Re: [PATCH V5 1/3] thermal/drivers/cpu_cooling: Add idle cooling device documentation

[Martin Kepplinger]

On 11.12.19 23:43, Daniel Lezcano wrote:
> Provide some documentation for the idle injection cooling effect in
> order to let people to understand the rational of the approach for the
> idle injection CPU cooling device.
> 
> Signed-off-by: Daniel Lezcano <daniel.lezcano@xxxxxxxxxx>
> Acked-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>

Reviewed-by: Martin Kepplinger <martin.kepplinger@xxxxxxx>

thanks

> ---
>   V4:
>     - Fixed typos, replaced 'period' per 'duty cycles', clarified some
>       wording (Amit Kucheria)
> ---
>  .../driver-api/thermal/cpu-idle-cooling.rst   | 189 ++++++++++++++++++
>  1 file changed, 189 insertions(+)
>  create mode 100644 Documentation/driver-api/thermal/cpu-idle-cooling.rst
> 
> diff --git a/Documentation/driver-api/thermal/cpu-idle-cooling.rst b/Documentation/driver-api/thermal/cpu-idle-cooling.rst
> new file mode 100644
> index 000000000000..13d7fe4e8de8
> --- /dev/null
> +++ b/Documentation/driver-api/thermal/cpu-idle-cooling.rst
> @@ -0,0 +1,189 @@
> +
> +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 OPPâs 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 stateâs 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 âCapacitanceâ and âUtilisationâ are a
> +fixed value, âVoltageâ and the âFrequencyâ are fixed artificially
> +because we donât want to change the OPP. We can group the
> +âCapacitanceâ and the âUtilisationâ into a single term which is the
> +âDynamic Power Coefficient (Cdyn)â 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 )
>