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

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Date:   Tue, 05 Dec 2023 12:00:04 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     pdurrant@...zon.co.uk
CC:     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


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? 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?