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tools: create power/x86/x86_energy_perf_policy
MSR_IA32_ENERGY_PERF_BIAS first became available on Westmere Xeon. It is implemented in all Sandy Bridge processors -- mobile, desktop and server. It is expected to become increasingly important in subsequent generations. x86_energy_perf_policy is a user-space utility to set the hardware energy vs performance policy hint in the processor. Most systems would benefit from "x86_energy_perf_policy normal" at system startup, as the hardware default is maximum performance at the expense of energy efficiency. See x86_energy_perf_policy.8 man page for more information. Background: Linux-2.6.36 added "epb" to /proc/cpuinfo to indicate if an x86 processor supports MSR_IA32_ENERGY_PERF_BIAS, without actually modifying the MSR. In March, 2010, Venkatesh Pallipadi proposed a small driver that programmed MSR_IA32_ENERGY_PERF_BIAS, based on the cpufreq governor in use. It also offered a boot-time cmdline option to override. http://lkml.org/lkml/2010/3/4/457 But hiding the hardware policy behind the governor choice was deemed "kinda icky". In June, 2010, I proposed a generic user/kernel API to generalize the power/performance policy trade-off. "RFC: /sys/power/policy_preference" http://lkml.org/lkml/2010/6/16/399 That is my preference for implementing this capability, but I received no support on the list. So in September, 2010, I sent x86_energy_perf_policy.c to LKML, a user-space utility that scribbles directly to the MSR. http://lkml.org/lkml/2010/9/28/246 Here is that same utility, after responding to some review feedback, to live in tools/power/, where it is easily found. Signed-off-by: Len Brown <len.brown@intel.com>
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‎tools/power/x86/x86_energy_perf_policy/Makefile

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x86_energy_perf_policy : x86_energy_perf_policy.c
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clean :
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rm -f x86_energy_perf_policy
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install :
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install x86_energy_perf_policy /usr/bin/
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install x86_energy_perf_policy.8 /usr/share/man/man8/

‎tools/power/x86/x86_energy_perf_policy/x86_energy_perf_policy.8

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.\" This page Copyright (C) 2010 Len Brown <len.brown@intel.com>
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.\" Distributed under the GPL, Copyleft 1994.
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.TH X86_ENERGY_PERF_POLICY 8
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.SH NAME
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x86_energy_perf_policy \- read or write MSR_IA32_ENERGY_PERF_BIAS
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.SH SYNOPSIS
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.ft B
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.B x86_energy_perf_policy
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.RB [ "\-c cpu" ]
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.RB [ "\-v" ]
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.RB "\-r"
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.br
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.B x86_energy_perf_policy
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.RB [ "\-c cpu" ]
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.RB [ "\-v" ]
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.RB 'performance'
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.br
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.B x86_energy_perf_policy
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.RB [ "\-c cpu" ]
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.RB [ "\-v" ]
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.RB 'normal'
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.br
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.B x86_energy_perf_policy
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.RB [ "\-c cpu" ]
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.RB [ "\-v" ]
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.RB 'powersave'
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.br
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.B x86_energy_perf_policy
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.RB [ "\-c cpu" ]
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.RB [ "\-v" ]
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.RB n
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.br
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.SH DESCRIPTION
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\fBx86_energy_perf_policy\fP
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allows software to convey
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its policy for the relative importance of performance
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versus energy savings to the processor.
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The processor uses this information in model-specific ways
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when it must select trade-offs between performance and
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energy efficiency.
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This policy hint does not supersede Processor Performance states
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(P-states) or CPU Idle power states (C-states), but allows
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software to have influence where it would otherwise be unable
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to express a preference.
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For example, this setting may tell the hardware how
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aggressively or conservatively to control frequency
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in the "turbo range" above the explicitly OS-controlled
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P-state frequency range. It may also tell the hardware
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how aggressively is should enter the OS requested C-states.
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Support for this feature is indicated by CPUID.06H.ECX.bit3
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per the Intel Architectures Software Developer's Manual.
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.SS Options
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\fB-c\fP limits operation to a single CPU.
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The default is to operate on all CPUs.
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Note that MSR_IA32_ENERGY_PERF_BIAS is defined per
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logical processor, but that the initial implementations
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of the MSR were shared among all processors in each package.
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.PP
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\fB-v\fP increases verbosity. By default
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x86_energy_perf_policy is silent.
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.PP
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\fB-r\fP is for "read-only" mode - the unchanged state
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is read and displayed.
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.PP
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.I performance
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Set a policy where performance is paramount.
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The processor will be unwilling to sacrifice any performance
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for the sake of energy saving. This is the hardware default.
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.PP
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.I normal
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Set a policy with a normal balance between performance and energy efficiency.
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The processor will tolerate minor performance compromise
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for potentially significant energy savings.
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This reasonable default for most desktops and servers.
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.PP
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.I powersave
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Set a policy where the processor can accept
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a measurable performance hit to maximize energy efficiency.
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.PP
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.I n
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Set MSR_IA32_ENERGY_PERF_BIAS to the specified number.
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The range of valid numbers is 0-15, where 0 is maximum
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performance and 15 is maximum energy efficiency.
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.SH NOTES
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.B "x86_energy_perf_policy "
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runs only as root.
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.SH FILES
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.ta
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.nf
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/dev/cpu/*/msr
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.fi
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.SH "SEE ALSO"
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msr(4)
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.PP
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.SH AUTHORS
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.nf
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Written by Len Brown <len.brown@intel.com>

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