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From: Daniel Lezcano <>
	Jonathan Corbet <>,
	"open list:DOCUMENTATION" <>
Subject: [PATCH v3 5/7] thermal/drivers/cpu_cooling: Add idle cooling device documentation
Date: Thu,  5 Apr 2018 18:16:42 +0200	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

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 <>
 Documentation/thermal/cpu-idle-cooling.txt | 166 +++++++++++++++++++++++++++++
 1 file changed, 166 insertions(+)
 create mode 100644 Documentation/thermal/cpu-idle-cooling.txt

diff --git a/Documentation/thermal/cpu-idle-cooling.txt b/Documentation/thermal/cpu-idle-cooling.txt
new file mode 100644
index 0000000..457cd99
--- /dev/null
+++ b/Documentation/thermal/cpu-idle-cooling.txt
@@ -0,0 +1,166 @@
+Under certain circumstances a SoC can reach the maximum temperature
+limit or 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
+maximum temperature is reached and to prevent a reboot or a shutdown,
+a decision must be taken to reduce the temperature under the critical
+threshold, that impacts the performance.
+Another situation is when the silicon reaches a certain temperature
+which continues to increase even if the dynamic leakage is reduced to
+its minimum by clock gating the component. The runaway phenomena will
+continue with the static leakage and only powering down the component,
+thus dropping the dynamic and static leakage 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 underuse the CPU, thus losing
+performances. In other words, one OPP under uses the CPU with a power
+lesser than the power budget and the next OPP exceed the power budget,
+an intermediate OPP could have been used if it were present.
+If we can remove the static and the dynamic leakage for a specific
+duration in a controlled period, the SoC temperature will
+decrease. Acting at 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 scenes.
+At a specific OPP, we can assume injecting idle cycle on all CPUs,
+belonging to the same cluster, with a duration greater than the
+cluster idle state target residency, we drop 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 cycle synchronously, the cluster can reach its power down
+state with a minimum power consumption and static leakage
+drop. However, these idle cycles injection will add extra latencies as
+the CPUs will have to wakeup from a deep sleep state.
+     ^
+     |
+     |
+     |-------       -------       -------
+     |_______|_____|_______|_____|_______|___________
+      <----->
+       idle  <---->
+              running
+With the fixed idle injection duration, we can give a value which is
+an acceptable performance drop off or latency when we reach a specific
+temperature and we begin to mitigate by varying the Idle injection
+The mitigation begins with a maximum period value which decrease when
+more cooling effect is requested. When the period duration is equal to
+the idle duration, then we are in a situation the platform can’t
+dissipate the heat enough and the mitigation fails. In this case the
+situation is considered critical and there is nothing to do. The idle
+injection duration must be changed by configuration and until we reach
+the cooling effect, otherwise an additionnal cooling device must be
+used or ultimately decrease the SoC performance by dropping the
+highest OPP point of the SoC.
+The idle injection duration value must comply with the constraints:
+- It is lesser 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.
+Minimum period
+The idle injection duration being fixed, it is obvious the minimum
+period can’t be lesser than that, otherwise we will be scheduling the
+idle injection task right before the idle injection duration is
+complete, so waking up the CPU to put it asleep again.
+Maximum period
+The maximum period is the initial period when the mitigation
+begins. Theoretically when we reach the thermal trip point, we have to
+sustain a specified power for specific temperature but at this time we
+ Power = Capacitance x Voltage^2 x Frequency x Utilisation
+... which is more than the sustainable power (or there is something
+wrong on 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 IPA 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 on
+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 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 lesses 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 )

  parent reply	other threads:[~2018-04-05 16:16 UTC|newest]

Thread overview: 46+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-04-05 16:16 [PATCH v3 0/7] CPU cooling device new strategies Daniel Lezcano
2018-04-05 16:16 ` [PATCH v3 1/7] thermal/drivers/cpu_cooling: Fixup the header and copyright Daniel Lezcano
     [not found]   ` <20180411061514.GL7671@vireshk-i7>
2018-04-11  8:56     ` Daniel Lezcano
2018-04-05 16:16 ` [PATCH v3 2/7] thermal/drivers/cpu_cooling: Add Software Package Data Exchange (SPDX) Daniel Lezcano
2018-04-05 16:16 ` [PATCH v3 3/7] thermal/drivers/cpu_cooling: Remove pointless field Daniel Lezcano
2018-04-05 16:16 ` [PATCH v3 4/7] thermal/drivers/Kconfig: Convert the CPU cooling device to a choice Daniel Lezcano
     [not found]   ` <20180411061851.GM7671@vireshk-i7>
2018-04-11  8:58     ` Daniel Lezcano
2018-04-05 16:16 ` Daniel Lezcano [this message]
2018-04-05 16:16 ` [PATCH v3 6/7] thermal/drivers/cpu_cooling: Introduce the cpu idle cooling driver Daniel Lezcano
2018-04-11  8:51   ` Viresh Kumar
2018-04-11  9:29     ` Daniel Lezcano
2018-04-13 11:23   ` Sudeep Holla
2018-04-13 11:47     ` Daniel Lezcano
2018-04-16  7:37       ` Viresh Kumar
2018-04-16  7:44         ` Daniel Lezcano
2018-04-16  9:34           ` Sudeep Holla
2018-04-16  9:37           ` Viresh Kumar
2018-04-16  9:45             ` Daniel Lezcano
2018-04-16  9:50               ` Viresh Kumar
2018-04-16 10:03                 ` Daniel Lezcano
2018-04-16 10:10                   ` Viresh Kumar
2018-04-16 12:10                     ` Daniel Lezcano
2018-04-16 12:30                       ` Lorenzo Pieralisi
2018-04-16 13:57                         ` Daniel Lezcano
2018-04-16 14:22                           ` Lorenzo Pieralisi
2018-04-17  7:17                             ` Daniel Lezcano
2018-04-17 10:24                               ` Lorenzo Pieralisi
2018-04-16 12:31                       ` Sudeep Holla
2018-04-16 12:49                         ` Daniel Lezcano
2018-04-16 13:03                           ` Sudeep Holla
2018-04-16 12:29                 ` Sudeep Holla
2018-04-13 11:38   ` Daniel Thompson
2018-04-13 11:46     ` Daniel Lezcano
2019-08-05  5:11   ` Martin Kepplinger
2019-08-05  6:53     ` Martin Kepplinger
2019-08-05  7:39       ` Daniel Lezcano
2019-08-05  7:42         ` Martin Kepplinger
2019-08-05  7:58           ` Daniel Lezcano
2019-10-25 11:22             ` Martin Kepplinger
2019-10-25 14:45               ` Daniel Lezcano
2019-10-26 18:23                 ` Martin Kepplinger
2019-10-28 15:16                   ` Daniel Lezcano
2019-08-05  7:37     ` Daniel Lezcano
2019-08-05  7:40       ` Martin Kepplinger
2018-04-05 16:16 ` [PATCH v3 7/7] cpuidle/drivers/cpuidle-arm: Register the cooling device Daniel Lezcano
2018-04-11  8:51   ` Viresh Kumar

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