280 lines
14 KiB
ReStructuredText
280 lines
14 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
|
|
.. include:: <isonum.txt>
|
|
|
|
========================
|
|
CPU Idle Time Management
|
|
========================
|
|
|
|
:Copyright: |copy| 2019 Intel Corporation
|
|
|
|
:Author: 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, 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.
|