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|
.. 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.
First, it does not use sleep length correction factors, but instead it attempts
to correlate the observed idle duration values with the available idle states
and use that information to pick up the idle state that is most likely to
"match" the upcoming CPU idle interval. Second, it does not take the tasks
that were running on the given CPU in the past and are waiting on some I/O
operations to complete now at all (there is no guarantee that they will run on
the same CPU when they become runnable again) and the pattern detection code in
it avoids taking timer wakeups into account. It also only uses idle duration
values less than the current time till the closest timer (with the scheduler
tick excluded) for that purpose.
Like in the ``menu`` governor `case <menu-gov_>`_, the first step is to obtain
the *sleep length*, which is the time until the closest timer event with the
assumption that the scheduler tick will be stopped (that also is the upper bound
on the time until the next CPU wakeup). That value is then used to preselect an
idle state on the basis of three metrics maintained for each idle state provided
by the ``CPUIdle`` driver: ``hits``, ``misses`` and ``early_hits``.
The ``hits`` and ``misses`` metrics measure the likelihood that a given idle
state will "match" the observed (post-wakeup) idle duration if it "matches" the
sleep length. They both are subject to decay (after a CPU wakeup) every time
the target residency of the idle state corresponding to them is less than or
equal to the sleep length and the target residency of the next idle state is
greater than the sleep length (that is, when the idle state corresponding to
them "matches" the sleep length). The ``hits`` metric is increased if the
former condition is satisfied and the target residency of the given idle state
is less than or equal to the observed idle duration and the target residency of
the next idle state is greater than the observed idle duration at the same time
(that is, it is increased when the given idle state "matches" both the sleep
length and the observed idle duration). In turn, the ``misses`` metric is
increased when the given idle state "matches" the sleep length only and the
observed idle duration is too short for its target residency.
The ``early_hits`` metric measures the likelihood that a given idle state will
"match" the observed (post-wakeup) idle duration if it does not "match" the
sleep length. It is subject to decay on every CPU wakeup and it is increased
when the idle state corresponding to it "matches" the observed (post-wakeup)
idle duration and the target residency of the next idle state is less than or
equal to the sleep length (i.e. the idle state "matching" the sleep length is
deeper than the given one).
The governor walks the list of idle states provided by the ``CPUIdle`` driver
and finds the last (deepest) one with the target residency less than or equal
to the sleep length. Then, the ``hits`` and ``misses`` metrics of that idle
state are compared with each other and it is preselected if the ``hits`` one is
greater (which means that that idle state is likely to "match" the observed idle
duration after CPU wakeup). If the ``misses`` one is greater, the governor
preselects the shallower idle state with the maximum ``early_hits`` metric
(or if there are multiple shallower idle states with equal ``early_hits``
metric which also is the maximum, the shallowest of them will be preselected).
[If there is a wakeup latency constraint coming from the `PM QoS framework
<cpu-pm-qos_>`_ which is hit before reaching the deepest idle state with the
target residency within the sleep length, the deepest idle state with the exit
latency within the constraint is preselected without consulting the ``hits``,
``misses`` and ``early_hits`` metrics.]
Next, the governor takes several idle duration values observed most recently
into consideration and if at least a half of them are greater than or equal to
the target residency of the preselected idle state, that idle state becomes the
final candidate to ask for. Otherwise, the average of the most recent idle
duration values below the target residency of the preselected idle state is
computed and the governor walks the idle states shallower than the preselected
one and finds the deepest of them with the target residency within that average.
That idle state is then taken as the final candidate to ask for.
Still, at this point the governor may need to refine the idle state selection if
it has not decided to `stop the scheduler tick <idle-cpus-and-tick_>`_. That
generally happens if the target residency of the idle state selected so far is
less than the tick period and the tick has not been stopped already (in a
previous iteration of the idle loop). Then, like in the ``menu`` governor
`case <menu-gov_>`_, 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, a shallower state with a suitable target residency may need to
be selected.
.. _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 ope
|