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

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Date:	Sat, 27 Feb 2016 00:15:47 +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 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.

>  }
>  
>  static void intel_bsp_resume(struct cpuinfo_x86 *c)
> 

Thanks,
Rafael