308 lines
11 KiB
Plaintext
308 lines
11 KiB
Plaintext
|
=======================
|
|||
|
|
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 moniors 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
|
|||
|
|
|
|||
|
|
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, which
|
|||
|
|
appear running to the scheduler. But the overall system idle is still
|
|||
|
|
reflected. 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).
|