Documentation: driver-api: PM: Add cpuidle document
Replace the remaining documents under Documentation/cpuidle/ with one more complete governor and driver API document for cpuidle under Documentation/driver-api/pm/. Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com> Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
This commit is contained in:
Rafael J. Wysocki
parent
b26bf6ab71
commit
3b7357663a
|
|
@ -1,37 +0,0 @@
|
|||
|
||||
|
||||
Supporting multiple CPU idle levels in kernel
|
||||
|
||||
cpuidle drivers
|
||||
|
||||
|
||||
|
||||
|
||||
cpuidle driver hooks into the cpuidle infrastructure and handles the
|
||||
architecture/platform dependent part of CPU idle states. Driver
|
||||
provides the platform idle state detection capability and also
|
||||
has mechanisms in place to support actual entry-exit into CPU idle states.
|
||||
|
||||
cpuidle driver initializes the cpuidle_device structure for each CPU device
|
||||
and registers with cpuidle using cpuidle_register_device.
|
||||
|
||||
If all the idle states are the same, the wrapper function cpuidle_register
|
||||
could be used instead.
|
||||
|
||||
It can also support the dynamic changes (like battery <-> AC), by using
|
||||
cpuidle_pause_and_lock, cpuidle_disable_device and cpuidle_enable_device,
|
||||
cpuidle_resume_and_unlock.
|
||||
|
||||
Interfaces:
|
||||
extern int cpuidle_register(struct cpuidle_driver *drv,
|
||||
const struct cpumask *const coupled_cpus);
|
||||
extern int cpuidle_unregister(struct cpuidle_driver *drv);
|
||||
extern int cpuidle_register_driver(struct cpuidle_driver *drv);
|
||||
extern void cpuidle_unregister_driver(struct cpuidle_driver *drv);
|
||||
extern int cpuidle_register_device(struct cpuidle_device *dev);
|
||||
extern void cpuidle_unregister_device(struct cpuidle_device *dev);
|
||||
|
||||
extern void cpuidle_pause_and_lock(void);
|
||||
extern void cpuidle_resume_and_unlock(void);
|
||||
extern int cpuidle_enable_device(struct cpuidle_device *dev);
|
||||
extern void cpuidle_disable_device(struct cpuidle_device *dev);
|
||||
|
|
@ -1,28 +0,0 @@
|
|||
|
||||
|
||||
|
||||
Supporting multiple CPU idle levels in kernel
|
||||
|
||||
cpuidle governors
|
||||
|
||||
|
||||
|
||||
|
||||
cpuidle governor is policy routine that decides what idle state to enter at
|
||||
any given time. cpuidle core uses different callbacks to the governor.
|
||||
|
||||
* enable() to enable governor for a particular device
|
||||
* disable() to disable governor for a particular device
|
||||
* select() to select an idle state to enter
|
||||
* reflect() called after returning from the idle state, which can be used
|
||||
by the governor for some record keeping.
|
||||
|
||||
More than one governor can be registered at the same time and
|
||||
users can switch between drivers using /sysfs interface (when enabled).
|
||||
More than one governor part is supported for developers to easily experiment
|
||||
with different governors. By default, most optimal governor based on your
|
||||
kernel configuration and platform will be selected by cpuidle.
|
||||
|
||||
Interfaces:
|
||||
extern int cpuidle_register_governor(struct cpuidle_governor *gov);
|
||||
struct cpuidle_governor
|
||||
|
|
@ -0,0 +1,282 @@
|
|||
.. |struct cpuidle_governor| replace:: :c:type:`struct cpuidle_governor <cpuidle_governor>`
|
||||
.. |struct cpuidle_device| replace:: :c:type:`struct cpuidle_device <cpuidle_device>`
|
||||
.. |struct cpuidle_driver| replace:: :c:type:`struct cpuidle_driver <cpuidle_driver>`
|
||||
.. |struct cpuidle_state| replace:: :c:type:`struct cpuidle_state <cpuidle_state>`
|
||||
|
||||
========================
|
||||
CPU Idle Time Management
|
||||
========================
|
||||
|
||||
::
|
||||
|
||||
Copyright (c) 2019 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com>
|
||||
|
||||
|
||||
CPU Idle Time Management Subsystem
|
||||
==================================
|
||||
|
||||
Every time one of the logical CPUs in the system (the entities that appear to
|
||||
fetch and execute instructions: hardware threads, if present, or processor
|
||||
cores) is idle after an interrupt or equivalent wakeup event, which means that
|
||||
there are no tasks to run on it except for the special "idle" task associated
|
||||
with it, there is an opportunity to save energy for the processor that it
|
||||
belongs to. That can be done by making the idle logical CPU stop fetching
|
||||
instructions from memory and putting some of the processor's functional units
|
||||
depended on by it into an idle state in which they will draw less power.
|
||||
|
||||
However, there may be multiple different idle states that can be used in such a
|
||||
situation in principle, so it may be necessary to find the most suitable one
|
||||
(from the kernel perspective) and ask the processor to use (or "enter") that
|
||||
particular idle state. That is the role of the CPU idle time management
|
||||
subsystem in the kernel, called ``CPUIdle``.
|
||||
|
||||
The design of ``CPUIdle`` is modular and based on the code duplication avoidance
|
||||
principle, so the generic code that in principle need not depend on the hardware
|
||||
or platform design details in it is separate from the code that interacts with
|
||||
the hardware. It generally is divided into three categories of functional
|
||||
units: *governors* responsible for selecting idle states to ask the processor
|
||||
to enter, *drivers* that pass the governors' decisions on to the hardware and
|
||||
the *core* providing a common framework for them.
|
||||
|
||||
|
||||
CPU Idle Time Governors
|
||||
=======================
|
||||
|
||||
A CPU idle time (``CPUIdle``) governor is a bundle of policy code invoked when
|
||||
one of the logical CPUs in the system turns out to be idle. Its role is to
|
||||
select an idle state to ask the processor to enter in order to save some energy.
|
||||
|
||||
``CPUIdle`` governors are generic and each of them can be used on any hardware
|
||||
platform that the Linux kernel can run on. For this reason, data structures
|
||||
operated on by them cannot depend on any hardware architecture or platform
|
||||
design details as well.
|
||||
|
||||
The governor itself is represented by a |struct cpuidle_governor| object
|
||||
containing four callback pointers, :c:member:`enable`, :c:member:`disable`,
|
||||
:c:member:`select`, :c:member:`reflect`, a :c:member:`rating` field described
|
||||
below, and a name (string) used for identifying it.
|
||||
|
||||
For the governor to be available at all, that object needs to be registered
|
||||
with the ``CPUIdle`` core by calling :c:func:`cpuidle_register_governor()` with
|
||||
a pointer to it passed as the argument. If successful, that causes the core to
|
||||
add the governor to the global list of available governors and, if it is the
|
||||
only one in the list (that is, the list was empty before) or the value of its
|
||||
:c:member:`rating` field is greater than the value of that field for the
|
||||
governor currently in use, or the name of the new governor was passed to the
|
||||
kernel as the value of the ``cpuidle.governor=`` command line parameter, the new
|
||||
governor will be used from that point on (there can be only one ``CPUIdle``
|
||||
governor in use at a time). Also, if ``cpuidle_sysfs_switch`` is passed to the
|
||||
kernel in the command line, user space can choose the ``CPUIdle`` governor to
|
||||
use at run time via ``sysfs``.
|
||||
|
||||
Once registered, ``CPUIdle`` governors cannot be unregistered, so it is not
|
||||
practical to put them into loadable kernel modules.
|
||||
|
||||
The interface between ``CPUIdle`` governors and the core consists of four
|
||||
callbacks:
|
||||
|
||||
:c:member:`enable`
|
||||
::
|
||||
|
||||
int (*enable) (struct cpuidle_driver *drv, struct cpuidle_device *dev);
|
||||
|
||||
The role of this callback is to prepare the governor for handling the
|
||||
(logical) CPU represented by the |struct cpuidle_device| object pointed
|
||||
to by the ``dev`` argument. The |struct cpuidle_driver| object pointed
|
||||
to by the ``drv`` argument represents the ``CPUIdle`` driver to be used
|
||||
with that CPU (among other things, it should contain the list of
|
||||
|struct cpuidle_state| objects representing idle states that the
|
||||
processor holding the given CPU can be asked to enter).
|
||||
|
||||
It may fail, in which case it is expected to return a negative error
|
||||
code, and that causes the kernel to run the architecture-specific
|
||||
default code for idle CPUs on the CPU in question instead of ``CPUIdle``
|
||||
until the ``->enable()`` governor callback is invoked for that CPU
|
||||
again.
|
||||
|
||||
:c:member:`disable`
|
||||
::
|
||||
|
||||
void (*disable) (struct cpuidle_driver *drv, struct cpuidle_device *dev);
|
||||
|
||||
Called to make the governor stop handling the (logical) CPU represented
|
||||
by the |struct cpuidle_device| object pointed to by the ``dev``
|
||||
argument.
|
||||
|
||||
It is expected to reverse any changes made by the ``->enable()``
|
||||
callback when it was last invoked for the target CPU, free all memory
|
||||
allocated by that callback and so on.
|
||||
|
||||
:c:member:`select`
|
||||
::
|
||||
|
||||
int (*select) (struct cpuidle_driver *drv, struct cpuidle_device *dev,
|
||||
bool *stop_tick);
|
||||
|
||||
Called to select an idle state for the processor holding the (logical)
|
||||
CPU represented by the |struct cpuidle_device| object pointed to by the
|
||||
``dev`` argument.
|
||||
|
||||
The list of idle states to take into consideration is represented by the
|
||||
:c:member:`states` array of |struct cpuidle_state| objects held by the
|
||||
|struct cpuidle_driver| object pointed to by the ``drv`` argument (which
|
||||
represents the ``CPUIdle`` driver to be used with the CPU at hand). The
|
||||
value returned by this callback is interpreted as an index into that
|
||||
array (unless it is a negative error code).
|
||||
|
||||
The ``stop_tick`` argument is used to indicate whether or not to stop
|
||||
the scheduler tick before asking the processor to enter the selected
|
||||
idle state. When the ``bool`` variable pointed to by it (which is set
|
||||
to ``true`` before invoking this callback) is cleared to ``false``, the
|
||||
processor will be asked to enter the selected idle state without
|
||||
stopping the scheduler tick on the given CPU (if the tick has been
|
||||
stopped on that CPU already, however, it will not be restarted before
|
||||
asking the processor to enter the idle state).
|
||||
|
||||
This callback is mandatory (i.e. the :c:member:`select` callback pointer
|
||||
in |struct cpuidle_governor| must not be ``NULL`` for the registration
|
||||
of the governor to succeed).
|
||||
|
||||
:c:member:`reflect`
|
||||
::
|
||||
|
||||
void (*reflect) (struct cpuidle_device *dev, int index);
|
||||
|
||||
Called to allow the governor to evaluate the accuracy of the idle state
|
||||
selection made by the ``->select()`` callback (when it was invoked last
|
||||
time) and possibly use the result of that to improve the accuracy of
|
||||
idle state selections in the future.
|
||||
|
||||
In addition, ``CPUIdle`` governors are required to take power management
|
||||
quality of service (PM QoS) constraints on the processor wakeup latency into
|
||||
account when selecting idle states. In order to obtain the current effective
|
||||
PM QoS wakeup latency constraint for a given CPU, a ``CPUIdle`` governor is
|
||||
expected to pass the number of the CPU to
|
||||
:c:func:`cpuidle_governor_latency_req()`. Then, the governor's ``->select()``
|
||||
callback must not return the index of an indle state whose
|
||||
:c:member:`exit_latency` value is greater than the number returned by that
|
||||
function.
|
||||
|
||||
|
||||
CPU Idle Time Management Drivers
|
||||
================================
|
||||
|
||||
CPU idle time management (``CPUIdle``) drivers provide an interface between the
|
||||
other parts of ``CPUIdle`` and the hardware.
|
||||
|
||||
First of all, a ``CPUIdle`` driver has to populate the :c:member:`states` array
|
||||
of |struct cpuidle_state| objects included in the |struct cpuidle_driver| object
|
||||
representing it. Going forward this array will represent the list of available
|
||||
idle states that the processor hardware can be asked to enter shared by all of
|
||||
the logical CPUs handled by the given driver.
|
||||
|
||||
The entries in the :c:member:`states` array are expected to be sorted by the
|
||||
value of the :c:member:`target_residency` field in |struct cpuidle_state| in
|
||||
the ascending order (that is, index 0 should correspond to the idle state with
|
||||
the minimum value of :c:member:`target_residency`). [Since the
|
||||
:c:member:`target_residency` value is expected to reflect the "depth" of the
|
||||
idle state represented by the |struct cpuidle_state| object holding it, this
|
||||
sorting order should be the same as the ascending sorting order by the idle
|
||||
state "depth".]
|
||||
|
||||
Three fields in |struct cpuidle_state| are used by the existing ``CPUIdle``
|
||||
governors for computations related to idle state selection:
|
||||
|
||||
:c:member:`target_residency`
|
||||
Minimum time to spend in this idle state including the time needed to
|
||||
enter it (which may be substantial) to save more energy than could
|
||||
be saved by staying in a shallower idle state for the same amount of
|
||||
time, in microseconds.
|
||||
|
||||
:c:member:`exit_latency`
|
||||
Maximum time it will take a CPU asking the processor to enter this idle
|
||||
state to start executing the first instruction after a wakeup from it,
|
||||
in microseconds.
|
||||
|
||||
:c:member:`flags`
|
||||
Flags representing idle state properties. Currently, governors only use
|
||||
the ``CPUIDLE_FLAG_POLLING`` flag which is set if the given object
|
||||
does not represent a real idle state, but an interface to a software
|
||||
"loop" that can be used in order to avoid asking the processor to enter
|
||||
any idle state at all. [There are other flags used by the ``CPUIdle``
|
||||
core in special situations.]
|
||||
|
||||
The :c:member:`enter` callback pointer in |struct cpuidle_state|, which must not
|
||||
be ``NULL``, points to the routine to execute in order to ask the processor to
|
||||
enter this particular idle state:
|
||||
|
||||
::
|
||||
|
||||
void (*enter) (struct cpuidle_device *dev, struct cpuidle_driver *drv,
|
||||
int index);
|
||||
|
||||
The first two arguments of it point to the |struct cpuidle_device| object
|
||||
representing the logical CPU running this callback and the
|
||||
|struct cpuidle_driver| object representing the driver itself, respectively,
|
||||
and the last one is an index of the |struct cpuidle_state| entry in the driver's
|
||||
:c:member:`states` array representing the idle state to ask the processor to
|
||||
enter.
|
||||
|
||||
The analogous ``->enter_s2idle()`` callback in |struct cpuidle_state| is used
|
||||
only for implementing the suspend-to-idle system-wide power management feature.
|
||||
The difference between in and ``->enter()`` is that it must not re-enable
|
||||
interrupts at any point (even temporarily) or attempt to change the states of
|
||||
clock event devices, which the ``->enter()`` callback may do sometimes.
|
||||
|
||||
Once the :c:member:`states` array has been populated, the number of valid
|
||||
entries in it has to be stored in the :c:member:`state_count` field of the
|
||||
|struct cpuidle_driver| object representing the driver. Moreover, if any
|
||||
entries in the :c:member:`states` array represent "coupled" idle states (that
|
||||
is, idle states that can only be asked for if multiple related logical CPUs are
|
||||
idle), the :c:member:`safe_state_index` field in |struct cpuidle_driver| needs
|
||||
to be the index of an idle state that is not "coupled" (that is, one that can be
|
||||
asked for if only one logical CPU is idle).
|
||||
|
||||
In addition to that, if the given ``CPUIdle`` driver is only going to handle a
|
||||
subset of logical CPUs in the system, the :c:member:`cpumask` field in its
|
||||
|struct cpuidle_driver| object must point to the set (mask) of CPUs that will be
|
||||
handled by it.
|
||||
|
||||
A ``CPUIdle`` driver can only be used after it has been registered. If there
|
||||
are no "coupled" idle state entries in the driver's :c:member:`states` array,
|
||||
that can be accomplished by passing the driver's |struct cpuidle_driver| object
|
||||
to :c:func:`cpuidle_register_driver()`. Otherwise, :c:func:`cpuidle_register()`
|
||||
should be used for this purpose.
|
||||
|
||||
However, it also is necessary to register |struct cpuidle_device| objects for
|
||||
all of the logical CPUs to be handled by the given ``CPUIdle`` driver with the
|
||||
help of :c:func:`cpuidle_register_device()` after the driver has been registered
|
||||
and :c:func:`cpuidle_register_driver()`, unlike :c:func:`cpuidle_register()`,
|
||||
does not do that automatically. For this reason, the drivers that use
|
||||
:c:func:`cpuidle_register_driver()` to register themselves must also take care
|
||||
of registering the |struct cpuidle_device| objects as needed, so it is generally
|
||||
recommended to use :c:func:`cpuidle_register()` for ``CPUIdle`` driver
|
||||
registration in all cases.
|
||||
|
||||
The registration of a |struct cpuidle_device| object causes the ``CPUIdle``
|
||||
``sysfs`` interface to be created and the governor's ``->enable()`` callback to
|
||||
be invoked for the logical CPU represented by it, so it must take place after
|
||||
registering the driver that will handle the CPU in question.
|
||||
|
||||
``CPUIdle`` drivers and |struct cpuidle_device| objects can be unregistered
|
||||
when they are not necessary any more which allows some resources associated with
|
||||
them to be released. Due to dependencies between them, all of the
|
||||
|struct cpuidle_device| objects representing CPUs handled by the given
|
||||
``CPUIdle`` driver must be unregistered, with the help of
|
||||
:c:func:`cpuidle_unregister_device()`, before calling
|
||||
:c:func:`cpuidle_unregister_driver()` to unregister the driver. Alternatively,
|
||||
:c:func:`cpuidle_unregister()` can be called to unregister a ``CPUIdle`` driver
|
||||
along with all of the |struct cpuidle_device| objects representing CPUs handled
|
||||
by it.
|
||||
|
||||
``CPUIdle`` drivers can respond to runtime system configuration changes that
|
||||
lead to modifications of the list of available processor idle states (which can
|
||||
happen, for example, when the system's power source is switched from AC to
|
||||
battery or the other way around). Upon a notification of such a change,
|
||||
a ``CPUIdle`` driver is expected to call :c:func:`cpuidle_pause_and_lock()` to
|
||||
turn ``CPUIdle`` off temporarily and then :c:func:`cpuidle_disable_device()` for
|
||||
all of the |struct cpuidle_device| objects representing CPUs affected by that
|
||||
change. Next, it can update its :c:member:`states` array in accordance with
|
||||
the new configuration of the system, call :c:func:`cpuidle_enable_device()` for
|
||||
all of the relevant |struct cpuidle_device| objects and invoke
|
||||
:c:func:`cpuidle_resume_and_unlock()` to allow ``CPUIdle`` to be used again.
|
||||
|
|
@ -1,9 +1,10 @@
|
|||
=======================
|
||||
Device Power Management
|
||||
=======================
|
||||
===============================
|
||||
CPU and Device Power Management
|
||||
===============================
|
||||
|
||||
.. toctree::
|
||||
|
||||
cpuidle
|
||||
devices
|
||||
notifiers
|
||||
types
|
||||
|
|
|
|||
|
|
@ -4016,6 +4016,7 @@ S: Maintained
|
|||
T: git git://git.kernel.org/pub/scm/linux/kernel/git/rafael/linux-pm.git
|
||||
B: https://bugzilla.kernel.org
|
||||
F: Documentation/admin-guide/pm/cpuidle.rst
|
||||
F: Documentation/driver-api/pm/cpuidle.rst
|
||||
F: drivers/cpuidle/*
|
||||
F: include/linux/cpuidle.h
|
||||
|
||||
|
|
|
|||
Loading…
Reference in New Issue