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From: "Durrant, Paul" <pdurrant@amazon.co.uk>
To: "Allister, Jack" <jalliste@amazon.co.uk>, "linux-kernel@vger.kernel.org"
	<linux-kernel@vger.kernel.org>
CC: "Allister, Jack" <jalliste@amazon.co.uk>, "Rafael J . Wysocki"
	<rafael@kernel.org>, "Wang, Jue" <juew@amazon.com>, Usama Arif
	<usama.arif@bytedance.com>, Jonathan Corbet <corbet@lwn.net>, Thomas Gleixner
	<tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, Borislav Petkov
	<bp@alien8.de>, Dave Hansen <dave.hansen@linux.intel.com>, "x86@kernel.org"
	<x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>, "Paul E. McKenney"
	<paulmck@kernel.org>, Randy Dunlap <rdunlap@infradead.org>, Tejun Heo
	<tj@kernel.org>, Peter Zijlstra <peterz@infradead.org>, Yan-Jie Wang
	<yanjiewtw@gmail.com>, Hans de Goede <hdegoede@redhat.com>,
	"linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>
Subject: RE: [PATCH v6] x86: intel_epb: Add earlyparam option to keep bias at
 performance
Thread-Topic: [PATCH v6] x86: intel_epb: Add earlyparam option to keep bias at
 performance
Thread-Index: AQHaPu06oUuGY3zUqk+MuinfJmm8L7DJYJiQ
Date: Thu, 4 Jan 2024 09:22:35 +0000
Message-ID: <12f10bbcba78463fa274e17fad305894@amazon.co.uk>
References: <ff3a0382-734d-4f46-bd35-ffa1f53a3ac3@intel.com>
 <20240104090551.46251-1-jalliste@amazon.com>
In-Reply-To: <20240104090551.46251-1-jalliste@amazon.com>
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> -----Original Message-----
> From: Jack Allister <jalliste@amazon.com>
> Sent: 04 January 2024 09:06
> Cc: Allister, Jack <jalliste@amazon.co.uk>; Rafael J . Wysocki
> <rafael@kernel.org>; Durrant, Paul <pdurrant@amazon.co.uk>; Wang, Jue
> <juew@amazon.com>; Usama Arif <usama.arif@bytedance.com>; Jonathan Corbet
> <corbet@lwn.net>; Thomas Gleixner <tglx@linutronix.de>; Ingo Molnar
> <mingo@redhat.com>; Borislav Petkov <bp@alien8.de>; Dave Hansen
> <dave.hansen@linux.intel.com>; x86@kernel.org; H. Peter Anvin
> <hpa@zytor.com>; Paul E. McKenney <paulmck@kernel.org>; Randy Dunlap
> <rdunlap@infradead.org>; Tejun Heo <tj@kernel.org>; Peter Zijlstra
> <peterz@infradead.org>; Yan-Jie Wang <yanjiewtw@gmail.com>; Hans de Goede
> <hdegoede@redhat.com>; linux-doc@vger.kernel.org; linux-
> kernel@vger.kernel.org
> Subject: [PATCH v6] x86: intel_epb: Add earlyparam option to keep bias at
> performance
>=20
> Buggy BIOSes may not set a sane boot-time Energy Performance Bias (EPB).
> A result of this may be overheating or excess power usage. The kernel
> overrides any boot-time EPB "performance" bias to "normal" to avoid this.
>=20
> When used in data centers it is preferable keep the EPB at "performance"
> when performing a live-update of the host kernel via a kexec to the new
> kernel. This is due to boot-time being critical when performing the kexec
> as running guest VMs will perceieve this as latency or downtime.
>=20
> On Intel Xeon Ice Lake platforms it has been observed that a combination
> of
> EPB being set to "normal" alongside HWP (Intel Hardware P-states) being
> enabled/configured during or close to the kexec causes an increases the
> live-update/kexec downtime by 7 times compared to when the EPB is set to
> "performance".
>=20
> Introduce a command-line parameter, "intel_epb=3Dpreserve", to skip the
> "performance" -> "normal" override/workaround. This maintains prior
> functionality when no parameter is set, but adds in the ability to stay a=
t
> performance for a speedy kexec if a user wishes.
>=20
> Signed-off-by: Jack Allister <jalliste@amazon.com>
> Acked-by: Rafael J. Wysocki <rafael@kernel.org>
> Cc: Paul Durrant <pdurrant@amazon.com>
> Cc: Jue Wang <juew@amazon.com>
> Cc: Usama Arif <usama.arif@bytedance.com>
> ---
>  .../admin-guide/kernel-parameters.txt         |  9 ++++++++
>  arch/x86/kernel/cpu/intel_epb.c               | 22 +++++++++++++++++--
>  2 files changed, 29 insertions(+), 2 deletions(-)
>=20

Reviewed-by: Paul Durrant <pdurrant@amazon.com>