linux-kernel - RE: [patch 2/2] x86: Manage ENERGY_PERF

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Date:	Fri, 5 Mar 2010 06:36:10 -0800
From:	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>
To:	Pavel Machek <pavel@....cz>
CC:	Randy Dunlap <rdunlap@...otime.net>, Ingo Molnar <mingo@...e.hu>,
	H Peter Anvin <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Len Brown <lenb@...nel.org>, Dave Jones <davej@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>
Subject: RE: [patch 2/2] x86: Manage ENERGY_PERF_BIAS based on cpufreq
	governor

 

>-----Original Message-----
>From: Pavel Machek [mailto:pavel@....cz] 
>Sent: Friday, March 05, 2010 1:20 AM
>To: Pallipadi, Venkatesh
>Cc: Randy Dunlap; Ingo Molnar; H Peter Anvin; Thomas Gleixner; 
>Len Brown; Dave Jones; linux-kernel@...r.kernel.org; 
>linux-acpi@...r.kernel.org
>Subject: Re: [patch 2/2] x86: Manage ENERGY_PERF_BIAS based on 
>cpufreq governor
>
>> On Wed, 2010-03-03 at 13:57 -0800, Pavel Machek wrote:
>> > Hi!
>> > 
>> > > index 8c666d8..4945add 100644
>> > > --- a/Documentation/kernel-parameters.txt
>> > > +++ b/Documentation/kernel-parameters.txt
>> > > @@ -749,6 +749,10 @@ and is between 256 and 4096 
>characters. It is defined in the file
>> > >  			Default value is 0.
>> > >  			Value can be changed at runtime 
>via /selinux/enforce.
>> > >  
>> > > +	epb		[X86] Control 
>IA32_ENERGY_PERF_BIAS setting
>> > > +			"disable" - Kernel will not 
>modify this MSR
>> > > +			<0..15> - Kernel will set this 
>MSR to i/p static value
>> > > +
>> > > 
>> > > 
>> > > Should be more like:
>> > > 
>> > > 	epb=		[X86] Control IA32_ENERGY_PERF_BIAS setting
>> > > 			Format: { disable | <0...15> }
>> > > 			"disable" - Kernel will not modify this MSR
>> > > 			<0..15> - Kernel will set this MSR to 
>i/p static value
>> > > 
>> > > 
>> > > But what is "i/p"?  Use whatever word it should be, please.
>> > > What do the values mean?
>> > > And what does IA32 have to do with this?  does it not 
>apply to x86_64?
>> > 
>> > Exactly. This is end user documentation, it should not 
>even talk about
>> > MSRs. Tell us what the setting does...
>> 
>> The not so good part of this feature is that the setting 
>here is opaque.
>> Software can set this based on its preference, for example 0 for
>> performance 15 for power and 7 for balanced. Different CPUs 
>can use this
>> information to do different optimizations or 
>power-performance tradeoffs
>> in the hardware. The only thing that user knows here is that there is
>> this dial with 16 possible values. I can remove the MSR name 
>here. But,
>> I think that will end up confusing the end user on what this thing is
>> and how it is related to all the other tunables we have in 
>the kernel.
>> Having the MSR name gives a hint.
>
>You should say what the setting does; you can mention below what MSR
>it corresponds to, but "Control IA32_ENERGY_PERF_BIAS setting" is not
>suitable user documentation.
>
>> Also, the expectation here is that kernel will do the right thing by
>> default. The option here is to the user who_knows_what_he_is_doing to
>> override the kernel default.
>
>You did not give user enough information to do anything intelligent...

I have rephrased it in the newer version sent yday with more info.

>> > Also... does it make change to tweak the setting during 
>runtime? Maybe
>> > different settings for AC and battery power?
>> 
>> Yes. Matthew mentioned in other response aboue setting this based on
>> freq. For the CPUs that support this feature currently, we don't see
>> advantage in setting this feature at run time.
>
>If the feature is useless, then why set it at all?

I just said changing it at run time doesn't give us benefits. Not that
the feature is useless. Having the default value for the tunable in
mid-range does increase energy-efficiency than the tunable being
at performance level.

Thanks,
Venki--
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