linux-kernel - Re: [PATCH] Do not modify MSR_IA32_ENERGY_PERF

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Date:	Wed, 02 Mar 2016 01:26:18 +0100
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Thomas Renninger <trenn@...e.de>
Cc:	Ingo Molnar <mingo@...nel.org>, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org, lenb@...nel.org,
	Thomas Renninger <trenn@...e.com>, x86@...nel.org
Subject: Re: [PATCH] Do not modify MSR_IA32_ENERGY_PERF_BIAS in kernel

On Tuesday, March 01, 2016 01:17:37 PM Thomas Renninger wrote:
> On Saturday, February 27, 2016 12:15:47 AM Rafael J. Wysocki wrote:
> > On Friday, February 26, 2016 05:38:00 PM Thomas Renninger wrote:
> > > The assumption that BIOSes never want to have this register being set to
> > > full performance (zero) is wrong.
> > > 
> > > While wrongly overruling this BIOS setting and set it from performance
> > > to normal did not hurt that much, because nobody really knew the effects
> > > inside Intel processors.
> > > 
> > > But with Broadwell-EP processor (E5-2687W v4) the CPU will not enter turbo
> > > modes if this value is not set to performance.
> > > 
> > > So switch logic to tell the user in a friendly way (info) that the CPU is
> > > in performance mode and how to switch via userspace if this is not
> > > intended.
> > > 
> > > But otherwise trust that the BIOS has set the correct value here and do
> > > not
> > > blindly overrule.
> > > 
> > > How this has been found: SLE11 had this patch, SLE12 it slipped through.
> > > It took quite some time to nail down that this patch missing is the reason
> > > for not entering turbo modes with this specific processor.
> > > 
> > > Signed-off-by: Thomas Renninger <trenn@...e.com>
> > > 
> > > --- a/arch/x86/kernel/cpu/intel.c	2016-02-26 17:19:55.731042972 +0100
> > > +++ b/arch/x86/kernel/cpu/intel.c	2016-02-26 17:20:48.598020581 +0100
> > > @@ -377,8 +377,12 @@ static void init_intel_energy_perf(struc
> > > 
> > >  	u64 epb;
> > >  	
> > >  	/*
> > > 
> > > -	 * Initialize MSR_IA32_ENERGY_PERF_BIAS if not already initialized.
> > > -	 * (x86_energy_perf_policy(8) is available to change it at run-time.)
> > > +	 * On server platforms energy bias typically is set to
> > > +	 * performance on purpose.
> > > +	 * On other platforms it may happen that MSR_IA32_ENERGY_PERF_BIAS
> > > +	 * did not get initialized properly by BIOS.
> > > +	 * Best is to to keep BIOS settings and give the user a hint whether
> > > +	 * to change it via cpupower-set(8) userspace tool at runtime.
> > > 
> > >  	 */
> > >  	
> > >  	if (!cpu_has(c, X86_FEATURE_EPB))
> > >  	
> > >  		return;
> > > 
> > > @@ -387,10 +391,8 @@ static void init_intel_energy_perf(struc
> > > 
> > >  	if ((epb & 0xF) != ENERGY_PERF_BIAS_PERFORMANCE)
> > >  	
> > >  		return;
> > > 
> > > -	pr_warn_once("ENERGY_PERF_BIAS: Set to 'normal', was 
> 'performance'\n");
> > > -	pr_warn_once("ENERGY_PERF_BIAS: View and update with
> > > x86_energy_perf_policy(8)\n"); -	epb = (epb & ~0xF) |
> > > ENERGY_PERF_BIAS_NORMAL;
> > > -	wrmsrl(MSR_IA32_ENERGY_PERF_BIAS, epb);
> > > +	pr_info_once("ENERGY_PERF_BIAS is set to 'performance'\n");
> > > +	pr_info_once("ENERGY_PERF_BIAS: Update with cpupower-set(8)\n");
> > 
> > This doesn't need to be cpupower-set IMO.
> 
> You mean why switch the message from:
> x86_energy_perf_policy to cpupower-set
> ?
> 
> IMO x86_energy_perf_policy should not exist. It has been introduce before
> cpupower set -b.
> Having an extra tool/binary for this functionality is an unneeded packaging 
> overhead for distros.
> Also having more and more of such CPU specific tools is not userfriendly.
> cpupower supports all power relevant features of your CPU and on all 
> architectures (or at least it should). People should know this one better
> than "x86_energy_perf_policy" and theoretically intuitively find it, even 
> without a message.
> 
> So it would be nice to get the message fixed as well.

My point is that since "cpupower set -b" is not the only way to set this,
it doesn't seem appropriate to refer to it explicitly from a kernel message.

I actually don't think the second message is necessary at all.

Thanks,
Rafael