1.. SPDX-License-Identifier: GPL-2.0 2 3================ 4CPU Idle Cooling 5================ 6 7Situation: 8---------- 9 10Under certain circumstances a SoC can reach a critical temperature 11limit and is unable to stabilize the temperature around a temperature 12control. When the SoC has to stabilize the temperature, the kernel can 13act on a cooling device to mitigate the dissipated power. When the 14critical temperature is reached, a decision must be taken to reduce 15the temperature, that, in turn impacts performance. 16 17Another situation is when the silicon temperature continues to 18increase even after the dynamic leakage is reduced to its minimum by 19clock gating the component. This runaway phenomenon can continue due 20to the static leakage. The only solution is to power down the 21component, thus dropping the dynamic and static leakage that will 22allow the component to cool down. 23 24Last but not least, the system can ask for a specific power budget but 25because of the OPP density, we can only choose an OPP with a power 26budget lower than the requested one and under-utilize the CPU, thus 27losing performance. In other words, one OPP under-utilizes the CPU 28with a power less than the requested power budget and the next OPP 29exceeds the power budget. An intermediate OPP could have been used if 30it were present. 31 32Solutions: 33---------- 34 35If we can remove the static and the dynamic leakage for a specific 36duration in a controlled period, the SoC temperature will 37decrease. Acting on the idle state duration or the idle cycle 38injection period, we can mitigate the temperature by modulating the 39power budget. 40 41The Operating Performance Point (OPP) density has a great influence on 42the control precision of cpufreq, however different vendors have a 43plethora of OPP density, and some have large power gap between OPPs, 44that will result in loss of performance during thermal control and 45loss of power in other scenarios. 46 47At a specific OPP, we can assume that injecting idle cycle on all CPUs 48belong to the same cluster, with a duration greater than the cluster 49idle state target residency, we lead to dropping the static and the 50dynamic leakage for this period (modulo the energy needed to enter 51this state). So the sustainable power with idle cycles has a linear 52relation with the OPP’s sustainable power and can be computed with a 53coefficient similar to:: 54 55 Power(IdleCycle) = Coef x Power(OPP) 56 57Idle Injection: 58--------------- 59 60The base concept of the idle injection is to force the CPU to go to an 61idle state for a specified time each control cycle, it provides 62another way to control CPU power and heat in addition to 63cpufreq. Ideally, if all CPUs belonging to the same cluster, inject 64their idle cycles synchronously, the cluster can reach its power down 65state with a minimum power consumption and reduce the static leakage 66to almost zero. However, these idle cycles injection will add extra 67latencies as the CPUs will have to wakeup from a deep sleep state. 68 69We use a fixed duration of idle injection that gives an acceptable 70performance penalty and a fixed latency. Mitigation can be increased 71or decreased by modulating the duty cycle of the idle injection. 72 73:: 74 75 ^ 76 | 77 | 78 |------- ------- 79 |_______|_______________________|_______|___________ 80 81 <------> 82 idle <----------------------> 83 running 84 85 <-----------------------------> 86 duty cycle 25% 87 88 89The implementation of the cooling device bases the number of states on 90the duty cycle percentage. When no mitigation is happening the cooling 91device state is zero, meaning the duty cycle is 0%. 92 93When the mitigation begins, depending on the governor's policy, a 94starting state is selected. With a fixed idle duration and the duty 95cycle (aka the cooling device state), the running duration can be 96computed. 97 98The governor will change the cooling device state thus the duty cycle 99and this variation will modulate the cooling effect. 100 101:: 102 103 ^ 104 | 105 | 106 |------- ------- 107 |_______|_______________|_______|___________ 108 109 <------> 110 idle <--------------> 111 running 112 113 <---------------------> 114 duty cycle 33% 115 116 117 ^ 118 | 119 | 120 |------- ------- 121 |_______|_______|_______|___________ 122 123 <------> 124 idle <------> 125 running 126 127 <-------------> 128 duty cycle 50% 129 130The idle injection duration value must comply with the constraints: 131 132- It is less than or equal to the latency we tolerate when the 133 mitigation begins. It is platform dependent and will depend on the 134 user experience, reactivity vs performance trade off we want. This 135 value should be specified. 136 137- It is greater than the idle state’s target residency we want to go 138 for thermal mitigation, otherwise we end up consuming more energy. 139 140Power considerations 141-------------------- 142 143When we reach the thermal trip point, we have to sustain a specified 144power for a specific temperature but at this time we consume:: 145 146 Power = Capacitance x Voltage^2 x Frequency x Utilisation 147 148... which is more than the sustainable power (or there is something 149wrong in the system setup). The ‘Capacitance’ and ‘Utilisation’ are a 150fixed value, ‘Voltage’ and the ‘Frequency’ are fixed artificially 151because we don’t want to change the OPP. We can group the 152‘Capacitance’ and the ‘Utilisation’ into a single term which is the 153‘Dynamic Power Coefficient (Cdyn)’ Simplifying the above, we have:: 154 155 Pdyn = Cdyn x Voltage^2 x Frequency 156 157The power allocator governor will ask us somehow to reduce our power 158in order to target the sustainable power defined in the device 159tree. So with the idle injection mechanism, we want an average power 160(Ptarget) resulting in an amount of time running at full power on a 161specific OPP and idle another amount of time. That could be put in a 162equation:: 163 164 P(opp)target = ((Trunning x (P(opp)running) + (Tidle x P(opp)idle)) / 165 (Trunning + Tidle) 166 167 ... 168 169 Tidle = Trunning x ((P(opp)running / P(opp)target) - 1) 170 171At this point if we know the running period for the CPU, that gives us 172the idle injection we need. Alternatively if we have the idle 173injection duration, we can compute the running duration with:: 174 175 Trunning = Tidle / ((P(opp)running / P(opp)target) - 1) 176 177Practically, if the running power is less than the targeted power, we 178end up with a negative time value, so obviously the equation usage is 179bound to a power reduction, hence a higher OPP is needed to have the 180running power greater than the targeted power. 181 182However, in this demonstration we ignore three aspects: 183 184 * The static leakage is not defined here, we can introduce it in the 185 equation but assuming it will be zero most of the time as it is 186 difficult to get the values from the SoC vendors 187 188 * The idle state wake up latency (or entry + exit latency) is not 189 taken into account, it must be added in the equation in order to 190 rigorously compute the idle injection 191 192 * The injected idle duration must be greater than the idle state 193 target residency, otherwise we end up consuming more energy and 194 potentially invert the mitigation effect 195 196So the final equation is:: 197 198 Trunning = (Tidle - Twakeup ) x 199 (((P(opp)dyn + P(opp)static ) - P(opp)target) / P(opp)target ) 200