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

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Message-ID: <35268BBA-E97F-4ACB-A1C1-04C94AFDEA65@infradead.org>
Date:   Tue, 05 Dec 2023 12:32:36 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
CC:     pdurrant@...zon.co.uk, bp@...en8.de, dave.hansen@...el.com,
        dave.hansen@...ux.intel.com, hdegoede@...hat.com, hpa@...or.com,
        jalliste@...zon.co.uk, juew@...zon.com, len.brown@...el.com,
        linux-kernel@...r.kernel.org, mingo@...hat.com,
        peterz@...radead.org, rafael.j.wysocki@...el.com,
        tglx@...utronix.de, usama.arif@...edance.com, x86@...nel.org
Subject: Re: [PATCH] x86: intel_epb: Add earlyparam option to keep bias at performance

On 5 December 2023 12:31:19 GMT, "Rafael J. Wysocki" <rafael@...nel.org> wrote:
>On Tue, Dec 5, 2023 at 1:15 PM David Woodhouse <dwmw2@...radead.org> wrote:
>>
>> On 5 December 2023 12:12:09 GMT, "Rafael J. Wysocki" <rafael@...nel.org> wrote:
>> >On Tue, Dec 5, 2023 at 1:00 PM David Woodhouse <dwmw2@...radead.org> wrote:
>> >>
>> >>
>> >> Paul writes:
>> >> > The problem is that this will take effect even on a kexec and hence it is throttling
>> >> > a system that set ENERGY_PERF_BIAS_PERFORMANCE prior to the kexec.  We use kexec to
>> >> > live update the host kernel of our systems whilst leaving virtual machines running.
>> >> > This resetting of the perf bias is having a very detrimental effect on the downtime
>> >> > of our systems across the live update - about a 7 fold increase.
>> >>
>> >> 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.
>> >
>> >> It's literally adding seconds to the startup time in some cases.
>> >>
>> >> And yes, we *particularly* care in the kexec case because guests experience it as excessive steal time. But it ain't great in the general case 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.
>>
>> We don't even care about setting it to an "initial value" during boot. We just want to leave it how it was already set up.
>
>Which does not work on some systems.
>
>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.

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