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[209.132.180.67]) by mx.google.com with ESMTP id n8si2010410otr.102.2019.12.11.14.44.06; Wed, 11 Dec 2019 14:44:06 -0800 (PST) Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; dkim=pass header.i=@linaro.org header.s=google header.b=adUlVTiT; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=linaro.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726890AbfLKWoF (ORCPT + 27 others); Wed, 11 Dec 2019 17:44:05 -0500 Received: from mail-wm1-f67.google.com ([209.85.128.67]:39534 "EHLO mail-wm1-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726592AbfLKWoE (ORCPT ); Wed, 11 Dec 2019 17:44:04 -0500 Received: by mail-wm1-f67.google.com with SMTP id d5so98391wmb.4 for ; Wed, 11 Dec 2019 14:44:02 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linaro.org; s=google; h=from:to:cc:subject:date:message-id:mime-version :content-transfer-encoding; bh=eHar8caGVk2CHYCleN7lzgXckcrdAjaPRcEzE+leMkc=; b=adUlVTiTz/iXrhnDM4CQmeVqAvo6g5aR4wZlXa2/qvQCfFdJBMaA2fP1Ktz4VgakJ9 BBO+16TMexseKta7EmbwHlfuSVpAUrBIWuLnAcmcbhRlSQYM6MMjAGNk+cmk3dzipVKv 5yCOUdWh0IB2vW2QZ/ipPZ6smv+S6mBHKcruTY9jukDzyAdeoICyWz1xtbaXYzMSfl/a NCjpGmD1r4vd2dBLCDnZiK1efTFD/2/OnfIvbI8G3NGDY4khNeRauKAu1pEEqxOH1049 UF4UA8Yy1Xme/7jfzyq9w2d9PQzmCjdGMfBXhYWITGHxf4AMCkQPj5C+0izWgcdzs+fw NJqQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:to:cc:subject:date:message-id:mime-version :content-transfer-encoding; bh=eHar8caGVk2CHYCleN7lzgXckcrdAjaPRcEzE+leMkc=; b=kq4kqR52uFUMrJ4VKhJsfrThZX3ifuFGrFcEbY0xpyo6Jw6rSlnxRRBzXFT/u/sWDv hKzeJxwJranbU2YeqSEPcL0folpktLLCIrjUdI54EdORCXhuS+KiNBQKZJdKz0+LzNjS Xp8RkS+h8+hbzaeluvrCp83hYjbSoQZ/88oXNQfoGh/kdhMhw37qWF+i12q0JcGym0Va NyMJ4uPI5ZrlSSFrw4xkebyL9988inRGocUGdbttxAwGTflYtgnFp70Vhk9Rdqd9k2VV zml4Y9YAH1C2Z/96Va7mRJXGIdgjYFYUfa5KsL2mD2CgLb5M8m3PCLg3iYIuNUZCvKNX 2k8A== X-Gm-Message-State: APjAAAWLjCaMZeEFoIG2y9qWGtKqfJk06X0IrDSJuEXvsFlLRM/gzGr0 UA2VMuhlfQ1yy6LTu6lbOqvWSw== X-Received: by 2002:a7b:c7d3:: with SMTP id z19mr2420119wmk.94.1576104241537; Wed, 11 Dec 2019 14:44:01 -0800 (PST) Received: from localhost.localdomain ([2a01:e34:ed2f:f020:48f7:48fb:71cb:f13a]) by smtp.gmail.com with ESMTPSA id s16sm3798809wrn.78.2019.12.11.14.44.00 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 11 Dec 2019 14:44:00 -0800 (PST) From: Daniel Lezcano To: rui.zhang@intel.com Cc: rjw@rjwysocki.net, linux-pm@vger.kernel.org, viresh.kumar@linaro.org, amit.kucheria@linaro.org, linux-kernel@vger.kernel.org, martin.kepplinger@puri.sm Subject: [PATCH V5 1/3] thermal/drivers/cpu_cooling: Add idle cooling device documentation Date: Wed, 11 Dec 2019 23:43:45 +0100 Message-Id: <20191211224347.1001-1-daniel.lezcano@linaro.org> X-Mailer: git-send-email 2.17.1 MIME-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 Acked-by: Viresh Kumar --- 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 -- 2.17.1 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 )