CPU Idle Time Management

Copyright:© 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, enable, disable, select, reflect, a 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 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 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:

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.

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.

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 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 select callback pointer in struct cpuidle_governor must not be NULL for the registration of the governor to succeed).

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 cpuidle_governor_latency_req(). Then, the governor’s ->select() callback must not return the index of an indle state whose 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 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 states array are expected to be sorted by the value of the 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 target_residency). [Since the 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:

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.
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.
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 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 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 states array has been populated, the number of valid entries in it has to be stored in the state_count field of the struct cpuidle_driver object representing the driver. Moreover, if any entries in the states array represent “coupled” idle states (that is, idle states that can only be asked for if multiple related logical CPUs are idle), the 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 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 states array, that can be accomplished by passing the driver’s struct cpuidle_driver object to cpuidle_register_driver(). Otherwise, 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 cpuidle_register_device() after the driver has been registered and cpuidle_register_driver(), unlike cpuidle_register(), does not do that automatically. For this reason, the drivers that use 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 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 cpuidle_unregister_device(), before calling cpuidle_unregister_driver() to unregister the driver. Alternatively, 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 cpuidle_pause_and_lock() to turn CPUIdle off temporarily and then cpuidle_disable_device() for all of the struct cpuidle_device objects representing CPUs affected by that change. Next, it can update its states array in accordance with the new configuration of the system, call cpuidle_enable_device() for all of the relevant struct cpuidle_device objects and invoke cpuidle_resume_and_unlock() to allow CPUIdle to be used again.