linux-kernel - Re: [PATCH] x86 intel power: Initialize MSR_IA32_ENERGY_PERF

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Message-ID: <20110415101712.GB28007@elte.hu>
Date:	Fri, 15 Apr 2011 12:17:12 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Len Brown <lenb@...nel.org>
Cc:	x86@...nel.org, linux-pm@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [PATCH] x86 intel power: Initialize MSR_IA32_ENERGY_PERF_BIAS


* Len Brown <lenb@...nel.org> wrote:

> 
> > > From: Len Brown <len.brown@...el.com>
> > > 
> > > Since 2.6.35 (23016bf0d25), Linux prints the existence of "epb" in /proc/cpuinfo,
> > > Since 2.6.38 (d5532ee7b40), the x86_energy_perf_policy(8) utility is available
> > > in-tree to update MSR_IA32_ENERGY_PERF_BIAS.
> > > 
> > > However, the typical BIOS fails to initialize the MSR,
> > > and the typical Linux distro neglects to invoke x86_energy_perf_policy(8).
> > > 
> > > The result is that some modern hardware is running in hardware default,
> > > which is "performance" mode, rather than the intended design default
> > > of "normal" mode.
> > > 
> > > Initialize the MSR to the "normal" setting during kernel boot.
> > > 
> > > Of course, x86_energy_perf_policy(8) is available to change
> > > the default after boot, should the user have a policy preference.
> > > 
> > > cc: stable@...nel.org
> > > Signed-off-by: Len Brown <len.brown@...el.com>
> > > ---
> > >  arch/x86/include/asm/msr-index.h |    3 +++
> > >  arch/x86/kernel/cpu/intel.c      |   14 ++++++++++++++
> > >  2 files changed, 17 insertions(+), 0 deletions(-)
> > > 
> ...
> > 
> > Dunno, this patch appears to silently modify the system to be slower than it 
> > was before under Linux.
> > 
> > Won't people report this as a regression if this change reduces performance for 
> > them?
> > 
> > They wont be able to see your comments in the code and in the changelog either, 
> > when this happens to them. They might look into /proc/cpuinfo and see 'epb' 
> > there but it wont tell them anything. They wont know about a utility available 
> > in tools/power/x86/ either.
> 
> This patch makes no change to the epb feature indicator
> /proc/cpuinfo.

I know. I reacted to this bit in the changelog:

> > > Since 2.6.35 (23016bf0d25), Linux prints the existence of "epb" in /proc/cpuinfo,

Printing the existence of a CPU feature does nothing to inform users.

> > So this patch has 'future trouble' written all over it i'm afraid.
> 
> EPB is limited to SNB and later.
> So the installed base as yet is small.
> (it also exists on WSM-EP, but doesn't do so much there)
> EPB will have a more significant effect on future hardware.
> 
> Linux currently trails competing operating systems in energy
> efficiency on SNB due to this setting, and Linux will trail
> competing operating systems even more on future hardware
> if this default is not fixed.
> 
> Will it be possible to measure a performance difference between
> "performance" and "normal"?  Yes, it will be possible.
> Will 99.9% of users notice?  Nope.  More likely they'll notice
> the the power savings that are disabled in "performance" mode.
> 
> I should have called it "benchmark" mode instead of "performance" mode...

That's all fair but does not address the concerns i raised. A silent change 
during bootup is asking for trouble.

So how about informing users, how about making it non-silent? An informative 
printk that also mentions the power configuration tool, etc. This solves the 
concerns i mentioned.

Thanks,

	Ingo
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