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

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Message-ID: <20100305091949.GA31554@atrey.karlin.mff.cuni.cz>
Date:	Fri, 5 Mar 2010 10:19:49 +0100
From:	Pavel Machek <pavel@....cz>
To:	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>
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

> 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...

> > 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?
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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