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