linux-kernel - Re: [PATCH v3] x86: intel_epb: Add earlyparam option to keep bias at performance

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Message-ID: <2e7de96c-0634-4c98-9d56-ceba19c06c98@intel.com>
Date:   Tue, 5 Dec 2023 07:26:20 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Jack Allister <jalliste@...zon.com>
Cc:     Paul Durrant <pdurrant@...zon.com>, Jue Wang <juew@...zon.com>,
        Usama Arif <usama.arif@...edance.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Hans de Goede <hdegoede@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] x86: intel_epb: Add earlyparam option to keep bias at
 performance

On 12/5/23 05:23, Jack Allister wrote:
> There are certain scenarios where it may be intentional that the EPB was
> set at to 0/ENERGY_PERF_BIAS_PERFORMANCE on kernel boot. For example, in
> data centers a kexec/live-update of the kernel may be performed regularly.
> 
> Usually this live-update is time critical and defaulting of the bias back
> to ENERGY_PERF_BIAS_NORMAL may actually be detrimental to the overall
> update time if processors' time to ramp up/boost are affected.

If this makes your kexecs 7 times faster, please say that here.

Could we also please make this less wishy-washy?  "May actually be
detrimental" does not scream how critical this is for you.

> This patch introduces a kernel command line "intel_epb_no_override"
> which will leave the EPB at performance if during the restoration code path
> it is detected as such.

No "this patch", please:

	https://www.kernel.org/doc/html/next/process/maintainer-tip.html

This also needs documentation of the parameter in
Documentation/admin-guide/kernel-parameters.txt.

Let me see if I can write a sane changelog, summarizing the discussion
here for posterity.  If there's confusion about a v1 patch that's
cleared up in the discussion, it would be wonderful to capture that in
the v2 changelog as opposed to making minimal changes.  How's this?  I
think it captures some of the things that Rafael related and also
additional information about the use case that motivated this effort.

--

Buggy BIOSes set a sane boot-time Energy Performance Bias (EPB) that
causes overheating.  The kernel overrides any boot-time EPB
"performance" bias to "normal" to avoid this.

<Hardware name here> platforms can tolerate a "performance" bias during
boot without overheating.  In addition, because of <root cause(s) here>,
a kexec with a "normal" bias is seven times slower than "performance" to
perform the kexec.  Boot time is critical when performing a
kexec/live-update of the kernel which is running guests VMs since boot
time appears as guest latency or downtime.

Introduce a command-line parameter, "intel_epb_no_override", to skip the
"performance"=>"normal" override.  This allows folks to get a speedy
kexec without exposing other folks with wonky BIOSes to overheating.