Re: [PATCH] Documentation: driver-api: PM: Add cpuidle document
From: Rafael J. Wysocki
Date: Wed Jan 16 2019 - 17:10:54 EST
On Wed, Jan 9, 2019 at 11:54 AM Rafael J. Wysocki <rjw@xxxxxxxxxxxxx> wrote:
>
> From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
>
> 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@xxxxxxxxx>
In the face of the lack of any feedback I'm assuming that this is fine
by everyone.
> ---
>
> On top of https://patchwork.kernel.org/patch/10747185/
>
> ---
> Documentation/cpuidle/driver.txt | 37 ----
> Documentation/cpuidle/governor.txt | 28 ---
> Documentation/driver-api/pm/cpuidle.rst | 282 ++++++++++++++++++++++++++++++++
> Documentation/driver-api/pm/index.rst | 7
> MAINTAINERS | 1
> 5 files changed, 287 insertions(+), 68 deletions(-)
>
> Index: linux-pm/Documentation/driver-api/pm/index.rst
> ===================================================================
> --- linux-pm.orig/Documentation/driver-api/pm/index.rst
> +++ linux-pm/Documentation/driver-api/pm/index.rst
> @@ -1,9 +1,10 @@
> -=======================
> -Device Power Management
> -=======================
> +===============================
> +CPU and Device Power Management
> +===============================
>
> .. toctree::
>
> + cpuidle
> devices
> notifiers
> types
> Index: linux-pm/Documentation/driver-api/pm/cpuidle.rst
> ===================================================================
> --- /dev/null
> +++ linux-pm/Documentation/driver-api/pm/cpuidle.rst
> @@ -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@xxxxxxxxx>
> +
> +
> +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.
> Index: linux-pm/Documentation/cpuidle/driver.txt
> ===================================================================
> --- linux-pm.orig/Documentation/cpuidle/driver.txt
> +++ /dev/null
> @@ -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);
> Index: linux-pm/Documentation/cpuidle/governor.txt
> ===================================================================
> --- linux-pm.orig/Documentation/cpuidle/governor.txt
> +++ /dev/null
> @@ -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
> Index: linux-pm/MAINTAINERS
> ===================================================================
> --- linux-pm.orig/MAINTAINERS
> +++ linux-pm/MAINTAINERS
> @@ -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
>
>