mirror of
https://github.com/torvalds/linux.git
synced 2024-12-06 19:11:31 +00:00
0c12018e01
Despite being named with .rst extension, this file doesn't match the ReST standard. It actually causes a crash at Sphinx: Sphinx parallel build error: docutils.utils.SystemMessage: /devel/v4l/docs/Documentation/driver-api/thermal/cpu-idle-cooling.rst:69: (SEVERE/4) Unexpected section title. Add needed markups for it to be properly parsed. Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org> Link: https://lore.kernel.org/r/7640755514809a7b5fe2756f3702613865877dcb.1592203650.git.mchehab+huawei@kernel.org Signed-off-by: Jonathan Corbet <corbet@lwn.net>
200 lines
7.1 KiB
ReStructuredText
200 lines
7.1 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
|
||
|
||
================
|
||
CPU Idle Cooling
|
||
================
|
||
|
||
Situation:
|
||
----------
|
||
|
||
Under certain circumstances a SoC can reach a critical temperature
|
||
limit and is unable to stabilize the temperature around a temperature
|
||
control. When the SoC has to stabilize the temperature, the kernel can
|
||
act on a cooling device to mitigate the dissipated power. When the
|
||
critical temperature is reached, a decision must be taken to reduce
|
||
the temperature, that, in turn impacts performance.
|
||
|
||
Another situation is when the silicon temperature continues to
|
||
increase even after the dynamic leakage is reduced to its minimum by
|
||
clock gating the component. This runaway phenomenon can continue due
|
||
to the static leakage. The only solution is to power down the
|
||
component, thus dropping the dynamic and static leakage that will
|
||
allow the component to cool down.
|
||
|
||
Last but not least, the system can ask for a specific power budget but
|
||
because of the OPP density, we can only choose an OPP with a power
|
||
budget lower than the requested one and under-utilize the CPU, thus
|
||
losing performance. In other words, one OPP under-utilizes the CPU
|
||
with a power less than the requested power budget and the next OPP
|
||
exceeds the power budget. An intermediate OPP could have been used if
|
||
it were present.
|
||
|
||
Solutions:
|
||
----------
|
||
|
||
If we can remove the static and the dynamic leakage for a specific
|
||
duration in a controlled period, the SoC temperature will
|
||
decrease. Acting on the idle state duration or the idle cycle
|
||
injection period, we can mitigate the temperature by modulating the
|
||
power budget.
|
||
|
||
The Operating Performance Point (OPP) density has a great influence on
|
||
the control precision of cpufreq, however different vendors have a
|
||
plethora of OPP density, and some have large power gap between OPPs,
|
||
that will result in loss of performance during thermal control and
|
||
loss of power in other scenarios.
|
||
|
||
At a specific OPP, we can assume that injecting idle cycle on all CPUs
|
||
belong to the same cluster, with a duration greater than the cluster
|
||
idle state target residency, we lead to dropping the static and the
|
||
dynamic leakage for this period (modulo the energy needed to enter
|
||
this state). So the sustainable power with idle cycles has a linear
|
||
relation with the OPP’s sustainable power and can be computed with a
|
||
coefficient similar to::
|
||
|
||
Power(IdleCycle) = Coef x Power(OPP)
|
||
|
||
Idle Injection:
|
||
---------------
|
||
|
||
The base concept of the idle injection is to force the CPU to go to an
|
||
idle state for a specified time each control cycle, it provides
|
||
another way to control CPU power and heat in addition to
|
||
cpufreq. Ideally, if all CPUs belonging to the same cluster, inject
|
||
their idle cycles synchronously, the cluster can reach its power down
|
||
state with a minimum power consumption and reduce the static leakage
|
||
to almost zero. However, these idle cycles injection will add extra
|
||
latencies as the CPUs will have to wakeup from a deep sleep state.
|
||
|
||
We use a fixed duration of idle injection that gives an acceptable
|
||
performance penalty and a fixed latency. Mitigation can be increased
|
||
or decreased by modulating the duty cycle of the idle injection.
|
||
|
||
::
|
||
|
||
^
|
||
|
|
||
|
|
||
|------- -------
|
||
|_______|_______________________|_______|___________
|
||
|
||
<------>
|
||
idle <---------------------->
|
||
running
|
||
|
||
<----------------------------->
|
||
duty cycle 25%
|
||
|
||
|
||
The implementation of the cooling device bases the number of states on
|
||
the duty cycle percentage. When no mitigation is happening the cooling
|
||
device state is zero, meaning the duty cycle is 0%.
|
||
|
||
When the mitigation begins, depending on the governor's policy, a
|
||
starting state is selected. With a fixed idle duration and the duty
|
||
cycle (aka the cooling device state), the running duration can be
|
||
computed.
|
||
|
||
The governor will change the cooling device state thus the duty cycle
|
||
and this variation will modulate the cooling effect.
|
||
|
||
::
|
||
|
||
^
|
||
|
|
||
|
|
||
|------- -------
|
||
|_______|_______________|_______|___________
|
||
|
||
<------>
|
||
idle <-------------->
|
||
running
|
||
|
||
<--------------------->
|
||
duty cycle 33%
|
||
|
||
|
||
^
|
||
|
|
||
|
|
||
|------- -------
|
||
|_______|_______|_______|___________
|
||
|
||
<------>
|
||
idle <------>
|
||
running
|
||
|
||
<------------->
|
||
duty cycle 50%
|
||
|
||
The idle injection duration value must comply with the constraints:
|
||
|
||
- It is less than or equal to the latency we tolerate when the
|
||
mitigation begins. It is platform dependent and will depend on the
|
||
user experience, reactivity vs performance trade off we want. This
|
||
value should be specified.
|
||
|
||
- It is greater than the idle state’s target residency we want to go
|
||
for thermal mitigation, otherwise we end up consuming more energy.
|
||
|
||
Power considerations
|
||
--------------------
|
||
|
||
When we reach the thermal trip point, we have to sustain a specified
|
||
power for a specific temperature but at this time we consume::
|
||
|
||
Power = Capacitance x Voltage^2 x Frequency x Utilisation
|
||
|
||
... which is more than the sustainable power (or there is something
|
||
wrong in the system setup). The ‘Capacitance’ and ‘Utilisation’ are a
|
||
fixed value, ‘Voltage’ and the ‘Frequency’ are fixed artificially
|
||
because we don’t want to change the OPP. We can group the
|
||
‘Capacitance’ and the ‘Utilisation’ into a single term which is the
|
||
‘Dynamic Power Coefficient (Cdyn)’ Simplifying the above, we have::
|
||
|
||
Pdyn = Cdyn x Voltage^2 x Frequency
|
||
|
||
The power allocator governor will ask us somehow to reduce our power
|
||
in order to target the sustainable power defined in the device
|
||
tree. So with the idle injection mechanism, we want an average power
|
||
(Ptarget) resulting in an amount of time running at full power on a
|
||
specific OPP and idle another amount of time. That could be put in a
|
||
equation::
|
||
|
||
P(opp)target = ((Trunning x (P(opp)running) + (Tidle x P(opp)idle)) /
|
||
(Trunning + Tidle)
|
||
|
||
...
|
||
|
||
Tidle = Trunning x ((P(opp)running / P(opp)target) - 1)
|
||
|
||
At this point if we know the running period for the CPU, that gives us
|
||
the idle injection we need. Alternatively if we have the idle
|
||
injection duration, we can compute the running duration with::
|
||
|
||
Trunning = Tidle / ((P(opp)running / P(opp)target) - 1)
|
||
|
||
Practically, if the running power is less than the targeted power, we
|
||
end up with a negative time value, so obviously the equation usage is
|
||
bound to a power reduction, hence a higher OPP is needed to have the
|
||
running power greater than the targeted power.
|
||
|
||
However, in this demonstration we ignore three aspects:
|
||
|
||
* The static leakage is not defined here, we can introduce it in the
|
||
equation but assuming it will be zero most of the time as it is
|
||
difficult to get the values from the SoC vendors
|
||
|
||
* The idle state wake up latency (or entry + exit latency) is not
|
||
taken into account, it must be added in the equation in order to
|
||
rigorously compute the idle injection
|
||
|
||
* The injected idle duration must be greater than the idle state
|
||
target residency, otherwise we end up consuming more energy and
|
||
potentially invert the mitigation effect
|
||
|
||
So the final equation is::
|
||
|
||
Trunning = (Tidle - Twakeup ) x
|
||
(((P(opp)dyn + P(opp)static ) - P(opp)target) / P(opp)target )
|