| Message ID | 20191211224347.1001-1-daniel.lezcano@linaro.org (mailing list archive) |
|---|---|
| State | Accepted |
| Delegated to: | Daniel Lezcano |
| Headers | show |
| Series | [V5,1/3] thermal/drivers/cpu_cooling: Add idle cooling device documentation | expand |
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@linaro.org> > Acked-by: Viresh Kumar <viresh.kumar@linaro.org> Reviewed-by: Martin Kepplinger <martin.kepplinger@puri.sm> 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 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 ) >
On 11/12/2019 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@linaro.org> > Acked-by: Viresh Kumar <viresh.kumar@linaro.org> > --- I will apply the series if nobody else has comments. Thanks
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 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 )