docs: thermal: add it to the driver API

The file contents mostly describes driver internals.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 320 insertions, 0 deletions

diff --git a/Documentation/driver-api/thermal/intel_powerclamp.rst b/Documentation/driver-api/thermal/intel_powerclamp.rst
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+=======================
+Intel Powerclamp Driver
+=======================
+
+By:
+  - Arjan van de Ven <arjan@linux.intel.com>
+  - Jacob Pan <jacob.jun.pan@linux.intel.com>
+
+.. Contents:
+
+	(*) Introduction
+	    - Goals and Objectives
+
+	(*) Theory of Operation
+	    - Idle Injection
+	    - Calibration
+
+	(*) Performance Analysis
+	    - Effectiveness and Limitations
+	    - Power vs Performance
+	    - Scalability
+	    - Calibration
+	    - Comparison with Alternative Techniques
+
+	(*) Usage and Interfaces
+	    - Generic Thermal Layer (sysfs)
+	    - Kernel APIs (TBD)
+
+INTRODUCTION
+============
+
+Consider the situation where a system’s power consumption must be
+reduced at runtime, due to power budget, thermal constraint, or noise
+level, and where active cooling is not preferred. Software managed
+passive power reduction must be performed to prevent the hardware
+actions that are designed for catastrophic scenarios.
+
+Currently, P-states, T-states (clock modulation), and CPU offlining
+are used for CPU throttling.
+
+On Intel CPUs, C-states provide effective power reduction, but so far
+they’re only used opportunistically, based on workload. With the
+development of intel_powerclamp driver, the method of synchronizing
+idle injection across all online CPU threads was introduced. The goal
+is to achieve forced and controllable C-state residency.
+
+Test/Analysis has been made in the areas of power, performance,
+scalability, and user experience. In many cases, clear advantage is
+shown over taking the CPU offline or modulating the CPU clock.
+
+
+THEORY OF OPERATION
+===================
+
+Idle Injection
+--------------
+
+On modern Intel processors (Nehalem or later), package level C-state
+residency is available in MSRs, thus also available to the kernel.
+
+These MSRs are::
+
+      #define MSR_PKG_C2_RESIDENCY      0x60D
+      #define MSR_PKG_C3_RESIDENCY      0x3F8
+      #define MSR_PKG_C6_RESIDENCY      0x3F9
+      #define MSR_PKG_C7_RESIDENCY      0x3FA
+
+If the kernel can also inject idle time to the system, then a
+closed-loop control system can be established that manages package
+level C-state. The intel_powerclamp driver is conceived as such a
+control system, where the target set point is a user-selected idle
+ratio (based on power reduction), and the error is the difference
+between the actual package level C-state residency ratio and the target idle
+ratio.
+
+Injection is controlled by high priority kernel threads, spawned for
+each online CPU.
+
+These kernel threads, with SCHED_FIFO class, are created to perform
+clamping actions of controlled duty ratio and duration. Each per-CPU
+thread synchronizes its idle time and duration, based on the rounding
+of jiffies, so accumulated errors can be prevented to avoid a jittery
+effect. Threads are also bound to the CPU such that they cannot be
+migrated, unless the CPU is taken offline. In this case, threads
+belong to the offlined CPUs will be terminated immediately.
+
+Running as SCHED_FIFO and relatively high priority, also allows such
+scheme to work for both preemptable and non-preemptable kernels.
+Alignment of idle time around jiffies ensures scalability for HZ
+values. This effect can be better visualized using a Perf timechart.
+The following diagram shows the behavior of kernel thread
+kidle_inject/cpu. During idle injection, it runs monitor/mwait idle
+for a given "duration", then relinquishes the CPU to other tasks,
+until the next time interval.
+
+The NOHZ schedule tick is disabled during idle time, but interrupts
+are not masked. Tests show that the extra wakeups from scheduler tick
+have a dramatic impact on the effectiveness of the powerclamp driver
+on large scale systems (Westmere system with 80 processors).
+
+::
+
+  CPU0
+		    ____________          ____________
+  kidle_inject/0   |   sleep    |  mwait |  sleep     |
+	  _________|            |________|            |_______
+				 duration
+  CPU1
+		    ____________          ____________
+  kidle_inject/1   |   sleep    |  mwait |  sleep     |
+	  _________|            |________|            |_______
+				^
+				|
+				|
+				roundup(jiffies, interval)
+
+Only one CPU is allowed to collect statistics and update global
+control parameters. This CPU is referred to as the controlling CPU in
+this document. The controlling CPU is elected at runtime, with a
+policy that favors BSP, taking into account the possibility of a CPU
+hot-plug.
+
+In terms of dynamics of the idle control system, package level idle
+time is considered largely as a non-causal system where its behavior
+cannot be based on the past or current input. Therefore, the
+intel_powerclamp driver attempts to enforce the desired idle time
+instantly as given input (target idle ratio). After injection,
+powerclamp monitors the actual idle for a given time window and adjust
+the next injection accordingly to avoid over/under correction.
+
+When used in a causal control system, such as a temperature control,
+it is up to the user of this driver to implement algorithms where
+past samples and outputs are included in the feedback. For example, a
+PID-based thermal controller can use the powerclamp driver to
+maintain a desired target temperature, based on integral and
+derivative gains of the past samples.
+
+
+
+Calibration
+-----------
+During scalability testing, it is observed that synchronized actions
+among CPUs become challenging as the number of cores grows. This is
+also true for the ability of a system to enter package level C-states.
+
+To make sure the intel_powerclamp driver scales well, online
+calibration is implemented. The goals for doing such a calibration
+are:
+
+a) determine the effective range of idle injection ratio
+b) determine the amount of compensation needed at each target ratio
+
+Compensation to each target ratio consists of two parts:
+
+	a) steady state error compensation
+	This is to offset the error occurring when the system can
+	enter idle without extra wakeups (such as external interrupts).
+
+	b) dynamic error compensation
+	When an excessive amount of wakeups occurs during idle, an
+	additional idle ratio can be added to quiet interrupts, by
+	slowing down CPU activities.
+
+A debugfs file is provided for the user to examine compensation
+progress and results, such as on a Westmere system::
+
+  [jacob@nex01 ~]$ cat
+  /sys/kernel/debug/intel_powerclamp/powerclamp_calib
+  controlling cpu: 0
+  pct confidence steady dynamic (compensation)
+  0       0       0       0
+  1       1       0       0
+  2       1       1       0
+  3       3       1       0
+  4       3       1       0
+  5       3       1       0
+  6       3       1       0
+  7       3       1       0
+  8       3       1       0
+  ...
+  30      3       2       0
+  31      3       2       0
+  32      3       1       0
+  33      3       2       0
+  34      3       1       0
+  35      3       2       0
+  36      3       1       0
+  37      3       2       0
+  38      3       1       0
+  39      3       2       0
+  40      3       3       0
+  41      3       1       0
+  42      3       2       0
+  43      3       1       0
+  44      3       1       0
+  45      3       2       0
+  46      3       3       0
+  47      3       0       0
+  48      3       2       0
+  49      3       3       0
+
+Calibration occurs during runtime. No offline method is available.
+Steady state compensation is used only when confidence levels of all
+adjacent ratios have reached satisfactory level. A confidence level
+is accumulated based on clean data collected at runtime. Data
+collected during a period without extra interrupts is considered
+clean.
+
+To compensate for excessive amounts of wakeup during idle, additional
+idle time is injected when such a condition is detected. Currently,
+we have a simple algorithm to double the injection ratio. A possible
+enhancement might be to throttle the offending IRQ, such as delaying
+EOI for level triggered interrupts. But it is a challenge to be
+non-intrusive to the scheduler or the IRQ core code.
+
+
+CPU Online/Offline
+------------------
+Per-CPU kernel threads are started/stopped upon receiving
+notifications of CPU hotplug activities. The intel_powerclamp driver
+keeps track of clamping kernel threads, even after they are migrated
+to other CPUs, after a CPU offline event.
+
+
+Performance Analysis
+====================
+This section describes the general performance data collected on
+multiple systems, including Westmere (80P) and Ivy Bridge (4P, 8P).
+
+Effectiveness and Limitations
+-----------------------------
+The maximum range that idle injection is allowed is capped at 50
+percent. As mentioned earlier, since interrupts are allowed during
+forced idle time, excessive interrupts could result in less
+effectiveness. The extreme case would be doing a ping -f to generated
+flooded network interrupts without much CPU acknowledgement. In this
+case, little can be done from the idle injection threads. In most
+normal cases, such as scp a large file, applications can be throttled
+by the powerclamp driver, since slowing down the CPU also slows down
+network protocol processing, which in turn reduces interrupts.
+
+When control parameters change at runtime by the controlling CPU, it
+may take an additional period for the rest of the CPUs to catch up
+with the changes. During this time, idle injection is out of sync,
+thus not able to enter package C- states at the expected ratio. But
+this effect is minor, in that in most cases change to the target
+ratio is updated much less frequently than the idle injection
+frequency.
+
+Scalability
+-----------
+Tests also show a minor, but measurable, difference between the 4P/8P
+Ivy Bridge system and the 80P Westmere server under 50% idle ratio.
+More compensation is needed on Westmere for the same amount of
+target idle ratio. The compensation also increases as the idle ratio
+gets larger. The above reason constitutes the need for the
+calibration code.
+
+On the IVB 8P system, compared to an offline CPU, powerclamp can
+achieve up to 40% better performance per watt. (measured by a spin
+counter summed over per CPU counting threads spawned for all running
+CPUs).
+
+Usage and Interfaces
+====================
+The powerclamp driver is registered to the generic thermal layer as a
+cooling device. Currently, it’s not bound to any thermal zones::
+
+  jacob@chromoly:/sys/class/thermal/cooling_device14$ grep . *
+  cur_state:0
+  max_state:50
+  type:intel_powerclamp
+
+cur_state allows user to set the desired idle percentage. Writing 0 to
+cur_state will stop idle injection. Writing a value between 1 and
+max_state will start the idle injection. Reading cur_state returns the
+actual and current idle percentage. This may not be the same value
+set by the user in that current idle percentage depends on workload
+and includes natural idle. When idle injection is disabled, reading
+cur_state returns value -1 instead of 0 which is to avoid confusing
+100% busy state with the disabled state.
+
+Example usage:
+- To inject 25% idle time::
+
+	$ sudo sh -c "echo 25 > /sys/class/thermal/cooling_device80/cur_state
+
+If the system is not busy and has more than 25% idle time already,
+then the powerclamp driver will not start idle injection. Using Top
+will not show idle injection kernel threads.
+
+If the system is busy (spin test below) and has less than 25% natural
+idle time, powerclamp kernel threads will do idle injection. Forced
+idle time is accounted as normal idle in that common code path is
+taken as the idle task.
+
+In this example, 24.1% idle is shown. This helps the system admin or
+user determine the cause of slowdown, when a powerclamp driver is in action::
+
+
+  Tasks: 197 total,   1 running, 196 sleeping,   0 stopped,   0 zombie
+  Cpu(s): 71.2%us,  4.7%sy,  0.0%ni, 24.1%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
+  Mem:   3943228k total,  1689632k used,  2253596k free,    74960k buffers
+  Swap:  4087804k total,        0k used,  4087804k free,   945336k cached
+
+    PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
+   3352 jacob     20   0  262m  644  428 S  286  0.0   0:17.16 spin
+   3341 root     -51   0     0    0    0 D   25  0.0   0:01.62 kidle_inject/0
+   3344 root     -51   0     0    0    0 D   25  0.0   0:01.60 kidle_inject/3
+   3342 root     -51   0     0    0    0 D   25  0.0   0:01.61 kidle_inject/1
+   3343 root     -51   0     0    0    0 D   25  0.0   0:01.60 kidle_inject/2
+   2935 jacob     20   0  696m 125m  35m S    5  3.3   0:31.11 firefox
+   1546 root      20   0  158m  20m 6640 S    3  0.5   0:26.97 Xorg
+   2100 jacob     20   0 1223m  88m  30m S    3  2.3   0:23.68 compiz
+
+Tests have shown that by using the powerclamp driver as a cooling
+device, a PID based userspace thermal controller can manage to
+control CPU temperature effectively, when no other thermal influence
+is added. For example, a UltraBook user can compile the kernel under
+certain temperature (below most active trip points).