particular, on ACPI-based
systems the kernel passes control to the platform firmware (BIOS) as the last
step during S2RAM transitions and that usually results in powering down some
more low-level components that are not directly controlled by the kernel.
The state of devices and CPUs is saved and held in memory. All devices are
suspended and put into low-power states. In many cases, all peripheral buses
lose power when entering S2RAM, so devices must be able to handle the transition
back to the "on" state.
On ACPI-based systems S2RAM requires some minimal boot-strapping code in the
platform firmware to resume the system from it. This may be the case on other
platforms too.
The set of devices that can wake up the system from S2RAM usually is reduced
relative to :ref:`suspend-to-idle <s2idle>` and :ref:`standby <standby>` and it
may be necessary to rely on the platform for setting up the wakeup functionality
as appropriate.
S2RAM is supported if the :c:macro:`CONFIG_SUSPEND` kernel configuration option
is set and the support for it is registered by the platform with the core system
suspend subsystem. On ACPI-based systems it is mapped to the S3 system state
defined by ACPI.
.. _hibernation:
Hibernation
-----------
This state (also referred to as Suspend-to-Disk or STD) offers the greatest
energy savings and can be used even in the absence of low-level platform support
for system suspend. However, it requires some low-level code for resuming the
system to be present for the underlying CPU architecture.
Hibernation is significantly different from any of the system suspend variants.
It takes three system state changes to put it into hibernation and two system
state changes to resume it.
First, when hibernation is triggered, the kernel stops all system activity and
creates a snapshot image of memory to be written into persistent storage. Next,
the system goes into a state in which the snapshot image can be saved, the image
is written out and finally the system goes into the target low-power state in
which power is cut from almost all of its hardware components, including memory,
except for a limited set of wakeup devices.
Once the snapshot image has been written out, the system may either enter a
special low-power state (like ACPI S4), or it may simply power down itself.
Powering down means minimum power draw and it allows this mechanism to work on
any system. However, entering a special low-power state may allow additional
means of system wakeup to be used (e.g. pressing a key on the keyboard or
opening a laptop lid).
After wakeup, control goes to the platform firmware that runs a boot loader
which boots a fresh instance of the kernel (control may also go directly to
the boot loader, depending on the system configuration, but anyway it causes
a fresh instance of the kernel to be booted). That new instance of the kernel
(referred to as the ``restore kernel``) looks for a hibernation image in
persistent storage and if one is found, it is loaded into memory. Next, all
activity in the system is stopped and the restore kernel overwrites itself with
the image contents and jumps into a special trampoline area in the original
kernel stored in the image (referred to as the ``image kernel``), which is where
the special architecture-specific low-level code is needed. Finally, the
image kernel restores the system to the pre-hibernation state and allows user
space to run again.
Hibernation is supported if the :c:macro:`CONFIG_HIBERNATION` kernel
configuration option is set. However, this option can only be set if support
for the given CPU architecture includes the low-level code for system resume.
Basic ``sysfs`` Interfaces for System Suspend and Hibernation
=============================================================
The following files located in the :file:`/sys/power/` directory can be used by
user space for sleep states control.
``state``
This file contains a list of strings representing sleep states supported
by the kernel. Writing one of these strings into it causes the kernel
to start a transition of the system into the sleep state represented by
that string.
In particular, the strings "disk", "freeze" and "standby" represent the
:ref:`hibernation <hibernation>`, :ref:`suspend-to-idle <s2idle>` and
:ref:`standby <standby>` sleep states, respectively. The string "mem"
is interpreted in accordance with the contents of the ``mem_sleep`` file
described below.
If the kernel does not support any system sleep states, this file is
not present.
``mem_sleep``
This file contains a list of strings representing supported system
suspend variants and allows user space to select the variant to be
associated with the "mem" string in the ``state`` file described above.
The strings that may be present in this file are "s2idle", "shallow"
and "deep". The string "s2idle" always represents :ref:`suspend-to-idle
<s2idle>` and, by convention, "shallow" and "deep" represent
:ref:`standby <standby>` and :ref:`suspend-to-RAM <s2ram>`,
respectively.
Writing one of the listed strings into this file causes the system
suspend variant represented by it to be associated with the "mem" string
in the ``state`` file. The string representing the suspend variant
currently associated with the "mem" string in the ``state`` file
is listed in square brackets.
If the kernel does not support system suspend, this file is not present.
``disk``
This file contains a list of strings representing different operations
that can be carried out after the hibernation image has been saved. The
possible options are as follows:
``platform``
Put the system into a special low-power state (e.g. ACPI S4) to
make additional wakeup options available and possibly allow the
platform firmware to take a simplified initialization path after
wakeup.
``shutdown``
Power off the system.
``reboot``
Reboot the system (useful for diagnostics mostly).
``suspend``
Hybrid system suspend. Put the system into the suspend sleep
state selected through the ``mem_sleep`` file described above.
If the system is successfully woken up from that state, discard
the hibernation image and continue. Otherwise, use the image
to restore the previous state of the system.
``test_resume``
Diagnostic operation. Load the image as though the system had
just woken up from hibernation and the currently running kernel
instance was a restore kernel and follow up with full system
resume.
Writing one of the listed strings into this file causes the option
represented by it to be selected.
The currently selected option is shown in square brackets which means
that the operation represented by it will be carried out after creating
and saving the image next time hibernation is triggered by writing
``disk`` to :file:`/sys/power/state`.
If the kernel does not support hibernation, this file is not present.
According to the above, there are two ways to make the system go into the
:ref:`suspend-to-idle <s2idle>` state. The first one is to write "freeze"
directly to :file:`/sys/power/state`. The second one is to write "s2idle" to
:file:`/sys/power/mem_sleep` and then to write "mem" to
:file:`/sys/power/state`. Likewise, there are two ways to make the system go
into the :ref:`standby <standby>` state (the strings to write to the control
files in that case are "standby" or "shallow" and "mem", respectively) if that
state is supported by the platform. However, there is only one way to make the
system go into the :ref:`suspend-to-RAM <s2ram>` state (write "deep" into
:file:`/sys/power/mem_sleep` and "mem" into :file:`/sys/power/state`).
The default suspend variant (ie. the one to be used without writing anything
into :file:`/sys/power/mem_sleep`) is either "deep" (on the majority of systems
supporting :ref:`suspend-to-RAM <s2ram>`) or "s2idle", but it can be overridden
by the value of the "mem_sleep_default" parameter in the kernel command line.
On some ACPI-based systems, depending on the information in the ACPI tables, the
default may be "s2idle" even if :ref:`suspend-to-RAM <s2ram>` is supported.

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@ -0,0 +1,52 @@
===========================
Power Management Strategies
===========================
::
Copyright (c) 2017 Intel Corp., Rafael J. Wysocki <[email protected]>
The Linux kernel supports two major high-level power management strategies.
One of them is based on using global low-power states of the whole system in
which user space code cannot be executed and the overall system activity is
significantly reduced, referred to as :doc:`sleep states <sleep-states>`. The
kernel puts the system into one of these states when requested by user space
and the system stays in it until a special signal is received from one of
designated devices, triggering a transition to the ``working state`` in which
user space code can run. Because sleep states are global and the whole system
is affected by the state changes, this strategy is referred to as the
:doc:`system-wide power management <system-wide>`.
The other strategy, referred to as the :doc:`working-state power management
<working-state>`, is based on adjusting the power states of individual hardware
components of the system, as needed, in the working state. In consequence, if
this strategy is in use, the working state of the system usually does not
correspond to any particular physical configuration of it, but can be treated as
a metastate covering a range of different power states of the system in which
the individual components of it can be either ``active`` (in use) or
``inactive`` (idle). If they are active, they have to be in power states
allowing them to process data and to be accessed by software. In turn, if they
are inactive, ideally, they should be in low-power states in which they may not
be accessible.
If all of the system components are active, the system as a whole is regarded as
"runtime active" and that situation typically corresponds to the maximum power
draw (or maximum energy usage) of it. If all of them are inactive, the system
as a whole is regarded as "runtime idle" which may be very close to a sleep
state from the physical system configuration and power draw perspective, but
then it takes much less time and effort to start executing user space code than
for the same system in a sleep state. However, transitions from sleep states
back to the working state can only be started by a limited set of devices, so
typically the system can spend much more time in a sleep state than it can be
runtime idle in one go. For this reason, systems usually use less energy in
sleep states than when they are runtime idle most of the time.
Moreover, the two power management strategies address different usage scenarios.
Namely, if the user indicates that the system will not be in use going forward,
for example by closing its lid (if the system is a laptop), it probably should
go into a sleep state at that point. On the other hand, if the user simply goes
away from the laptop keyboard, it probably should stay in the working state and
use the working-state power management in case it becomes idle, because the user
may come back to it at any time and then may want the system to be immediately
accessible.

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============================
System-Wide Power Management
============================
.. toctree::
:maxdepth: 2
sleep-states

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==============================
Working-State Power Management
==============================
.. toctree::
:maxdepth: 2
cpufreq
intel_pstate