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diff --git a/Documentation/thermal/intel_powerclamp.txt b/Documentation/thermal/intel_powerclamp.txt deleted file mode 100644 index b5df21168fbc..000000000000 --- a/Documentation/thermal/intel_powerclamp.txt +++ /dev/null @@ -1,317 +0,0 @@ - ======================= - 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). |