linux-kernel - Re: [PATCH] x86/tsc: Add tsc_guess flag disabling CPUID.16h use for tsc calibration

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Message-ID: <CALCETrVBbj+tKxu0sPwQTPMS+UV6_6cRVPXF8tqfGFOaxtH17A@mail.gmail.com>
Date:   Wed, 15 Jan 2020 12:15:36 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Krzysztof Piecuch <piecuch@...tonmail.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "juri.lelli@...hat.com" <juri.lelli@...hat.com>,
        "malat@...ian.org" <malat@...ian.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "mzhivich@...mai.com" <mzhivich@...mai.com>,
        "viresh.kumar@...aro.org" <viresh.kumar@...aro.org>,
        "drake@...lessm.com" <drake@...lessm.com>,
        "rafael.j.wysocki@...el.com" <rafael.j.wysocki@...el.com>,
        "x86@...nel.org" <x86@...nel.org>, "hpa@...or.com" <hpa@...or.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>
Subject: Re: [PATCH] x86/tsc: Add tsc_guess flag disabling CPUID.16h use for
 tsc calibration

On Tue, Jan 14, 2020 at 6:39 AM Krzysztof Piecuch
<piecuch@...tonmail.com> wrote:
>
> Changing base clock frequency directly impacts tsc hz but not CPUID.16h
> values. An overclocked CPU supporting CPUID.16h and partial CPUID.15h
> support will set tsc hz according to "best guess" given by CPUID.16h
> relying on tsc_refine_calibration_work to give better numbers later.
> tsc_refine_calibration_work will refuse to do its work when the outcome is
> off the early tsc hz value by more than 1% which is certain to happen on an
> overclocked system.
>
> Signed-off-by: Krzysztof Piecuch <piecuch@...tonmail.com>
> ---
>  Documentation/admin-guide/kernel-parameters.txt |  6 ++++++
>  arch/x86/kernel/tsc.c                           | 12 ++++++++++--
>  2 files changed, 16 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index ade4e6ec23e0..54ae9e153a19 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -4905,6 +4905,12 @@
>                         interruptions from clocksource watchdog are not
>                         acceptable).
>
> +       tsc_guess=      [X86,INTEL] Don't use data provided by CPUID.16h during
> +                       early tsc calibration. Disabling this may be useful for
> +                       CPUs with altered base clocks.
> +                       Format: <bool> (1/Y/y=enable, 0/N/n=disable)
> +                       default: enabled

This has more negatives than makes sense.  I also think it's wrong.  How about:

tsc_guess= ... Use data provided by CPUID.16h ...