| .. SPDX-License-Identifier: GPL-2.0 |
| .. include:: <isonum.txt> |
| |
| .. |struct cpuidle_state| replace:: :c:type:`struct cpuidle_state <cpuidle_state>` |
| .. |cpufreq| replace:: :doc:`CPU Performance Scaling <cpufreq>` |
| |
| ======================== |
| CPU Idle Time Management |
| ======================== |
| |
| :Copyright: |copy| 2018 Intel Corporation |
| |
| :Author: Rafael J. Wysocki <rafael.j.wysocki@intel.com> |
| |
| |
| Concepts |
| ======== |
| |
| Modern processors are generally able to enter states in which the execution of |
| a program is suspended and instructions belonging to it are not fetched from |
| memory or executed. Those states are the *idle* states of the processor. |
| |
| Since part of the processor hardware is not used in idle states, entering them |
| generally allows power drawn by the processor to be reduced and, in consequence, |
| it is an opportunity to save energy. |
| |
| CPU idle time management is an energy-efficiency feature concerned about using |
| the idle states of processors for this purpose. |
| |
| Logical CPUs |
| ------------ |
| |
| CPU idle time management operates on CPUs as seen by the *CPU scheduler* (that |
| is the part of the kernel responsible for the distribution of computational |
| work in the system). In its view, CPUs are *logical* units. That is, they need |
| not be separate physical entities and may just be interfaces appearing to |
| software as individual single-core processors. In other words, a CPU is an |
| entity which appears to be fetching instructions that belong to one sequence |
| (program) from memory and executing them, but it need not work this way |
| physically. Generally, three different cases can be consider here. |
| |
| First, if the whole processor can only follow one sequence of instructions (one |
| program) at a time, it is a CPU. In that case, if the hardware is asked to |
| enter an idle state, that applies to the processor as a whole. |
| |
| Second, if the processor is multi-core, each core in it is able to follow at |
| least one program at a time. The cores need not be entirely independent of each |
| other (for example, they may share caches), but still most of the time they |
| work physically in parallel with each other, so if each of them executes only |
| one program, those programs run mostly independently of each other at the same |
| time. The entire cores are CPUs in that case and if the hardware is asked to |
| enter an idle state, that applies to the core that asked for it in the first |
| place, but it also may apply to a larger unit (say a "package" or a "cluster") |
| that the core belongs to (in fact, it may apply to an entire hierarchy of larger |
| units containing the core). Namely, if all of the cores in the larger unit |
| except for one have been put into idle states at the "core level" and the |
| remaining core asks the processor to enter an idle state, that may trigger it |
| to put the whole larger unit into an idle state which also will affect the |
| other cores in that unit. |
| |
| Finally, each core in a multi-core processor may be able to follow more than one |
| program in the same time frame (that is, each core may be able to fetch |
| instructions from multiple locations in memory and execute them in the same time |
| frame, but not necessarily entirely in parallel with each other). In that case |
| the cores present themselves to software as "bundles" each consisting of |
| multiple individual single-core "processors", referred to as *hardware threads* |
| (or hyper-threads specifically on Intel hardware), that each can follow one |
| sequence of instructions. Then, the hardware threads are CPUs from the CPU idle |
| time management perspective and if the processor is asked to enter an idle state |
| by one of them, the hardware thread (or CPU) that asked for it is stopped, but |
| nothing more happens, unless all of the other hardware threads within the same |
| core also have asked the processor to enter an idle state. In that situation, |
| the core may be put into an idle state individually or a larger unit containing |
| it may be put into an idle state as a whole (if the other cores within the |
| larger unit are in idle states already). |
| |
| Idle CPUs |
| --------- |
| |
| Logical CPUs, simply referred to as "CPUs" in what follows, are regarded as |
| *idle* by the Linux kernel when there are no tasks to run on them except for the |
| special "idle" task. |
| |
| Tasks are the CPU scheduler's representation of work. Each task consists of a |
| sequence of instructions to execute, or code, data to be manipulated while |
| running that code, and some context information that needs to be loaded into the |
| processor every time the task's code is run by a CPU. The CPU scheduler |
| distributes work by assigning tasks to run to the CPUs present in the system. |
| |
| Tasks can be in various states. In particular, they are *runnable* if there are |
| no specific conditions preventing their code from being run by a CPU as long as |
| there is a CPU available for that (for example, they are not waiting for any |
| events to occur or similar). When a task becomes runnable, the CPU scheduler |
| assigns it to one of the available CPUs to run and if there are no more runnable |
| tasks assigned to it, the CPU will load the given task's context and run its |
| code (from the instruction following the last one executed so far, possibly by |
| another CPU). [If there are multiple runnable tasks assigned to one CPU |
| simultaneously, they will be subject to prioritization and time sharing in order |
| to allow them to make some progress over time.] |
| |
| The special "idle" task becomes runnable if there are no other runnable tasks |
| assigned to the given CPU and the CPU is then regarded as idle. In other words, |
| in Linux idle CPUs run the code of the "idle" task called *the idle loop*. That |
| code may cause the processor to be put into one of its idle states, if they are |
| supported, in order to save energy, but if the processor does not support any |
| idle states, or there is not enough time to spend in an idle state before the |
| next wakeup event, or there are strict latency constraints preventing any of the |
| available idle states from being used, the CPU will simply execute more or less |
| useless instructions in a loop until it is assigned a new task to run. |
| |
| |
| .. _idle-loop: |
| |
| The Idle Loop |
| ============= |
| |
| The idle loop code takes two major steps in every iteration of it. First, it |
| calls into a code module referred to as the *governor* that belongs to the CPU |
| idle time management subsystem called ``CPUIdle`` to select an idle state for |
| the CPU to ask the hardware to enter. Second, it invokes another code module |
| from the ``CPUIdle`` subsystem, called the *driver*, to actually ask the |
| processor hardware to enter the idle state selected by the governor. |
| |
| The role of the governor is to find an idle state most suitable for the |
| conditions at hand. For this purpose, idle states that the hardware can be |
| asked to enter by logical CPUs are represented in an abstract way independent of |
| the platform or the processor architecture and organized in a one-dimensional |
| (linear) array. That array has to be prepared and supplied by the ``CPUIdle`` |
| driver matching the platform the kernel is running on at the initialization |
| time. This allows ``CPUIdle`` governors to be independent of the underlying |
| hardware and to work with any platforms that the Linux kernel can run on. |
| |
| Each idle state present in that array is characterized by two parameters to be |
| taken into account by the governor, the *target residency* and the (worst-case) |
| *exit latency*. The target residency is the minimum time the hardware must |
| spend in the given state, including the time needed to enter it (which may be |
| substantial), in order to save more energy than it would save by entering one of |
| the shallower idle states instead. [The "depth" of an idle state roughly |
| corresponds to the power drawn by the processor in that state.] The exit |
| latency, in turn, is the maximum time it will take a CPU asking the processor |
| hardware to enter an idle state to start executing the first instruction after a |
| wakeup from that state. Note that in general the exit latency also must cover |
| the time needed to enter the given state in case the wakeup occurs when the |
| hardware is entering it and it must be entered completely to be exited in an |
| ordered manner. |
| |
| There are two types of information that can influence the governor's decisions. |
| First of all, the governor knows the time until the closest timer event. That |
| time is known exactly, because the kernel programs timers and it knows exactly |
| when they will trigger, and it is the maximum time the hardware that the given |
| CPU depends on can spend in an idle state, including the time necessary to enter |
| and exit it. However, the CPU may be woken up by a non-timer event at any time |
| (in particular, before the closest timer triggers) and it generally is not known |
| when that may happen. The governor can only see how much time the CPU actually |
| was idle after it has been woken up (that time will be referred to as the *idle |
| duration* from now on) and it can use that information somehow along with the |
| time until the closest timer to estimate the idle duration in future. How the |
| governor uses that information depends on what algorithm is implemented by it |
| and that is the primary reason for having more than one governor in the |
| ``CPUIdle`` subsystem. |
| |
| There are four ``CPUIdle`` governors available, ``menu``, `TEO <teo-gov_>`_, |
| ``ladder`` and ``haltpoll``. Which of them is used by default depends on the |
| configuration of the kernel and in particular on whether or not the scheduler |
| tick can be `stopped by the idle loop <idle-cpus-and-tick_>`_. Available |
| governors can be read from the :file:`available_governors`, and the governor |
| can be changed at runtime. The name of the ``CPUIdle`` governor currently |
| used by the kernel can be read from the :file:`current_governor_ro` or |
| :file:`current_governor` file under :file:`/sys/devices/system/cpu/cpuidle/` |
| in ``sysfs``. |
| |
| Which ``CPUIdle`` driver is used, on the other hand, usually depends on the |
| platform the kernel is running on, but there are platforms with more than one |
| matching driver. For example, there are two drivers that can work with the |
| majority of Intel platforms, ``intel_idle`` and ``acpi_idle``, one with |
| hardcoded idle states information and the other able to read that information |
| from the system's ACPI tables, respectively. Still, even in those cases, the |
| driver chosen at the system initialization time cannot be replaced later, so the |
| decision on which one of them to use has to be made early (on Intel platforms |
| the ``acpi_idle`` driver will be used if ``intel_idle`` is disabled for some |
| reason or if it does not recognize the processor). The name of the ``CPUIdle`` |
| driver currently used by the kernel can be read from the :file:`current_driver` |
| file under :file:`/sys/devices/system/cpu/cpuidle/` in ``sysfs``. |
| |
| |
| .. _idle-cpus-and-tick: |
| |
| Idle CPUs and The Scheduler Tick |
| ================================ |
| |
| The scheduler tick is a timer that triggers periodically in order to implement |
| the time sharing strategy of the CPU scheduler. Of course, if there are |
| multiple runnable tasks assigned to one CPU at the same time, the only way to |
| allow them to make reasonable progress in a given time frame is to make them |
| share the available CPU time. Namely, in rough approximation, each task is |
| given a slice of the CPU time to run its code, subject to the scheduling class, |
| prioritization and so on and when that time slice is used up, the CPU should be |
| switched over to running (the code of) another task. The currently running task |
| may not want to give the CPU away voluntarily, however, and the scheduler tick |
| is there to make the switch happen regardless. That is not the only role of the |
| tick, but it is the primary reason for using it. |
| |
| The scheduler tick is problematic from the CPU idle time management perspective, |
| because it triggers periodically and relatively often (depending on the kernel |
| configuration, the length of the tick period is between 1 ms and 10 ms). |
| Thus, if the tick is allowed to trigger on idle CPUs, it will not make sense |
| for them to ask the hardware to enter idle states with target residencies above |
| the tick period length. Moreover, in that case the idle duration of any CPU |
| will never exceed the tick period length and the energy used for entering and |
| exiting idle states due to the tick wakeups on idle CPUs will be wasted. |
| |
| Fortunately, it is not really necessary to allow the tick to trigger on idle |
| CPUs, because (by definition) they have no tasks to run except for the special |
| "idle" one. In other words, from the CPU scheduler perspective, the only user |
| of the CPU time on them is the idle loop. Since the time of an idle CPU need |
| not be shared between multiple runnable tasks, the primary reason for using the |
| tick goes away if the given CPU is idle. Consequently, it is possible to stop |
| the scheduler tick entirely on idle CPUs in principle, even though that may not |
| always be worth the effort. |
| |
| Whether or not it makes sense to stop the scheduler tick in the idle loop |
| depends on what is expected by the governor. First, if there is another |
| (non-tick) timer due to trigger within the tick range, stopping the tick clearly |
| would be a waste of time, even though the timer hardware may not need to be |
| reprogrammed in that case. Second, if the governor is expecting a non-timer |
| wakeup within the tick range, stopping the tick is not necessary and it may even |
| be harmful. Namely, in that case the governor will select an idle state with |
| the target residency within the time until the expected wakeup, so that state is |
| going to be relatively shallow. The governor really cannot select a deep idle |
| state then, as that would contradict its own expectation of a wakeup in short |
| order. Now, if the wakeup really occurs shortly, stopping the tick would be a |
| waste of time and in this case the timer hardware would need to be reprogrammed, |
| which is expensive. On the other hand, if the tick is stopped and the wakeup |
| does not occur any time soon, the hardware may spend indefinite amount of time |
| in the shallow idle state selected by the governor, which will be a waste of |
| energy. Hence, if the governor is expecting a wakeup of any kind within the |
| tick range, it is better to allow the tick trigger. Otherwise, however, the |
| governor will select a relatively deep idle state, so the tick should be stopped |
| so that it does not wake up the CPU too early. |
| |
| In any case, the governor knows what it is expecting and the decision on whether |
| or not to stop the scheduler tick belongs to it. Still, if the tick has been |
| stopped already (in one of the previous iterations of the loop), it is better |
| to leave it as is and the governor needs to take that into account. |
| |
| The kernel can be configured to disable stopping the scheduler tick in the idle |
| loop altogether. That can be done through the build-time configuration of it |
| (by unsetting the ``CONFIG_NO_HZ_IDLE`` configuration option) or by passing |
| ``nohz=off`` to it in the command line. In both cases, as the stopping of the |
| scheduler tick is disabled, the governor's decisions regarding it are simply |
| ignored by the idle loop code and the tick is never stopped. |
| |
| The systems that run kernels configured to allow the scheduler tick to be |
| stopped on idle CPUs are referred to as *tickless* systems and they are |
| generally regarded as more energy-efficient than the systems running kernels in |
| which the tick cannot be stopped. If the given system is tickless, it will use |
| the ``menu`` governor by default and if it is not tickless, the default |
| ``CPUIdle`` governor on it will be ``ladder``. |
| |
| |
| .. _menu-gov: |
| |
| The ``menu`` Governor |
| ===================== |
| |
| The ``menu`` governor is the default ``CPUIdle`` governor for tickless systems. |
| It is quite complex, but the basic principle of its design is straightforward. |
| Namely, when invoked to select an idle state for a CPU (i.e. an idle state that |
| the CPU will ask the processor hardware to enter), it attempts to predict the |
| idle duration and uses the predicted value for idle state selection. |
| |
| It first obtains the time until the closest timer event with the assumption |
| that the scheduler tick will be stopped. That time, referred to as the *sleep |
| length* in what follows, is the upper bound on the time before the next CPU |
| wakeup. It is used to determine the sleep length range, which in turn is needed |
| to get the sleep length correction factor. |
| |
| The ``menu`` governor maintains two arrays of sleep length correction factors. |
| One of them is used when tasks previously running on the given CPU are waiting |
| for some I/O operations to complete and the other one is used when that is not |
| the case. Each array contains several correction factor values that correspond |
| to different sleep length ranges organized so that each range represented in the |
| array is approximately 10 times wider than the previous one. |
| |
| The correction factor for the given sleep length range (determined before |
| selecting the idle state for the CPU) is updated after the CPU has been woken |
| up and the closer the sleep length is to the observed idle duration, the closer |
| to 1 the correction factor becomes (it must fall between 0 and 1 inclusive). |
| The sleep length is multiplied by the correction factor for the range that it |
| falls into to obtain the first approximation of the predicted idle duration. |
| |
| Next, the governor uses a simple pattern recognition algorithm to refine its |
| idle duration prediction. Namely, it saves the last 8 observed idle duration |
| values and, when predicting the idle duration next time, it computes the average |
| and variance of them. If the variance is small (smaller than 400 square |
| milliseconds) or it is small relative to the average (the average is greater |
| that 6 times the standard deviation), the average is regarded as the "typical |
| interval" value. Otherwise, the longest of the saved observed idle duration |
| values is discarded and the computation is repeated for the remaining ones. |
| Again, if the variance of them is small (in the above sense), the average is |
| taken as the "typical interval" value and so on, until either the "typical |
| interval" is determined or too many data points are disregarded, in which case |
| the "typical interval" is assumed to equal "infinity" (the maximum unsigned |
| integer value). The "typical interval" computed this way is compared with the |
| sleep length multiplied by the correction factor and the minimum of the two is |
| taken as the predicted idle duration. |
| |
| Then, the governor computes an extra latency limit to help "interactive" |
| workloads. It uses the observation that if the exit latency of the selected |
| idle state is comparable with the predicted idle duration, the total time spent |
| in that state probably will be very short and the amount of energy to save by |
| entering it will be relatively small, so likely it is better to avoid the |
| overhead related to entering that state and exiting it. Thus selecting a |
| shallower state is likely to be a better option then. The first approximation |
| of the extra latency limit is the predicted idle duration itself which |
| additionally is divided by a value depending on the number of tasks that |
| previously ran on the given CPU and now they are waiting for I/O operations to |
| complete. The result of that division is compared with the latency limit coming |
| from the power management quality of service, or `PM QoS <cpu-pm-qos_>`_, |
| framework and the minimum of the two is taken as the limit for the idle states' |
| exit latency. |
| |
| Now, the governor is ready to walk the list of idle states and choose one of |
| them. For this purpose, it compares the target residency of each state with |
| the predicted idle duration and the exit latency of it with the computed latency |
| limit. It selects the state with the target residency closest to the predicted |
| idle duration, but still below it, and exit latency that does not exceed the |
| limit. |
| |
| In the final step the governor may still need to refine the idle state selection |
| if it has not decided to `stop the scheduler tick <idle-cpus-and-tick_>`_. That |
| happens if the idle duration predicted by it is less than the tick period and |
| the tick has not been stopped already (in a previous iteration of the idle |
| loop). Then, the sleep length used in the previous computations may not reflect |
| the real time until the closest timer event and if it really is greater than |
| that time, the governor may need to select a shallower state with a suitable |
| target residency. |
| |
| |
| .. _teo-gov: |
| |
| The Timer Events Oriented (TEO) Governor |
| ======================================== |
| |
| The timer events oriented (TEO) governor is an alternative ``CPUIdle`` governor |
| for tickless systems. It follows the same basic strategy as the ``menu`` `one |
| <menu-gov_>`_: it always tries to find the deepest idle state suitable for the |
| given conditions. However, it applies a different approach to that problem. |
| |
| .. kernel-doc:: drivers/cpuidle/governors/teo.c |
| :doc: teo-description |
| |
| .. _idle-states-representation: |
| |
| Representation of Idle States |
| ============================= |
| |
| For the CPU idle time management purposes all of the physical idle states |
| supported by the processor have to be represented as a one-dimensional array of |
| |struct cpuidle_state| objects each allowing an individual (logical) CPU to ask |
| the processor hardware to enter an idle state of certain properties. If there |
| is a hierarchy of units in the processor, one |struct cpuidle_state| object can |
| cover a combination of idle states supported by the units at different levels of |
| the hierarchy. In that case, the `target residency and exit latency parameters |
| of it <idle-loop_>`_, must reflect the properties of the idle state at the |
| deepest level (i.e. the idle state of the unit containing all of the other |
| units). |
| |
| For example, take a processor with two cores in a larger unit referred to as |
| a "module" and suppose that asking the hardware to enter a specific idle state |
| (say "X") at the "core" level by one core will trigger the module to try to |
| enter a specific idle state of its own (say "MX") if the other core is in idle |
| state "X" already. In other words, asking for idle state "X" at the "core" |
| level gives the hardware a license to go as deep as to idle state "MX" at the |
| "module" level, but there is no guarantee that this is going to happen (the core |
| asking for idle state "X" may just end up in that state by itself instead). |
| Then, the target residency of the |struct cpuidle_state| object representing |
| idle state "X" must reflect the minimum time to spend in idle state "MX" of |
| the module (including the time needed to enter it), because that is the minimum |
| time the CPU needs to be idle to save any energy in case the hardware enters |
| that state. Analogously, the exit latency parameter of that object must cover |
| the exit time of idle state "MX" of the module (and usually its entry time too), |
| because that is the maximum delay between a wakeup signal and the time the CPU |
| will start to execute the first new instruction (assuming that both cores in the |
| module will always be ready to execute instructions as soon as the module |
| becomes operational as a whole). |
| |
| There are processors without direct coordination between different levels of the |
| hierarchy of units inside them, however. In those cases asking for an idle |
| state at the "core" level does not automatically affect the "module" level, for |
| example, in any way and the ``CPUIdle`` driver is responsible for the entire |
| handling of the hierarchy. Then, the definition of the idle state objects is |
| entirely up to the driver, but still the physical properties of the idle state |
| that the processor hardware finally goes into must always follow the parameters |
| used by the governor for idle state selection (for instance, the actual exit |
| latency of that idle state must not exceed the exit latency parameter of the |
| idle state object selected by the governor). |
| |
| In addition to the target residency and exit latency idle state parameters |
| discussed above, the objects representing idle states each contain a few other |
| parameters describing the idle state and a pointer to the function to run in |
| order to ask the hardware to enter that state. Also, for each |
| |struct cpuidle_state| object, there is a corresponding |
| :c:type:`struct cpuidle_state_usage <cpuidle_state_usage>` one containing usage |
| statistics of the given idle state. That information is exposed by the kernel |
| via ``sysfs``. |
| |
| For each CPU in the system, there is a :file:`/sys/devices/system/cpu/cpu<N>/cpuidle/` |
| directory in ``sysfs``, where the number ``<N>`` is assigned to the given |
| CPU at the initialization time. That directory contains a set of subdirectories |
| called :file:`state0`, :file:`state1` and so on, up to the number of idle state |
| objects defined for the given CPU minus one. Each of these directories |
| corresponds to one idle state object and the larger the number in its name, the |
| deeper the (effective) idle state represented by it. Each of them contains |
| a number of files (attributes) representing the properties of the idle state |
| object corresponding to it, as follows: |
| |
| ``above`` |
| Total number of times this idle state had been asked for, but the |
| observed idle duration was certainly too short to match its target |
| residency. |
| |
| ``below`` |
| Total number of times this idle state had been asked for, but certainly |
| a deeper idle state would have been a better match for the observed idle |
| duration. |
| |
| ``desc`` |
| Description of the idle state. |
| |
| ``disable`` |
| Whether or not this idle state is disabled. |
| |
| ``default_status`` |
| The default status of this state, "enabled" or "disabled". |
| |
| ``latency`` |
| Exit latency of the idle state in microseconds. |
| |
| ``name`` |
| Name of the idle state. |
| |
| ``power`` |
| Power drawn by hardware in this idle state in milliwatts (if specified, |
| 0 otherwise). |
| |
| ``residency`` |
| Target residency of the idle state in microseconds. |
| |
| ``time`` |
| Total time spent in this idle state by the given CPU (as measured by the |
| kernel) in microseconds. |
| |
| ``usage`` |
| Total number of times the hardware has been asked by the given CPU to |
| enter this idle state. |
| |
| ``rejected`` |
| Total number of times a request to enter this idle state on the given |
| CPU was rejected. |
| |
| The :file:`desc` and :file:`name` files both contain strings. The difference |
| between them is that the name is expected to be more concise, while the |
| description may be longer and it may contain white space or special characters. |
| The other files listed above contain integer numbers. |
| |
| The :file:`disable` attribute is the only writeable one. If it contains 1, the |
| given idle state is disabled for this particular CPU, which means that the |
| governor will never select it for this particular CPU and the ``CPUIdle`` |
| driver will never ask the hardware to enter it for that CPU as a result. |
| However, disabling an idle state for one CPU does not prevent it from being |
| asked for by the other CPUs, so it must be disabled for all of them in order to |
| never be asked for by any of them. [Note that, due to the way the ``ladder`` |
| governor is implemented, disabling an idle state prevents that governor from |
| selecting any idle states deeper than the disabled one too.] |
| |
| If the :file:`disable` attribute contains 0, the given idle state is enabled for |
| this particular CPU, but it still may be disabled for some or all of the other |
| CPUs in the system at the same time. Writing 1 to it causes the idle state to |
| be disabled for this particular CPU and writing 0 to it allows the governor to |
| take it into consideration for the given CPU and the driver to ask for it, |
| unless that state was disabled globally in the driver (in which case it cannot |
| be used at all). |
| |
| The :file:`power` attribute is not defined very well, especially for idle state |
| objects representing combinations of idle states at different levels of the |
| hierarchy of units in the processor, and it generally is hard to obtain idle |
| state power numbers for complex hardware, so :file:`power` often contains 0 (not |
| available) and if it contains a nonzero number, that number may not be very |
| accurate and it should not be relied on for anything meaningful. |
| |
| The number in the :file:`time` file generally may be greater than the total time |
| really spent by the given CPU in the given idle state, because it is measured by |
| the kernel and it may not cover the cases in which the hardware refused to enter |
| this idle state and entered a shallower one instead of it (or even it did not |
| enter any idle state at all). The kernel can only measure the time span between |
| asking the hardware to enter an idle state and the subsequent wakeup of the CPU |
| and it cannot say what really happened in the meantime at the hardware level. |
| Moreover, if the idle state object in question represents a combination of idle |
| states at different levels of the hierarchy of units in the processor, |
| the kernel can never say how deep the hardware went down the hierarchy in any |
| particular case. For these reasons, the only reliable way to find out how |
| much time has been spent by the hardware in different idle states supported by |
| it is to use idle state residency counters in the hardware, if available. |
| |
| Generally, an interrupt received when trying to enter an idle state causes the |
| idle state entry request to be rejected, in which case the ``CPUIdle`` driver |
| may return an error code to indicate that this was the case. The :file:`usage` |
| and :file:`rejected` files report the number of times the given idle state |
| was entered successfully or rejected, respectively. |
| |
| .. _cpu-pm-qos: |
| |
| Power Management Quality of Service for CPUs |
| ============================================ |
| |
| The power management quality of service (PM QoS) framework in the Linux kernel |
| allows kernel code and user space processes to set constraints on various |
| energy-efficiency features of the kernel to prevent performance from dropping |
| below a required level. |
| |
| CPU idle time management can be affected by PM QoS in two ways, through the |
| global CPU latency limit and through the resume latency constraints for |
| individual CPUs. Kernel code (e.g. device drivers) can set both of them with |
| the help of special internal interfaces provided by the PM QoS framework. User |
| space can modify the former by opening the :file:`cpu_dma_latency` special |
| device file under :file:`/dev/` and writing a binary value (interpreted as a |
| signed 32-bit integer) to it. In turn, the resume latency constraint for a CPU |
| can be modified from user space by writing a string (representing a signed |
| 32-bit integer) to the :file:`power/pm_qos_resume_latency_us` file under |
| :file:`/sys/devices/system/cpu/cpu<N>/` in ``sysfs``, where the CPU number |
| ``<N>`` is allocated at the system initialization time. Negative values |
| will be rejected in both cases and, also in both cases, the written integer |
| number will be interpreted as a requested PM QoS constraint in microseconds. |
| |
| The requested value is not automatically applied as a new constraint, however, |
| as it may be less restrictive (greater in this particular case) than another |
| constraint previously requested by someone else. For this reason, the PM QoS |
| framework maintains a list of requests that have been made so far for the |
| global CPU latency limit and for each individual CPU, aggregates them and |
| applies the effective (minimum in this particular case) value as the new |
| constraint. |
| |
| In fact, opening the :file:`cpu_dma_latency` special device file causes a new |
| PM QoS request to be created and added to a global priority list of CPU latency |
| limit requests and the file descriptor coming from the "open" operation |
| represents that request. If that file descriptor is then used for writing, the |
| number written to it will be associated with the PM QoS request represented by |
| it as a new requested limit value. Next, the priority list mechanism will be |
| used to determine the new effective value of the entire list of requests and |
| that effective value will be set as a new CPU latency limit. Thus requesting a |
| new limit value will only change the real limit if the effective "list" value is |
| affected by it, which is the case if it is the minimum of the requested values |
| in the list. |
| |
| The process holding a file descriptor obtained by opening the |
| :file:`cpu_dma_latency` special device file controls the PM QoS request |
| associated with that file descriptor, but it controls this particular PM QoS |
| request only. |
| |
| Closing the :file:`cpu_dma_latency` special device file or, more precisely, the |
| file descriptor obtained while opening it, causes the PM QoS request associated |
| with that file descriptor to be removed from the global priority list of CPU |
| latency limit requests and destroyed. If that happens, the priority list |
| mechanism will be used again, to determine the new effective value for the whole |
| list and that value will become the new limit. |
| |
| In turn, for each CPU there is one resume latency PM QoS request associated with |
| the :file:`power/pm_qos_resume_latency_us` file under |
| :file:`/sys/devices/system/cpu/cpu<N>/` in ``sysfs`` and writing to it causes |
| this single PM QoS request to be updated regardless of which user space |
| process does that. In other words, this PM QoS request is shared by the entire |
| user space, so access to the file associated with it needs to be arbitrated |
| to avoid confusion. [Arguably, the only legitimate use of this mechanism in |
| practice is to pin a process to the CPU in question and let it use the |
| ``sysfs`` interface to control the resume latency constraint for it.] It is |
| still only a request, however. It is an entry in a priority list used to |
| determine the effective value to be set as the resume latency constraint for the |
| CPU in question every time the list of requests is updated this way or another |
| (there may be other requests coming from kernel code in that list). |
| |
| CPU idle time governors are expected to regard the minimum of the global |
| (effective) CPU latency limit and the effective resume latency constraint for |
| the given CPU as the upper limit for the exit latency of the idle states that |
| they are allowed to select for that CPU. They should never select any idle |
| states with exit latency beyond that limit. |
| |
| |
| Idle States Control Via Kernel Command Line |
| =========================================== |
| |
| In addition to the ``sysfs`` interface allowing individual idle states to be |
| `disabled for individual CPUs <idle-states-representation_>`_, there are kernel |
| command line parameters affecting CPU idle time management. |
| |
| The ``cpuidle.off=1`` kernel command line option can be used to disable the |
| CPU idle time management entirely. It does not prevent the idle loop from |
| running on idle CPUs, but it prevents the CPU idle time governors and drivers |
| from being invoked. If it is added to the kernel command line, the idle loop |
| will ask the hardware to enter idle states on idle CPUs via the CPU architecture |
| support code that is expected to provide a default mechanism for this purpose. |
| That default mechanism usually is the least common denominator for all of the |
| processors implementing the architecture (i.e. CPU instruction set) in question, |
| however, so it is rather crude and not very energy-efficient. For this reason, |
| it is not recommended for production use. |
| |
| The ``cpuidle.governor=`` kernel command line switch allows the ``CPUIdle`` |
| governor to use to be specified. It has to be appended with a string matching |
| the name of an available governor (e.g. ``cpuidle.governor=menu``) and that |
| governor will be used instead of the default one. It is possible to force |
| the ``menu`` governor to be used on the systems that use the ``ladder`` governor |
| by default this way, for example. |
| |
| The other kernel command line parameters controlling CPU idle time management |
| described below are only relevant for the *x86* architecture and references |
| to ``intel_idle`` affect Intel processors only. |
| |
| The *x86* architecture support code recognizes three kernel command line |
| options related to CPU idle time management: ``idle=poll``, ``idle=halt``, |
| and ``idle=nomwait``. The first two of them disable the ``acpi_idle`` and |
| ``intel_idle`` drivers altogether, which effectively causes the entire |
| ``CPUIdle`` subsystem to be disabled and makes the idle loop invoke the |
| architecture support code to deal with idle CPUs. How it does that depends on |
| which of the two parameters is added to the kernel command line. In the |
| ``idle=halt`` case, the architecture support code will use the ``HLT`` |
| instruction of the CPUs (which, as a rule, suspends the execution of the program |
| and causes the hardware to attempt to enter the shallowest available idle state) |
| for this purpose, and if ``idle=poll`` is used, idle CPUs will execute a |
| more or less "lightweight" sequence of instructions in a tight loop. [Note |
| that using ``idle=poll`` is somewhat drastic in many cases, as preventing idle |
| CPUs from saving almost any energy at all may not be the only effect of it. |
| For example, on Intel hardware it effectively prevents CPUs from using |
| P-states (see |cpufreq|) that require any number of CPUs in a package to be |
| idle, so it very well may hurt single-thread computations performance as well as |
| energy-efficiency. Thus using it for performance reasons may not be a good idea |
| at all.] |
| |
| The ``idle=nomwait`` option prevents the use of ``MWAIT`` instruction of |
| the CPU to enter idle states. When this option is used, the ``acpi_idle`` |
| driver will use the ``HLT`` instruction instead of ``MWAIT``. On systems |
| running Intel processors, this option disables the ``intel_idle`` driver |
| and forces the use of the ``acpi_idle`` driver instead. Note that in either |
| case, ``acpi_idle`` driver will function only if all the information needed |
| by it is in the system's ACPI tables. |
| |
| In addition to the architecture-level kernel command line options affecting CPU |
| idle time management, there are parameters affecting individual ``CPUIdle`` |
| drivers that can be passed to them via the kernel command line. Specifically, |
| the ``intel_idle.max_cstate=<n>`` and ``processor.max_cstate=<n>`` parameters, |
| where ``<n>`` is an idle state index also used in the name of the given |
| state's directory in ``sysfs`` (see |
| `Representation of Idle States <idle-states-representation_>`_), causes the |
| ``intel_idle`` and ``acpi_idle`` drivers, respectively, to discard all of the |
| idle states deeper than idle state ``<n>``. In that case, they will never ask |
| for any of those idle states or expose them to the governor. [The behavior of |
| the two drivers is different for ``<n>`` equal to ``0``. Adding |
| ``intel_idle.max_cstate=0`` to the kernel command line disables the |
| ``intel_idle`` driver and allows ``acpi_idle`` to be used, whereas |
| ``processor.max_cstate=0`` is equivalent to ``processor.max_cstate=1``. |
| Also, the ``acpi_idle`` driver is part of the ``processor`` kernel module that |
| can be loaded separately and ``max_cstate=<n>`` can be passed to it as a module |
| parameter when it is loaded.] |