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Date:   Tue, 05 Dec 2023 12:32:36 +0000
From:   David Woodhouse <dwmw2@infradead.org>
To:     "Rafael J. Wysocki" <rafael@kernel.org>
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Subject: =?US-ASCII?Q?Re=3A_=5BPATCH=5D_x86=3A_intel=5Fepb=3A_Add_earlyp?= =?US-ASCII?Q?aram_option_to_keep_bias_at_performance?=
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On 5 December 2023 12:31:19 GMT, "Rafael J=2E Wysocki" <rafael@kernel=2Eorg=
> wrote:
>On Tue, Dec 5, 2023 at 1:15=E2=80=AFPM David Woodhouse <dwmw2@infradead=
=2Eorg> wrote:
>>
>> On 5 December 2023 12:12:09 GMT, "Rafael J=2E Wysocki" <rafael@kernel=
=2Eorg> wrote:
>> >On Tue, Dec 5, 2023 at 1:00=E2=80=AFPM David Woodhouse <dwmw2@infradea=
d=2Eorg> wrote:
>> >>
>> >>
>> >> Paul writes:
>> >> > The problem is that this will take effect even on a kexec and henc=
e it is throttling
>> >> > a system that set ENERGY_PERF_BIAS_PERFORMANCE prior to the kexec=
=2E  We use kexec to
>> >> > live update the host kernel of our systems whilst leaving virtual =
machines running=2E
>> >> > This resetting of the perf bias is having a very detrimental effec=
t on the downtime
>> >> > of our systems across the live update - about a 7 fold increase=2E
>> >>
>> >> It isn't just about kexec, is it? Even in a clean boot why wouldn't =
we want to stay in performance mode until the kernel has *finished* booting=
?
>> >
>> >Because it may overheat during that period=2E
>> >
>> >> It's literally adding seconds to the startup time in some cases=2E
>> >>
>> >> And yes, we *particularly* care in the kexec case because guests exp=
erience it as excessive steal time=2E But it ain't great in the general cas=
e either, surely?
>> >
>> >So IMV it would be perfectly fine to add a command line arg to provide
>> >the initial value of energy_perf_bias for the ones who know what they
>> >are doing=2E
>>
>> We don't even care about setting it to an "initial value" during boot=
=2E We just want to leave it how it was already set up=2E
>
>Which does not work on some systems=2E
>
>The problem here is that the BIOS cannot be trusted to set the initial
>value that makes sense for the given platform and that's why the code
>is the way it is=2E

Yeah, I understand why we have the existing hack=2E We just need a way to =
disable it when it's doing the wrong thing=2E