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

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Message-ID: <CAJZ5v0iVvMLK_VcPRJ4sW1eOh0EtfcFvKjH5j1y1GbA0Y6q--Q@mail.gmail.com>
Date:   Tue, 5 Dec 2023 13:12:09 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     David Woodhouse <dwmw2@...radead.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,
        rafael@...nel.org, 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 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.