Lines Matching refs:idle
21 memory or executed. Those states are the *idle* states of the processor.
23 Since part of the processor hardware is not used in idle states, entering them
27 CPU idle time management is an energy-efficiency feature concerned about using
28 the idle states of processors for this purpose.
33 CPU idle time management operates on CPUs as seen by the *CPU scheduler* (that
44 enter an idle state, that applies to the processor as a whole.
52 enter an idle state, that applies to the core that asked for it in the first
56 except for one have been put into idle states at the "core level" and the
57 remaining core asks the processor to enter an idle state, that may trigger it
58 to put the whole larger unit into an idle state which also will affect the
68 sequence of instructions. Then, the hardware threads are CPUs from the CPU idle
69 time management perspective and if the processor is asked to enter an idle state
72 core also have asked the processor to enter an idle state. In that situation,
73 the core may be put into an idle state individually or a larger unit containing
74 it may be put into an idle state as a whole (if the other cores within the
75 larger unit are in idle states already).
81 *idle* by the Linux kernel when there are no tasks to run on them except for the
82 special "idle" task.
101 The special "idle" task becomes runnable if there are no other runnable tasks
102 assigned to the given CPU and the CPU is then regarded as idle. In other words,
103 in Linux idle CPUs run the code of the "idle" task called *the idle loop*. That
104 code may cause the processor to be put into one of its idle states, if they are
106 idle states, or there is not enough time to spend in an idle state before the
108 available idle states from being used, the CPU will simply execute more or less
117 The idle loop code takes two major steps in every iteration of it. First, it
119 idle time management subsystem called ``CPUIdle`` to select an idle state for
122 processor hardware to enter the idle state selected by the governor.
124 The role of the governor is to find an idle state most suitable for the
125 conditions at hand. For this purpose, idle states that the hardware can be
133 Each idle state present in that array is characterized by two parameters to be
138 the shallower idle states instead. [The "depth" of an idle state roughly
141 hardware to enter an idle state to start executing the first instruction after a
151 CPU depends on can spend in an idle state, including the time necessary to enter
155 was idle after it has been woken up (that time will be referred to as the *idle
157 time until the closest timer to estimate the idle duration in future. How the
165 tick can be `stopped by the idle loop <idle-cpus-and-tick_>`_. Available
176 hardcoded idle states information and the other able to read that information
203 The scheduler tick is problematic from the CPU idle time management perspective,
206 Thus, if the tick is allowed to trigger on idle CPUs, it will not make sense
207 for them to ask the hardware to enter idle states with target residencies above
208 the tick period length. Moreover, in that case the idle duration of any CPU
210 exiting idle states due to the tick wakeups on idle CPUs will be wasted.
212 Fortunately, it is not really necessary to allow the tick to trigger on idle
214 "idle" one. In other words, from the CPU scheduler perspective, the only user
215 of the CPU time on them is the idle loop. Since the time of an idle CPU need
217 tick goes away if the given CPU is idle. Consequently, it is possible to stop
218 the scheduler tick entirely on idle CPUs in principle, even though that may not
221 Whether or not it makes sense to stop the scheduler tick in the idle loop
227 be harmful. Namely, in that case the governor will select an idle state with
229 going to be relatively shallow. The governor really cannot select a deep idle
235 in the shallow idle state selected by the governor, which will be a waste of
238 governor will select a relatively deep idle state, so the tick should be stopped
246 The kernel can be configured to disable stopping the scheduler tick in the idle
251 ignored by the idle loop code and the tick is never stopped.
254 stopped on idle CPUs are referred to as *tickless* systems and they are
268 Namely, when invoked to select an idle state for a CPU (i.e. an idle state that
270 idle duration and uses the predicted value for idle state selection.
286 selecting the idle state for the CPU) is updated after the CPU has been woken
287 up and the closer the sleep length is to the observed idle duration, the closer
290 falls into to obtain the first approximation of the predicted idle duration.
293 idle duration prediction. Namely, it saves the last 8 observed idle duration
294 values and, when predicting the idle duration next time, it computes the average
298 interval" value. Otherwise, the longest of the saved observed idle duration
306 taken as the predicted idle duration.
310 idle state is comparable with the predicted idle duration, the total time spent
315 of the extra latency limit is the predicted idle duration itself which
320 framework and the minimum of the two is taken as the limit for the idle states'
323 Now, the governor is ready to walk the list of idle states and choose one of
325 the predicted idle duration and the exit latency of it with the computed latency
327 idle duration, but still below it, and exit latency that does not exceed the
330 In the final step the governor may still need to refine the idle state selection
331 if it has not decided to `stop the scheduler tick <idle-cpus-and-tick_>`_. That
332 happens if the idle duration predicted by it is less than the tick period and
333 the tick has not been stopped already (in a previous iteration of the idle
347 <menu-gov_>`_: it always tries to find the deepest idle state suitable for the
358 For the CPU idle time management purposes all of the physical idle states
361 the processor hardware to enter an idle state of certain properties. If there
363 cover a combination of idle states supported by the units at different levels of
365 of it <idle-loop_>`_, must reflect the properties of the idle state at the
366 deepest level (i.e. the idle state of the unit containing all of the other
370 a "module" and suppose that asking the hardware to enter a specific idle state
372 enter a specific idle state of its own (say "MX") if the other core is in idle
373 state "X" already. In other words, asking for idle state "X" at the "core"
374 level gives the hardware a license to go as deep as to idle state "MX" at the
376 asking for idle state "X" may just end up in that state by itself instead).
378 idle state "X" must reflect the minimum time to spend in idle state "MX" of
380 time the CPU needs to be idle to save any energy in case the hardware enters
382 the exit time of idle state "MX" of the module (and usually its entry time too),
389 hierarchy of units inside them, however. In those cases asking for an idle
392 handling of the hierarchy. Then, the definition of the idle state objects is
393 entirely up to the driver, but still the physical properties of the idle state
395 used by the governor for idle state selection (for instance, the actual exit
396 latency of that idle state must not exceed the exit latency parameter of the
397 idle state object selected by the governor).
399 In addition to the target residency and exit latency idle state parameters
400 discussed above, the objects representing idle states each contain a few other
401 parameters describing the idle state and a pointer to the function to run in
405 statistics of the given idle state. That information is exposed by the kernel
411 called :file:`state0`, :file:`state1` and so on, up to the number of idle state
413 corresponds to one idle state object and the larger the number in its name, the
414 deeper the (effective) idle state represented by it. Each of them contains
415 a number of files (attributes) representing the properties of the idle state
419 Total number of times this idle state had been asked for, but the
420 observed idle duration was certainly too short to match its target
424 Total number of times this idle state had been asked for, but certainly
425 a deeper idle state would have been a better match for the observed idle
429 Description of the idle state.
432 Whether or not this idle state is disabled.
438 Exit latency of the idle state in microseconds.
441 Name of the idle state.
444 Power drawn by hardware in this idle state in milliwatts (if specified,
448 Target residency of the idle state in microseconds.
451 Total time spent in this idle state by the given CPU (as measured by the
456 enter this idle state.
459 Total number of times a request to enter this idle state on the given
468 given idle state is disabled for this particular CPU, which means that the
471 However, disabling an idle state for one CPU does not prevent it from being
474 governor is implemented, disabling an idle state prevents that governor from
475 selecting any idle states deeper than the disabled one too.]
477 If the :file:`disable` attribute contains 0, the given idle state is enabled for
479 CPUs in the system at the same time. Writing 1 to it causes the idle state to
485 The :file:`power` attribute is not defined very well, especially for idle state
486 objects representing combinations of idle states at different levels of the
487 hierarchy of units in the processor, and it generally is hard to obtain idle
493 really spent by the given CPU in the given idle state, because it is measured by
495 this idle state and entered a shallower one instead of it (or even it did not
496 enter any idle state at all). The kernel can only measure the time span between
497 asking the hardware to enter an idle state and the subsequent wakeup of the CPU
499 Moreover, if the idle state object in question represents a combination of idle
503 much time has been spent by the hardware in different idle states supported by
504 it is to use idle state residency counters in the hardware, if available.
506 Generally, an interrupt received when trying to enter an idle state causes the
507 idle state entry request to be rejected, in which case the ``CPUIdle`` driver
509 and :file:`rejected` files report the number of times the given idle state
522 CPU idle time management can be affected by PM QoS in two ways, through the
582 CPU idle time governors are expected to regard the minimum of the global
584 the given CPU as the upper limit for the exit latency of the idle states that
585 they are allowed to select for that CPU. They should never select any idle
592 In addition to the ``sysfs`` interface allowing individual idle states to be
593 `disabled for individual CPUs <idle-states-representation_>`_, there are kernel
594 command line parameters affecting CPU idle time management.
597 CPU idle time management entirely. It does not prevent the idle loop from
598 running on idle CPUs, but it prevents the CPU idle time governors and drivers
599 from being invoked. If it is added to the kernel command line, the idle loop
600 will ask the hardware to enter idle states on idle CPUs via the CPU architecture
614 The other kernel command line parameters controlling CPU idle time management
619 options related to CPU idle time management: ``idle=poll``, ``idle=halt``,
620 and ``idle=nomwait``. The first two of them disable the ``acpi_idle`` and
622 ``CPUIdle`` subsystem to be disabled and makes the idle loop invoke the
623 architecture support code to deal with idle CPUs. How it does that depends on
625 ``idle=halt`` case, the architecture support code will use the ``HLT``
627 and causes the hardware to attempt to enter the shallowest available idle state)
628 for this purpose, and if ``idle=poll`` is used, idle CPUs will execute a
630 that using ``idle=poll`` is somewhat drastic in many cases, as preventing idle
634 idle, so it very well may hurt single-thread computations performance as well as
638 The ``idle=nomwait`` option disables the ``intel_idle`` driver and causes
641 ``MWAIT`` instruction of the CPUs to ask the hardware to enter idle states.
644 idle time management, there are parameters affecting individual ``CPUIdle``
647 where ``<n>`` is an idle state index also used in the name of the given
649 `Representation of Idle States <idle-states-representation_>`_), causes the
651 idle states deeper than idle state ``<n>``. In that case, they will never ask
652 for any of those idle states or expose them to the governor. [The behavior of