linux-kernel - Re: [PATCH 1/2] PM / arch: x86: Rework the MSR_IA32_ENERGY_PERF

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Date:   Mon, 25 Mar 2019 12:31:23 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     x86 <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        Len Brown <len.brown@...el.com>,
        Linux PM <linux-pm@...r.kernel.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Laura Abbott <labbott@...oraproject.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Simon Schricker <sschricker@...e.de>,
        Hannes Reinecke <hare@...e.de>
Subject: Re: [PATCH 1/2] PM / arch: x86: Rework the MSR_IA32_ENERGY_PERF_BIAS
 handling

On Thu, Mar 21, 2019 at 11:18:01PM +0100, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> The current handling of MSR_IA32_ENERGY_PERF_BIAS in the kernel is
> problematic, because it may cause changes made by user space to that
> MSR (with the help of the x86_energy_perf_policy tool, for example)
> to be lost every time a CPU goes offline and then back online as well
> as during system-wide power management transitions into sleep states
> and back into the working state.
> 
> The first problem is that if the current EPB value for a CPU going
> online is 0 ('performance'), the kernel will change it to 6 ('normal')
> regardless of whether or not this is the first bring-up of that CPU.
> That also happens during system-wide resume from sleep states
> (including, but not limited to, hibernation).  However, the EPB may
> have been adjusted by user space this way and the kernel should not
> blindly override that setting.
> 
> The second problem is that if the platform firmware resets the EPB
> values for any CPUs during system-wide resume from a sleep state,
> the kernel will not restore their previous EPB values that may
> have been set by user space before the preceding system-wide
> suspend transition.  Again, that behavior may at least be confusing
> from the user space perspective.
> 
> In order to address these issues, rework the handling of
> MSR_IA32_ENERGY_PERF_BIAS so that the EPB value is saved on CPU
> offline and restored on CPU online as well as (for the boot CPU)
> during the syscore stages of system-wide suspend and resume
> transitions, respectively.
> 
> However, retain the policy by which the EPB is set to 6 ('normal')
> on the first bring-up of each CPU if its initial value is 0, based
> on the observation that 0 may mean 'not initialized' just as well as
> 'performance' in that case.
> 
> While at it, move the MSR_IA32_ENERGY_PERF_BIAS handling code into
> a separate file and document it in Documentation/admin-guide.
> 
> Fixes: abe48b108247 (x86, intel, power: Initialize MSR_IA32_ENERGY_PERF_BIAS)
> Fixes: b51ef52df71c (x86/cpu: Restore MSR_IA32_ENERGY_PERF_BIAS after resume)
> Reported-by: Thomas Renninger <trenn@...e.de>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

Acked-by: Borislav Petkov <bp@...e.de>

-- 
Regards/Gruss,
    Boris.

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