linux-kernel - Re: [PATCH RFC] x86/tsc: Make recalibration default on for TSC_KNOWN

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Message-ID: <20230523081115.GG4253@hirez.programming.kicks-ass.net>
Date:   Tue, 23 May 2023 10:11:15 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Feng Tang <feng.tang@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...el.com>,
        "H . Peter Anvin" <hpa@...or.com>, paulmck@...nel.org,
        rui.zhang@...el.com, x86@...nel.org, linux-kernel@...r.kernel.org,
        bin.gao@...el.com
Subject: Re: [PATCH RFC] x86/tsc: Make recalibration default on for
 TSC_KNOWN_FREQ cases

On Tue, May 23, 2023 at 09:18:23AM +0800, Feng Tang wrote:
> On Mon, May 22, 2023 at 06:11:58PM +0200, Peter Zijlstra wrote:
> > On Mon, May 22, 2023 at 11:20:15PM +0800, Feng Tang wrote:
> > 
> > > And I don't understand the commit log: "On Intel GOLDMONT Atom SoC
> > > TSC is the only reliable clocksource. We mark TSC reliable to avoid
> > > watchdog on it."
> > > 
> > > Clearly the Denventon I found today has both HPET and ACPI_PM timer:
> > > 
> > >   [root@...0 ~]# grep .  /sys/devices/system/clocksource/clocksource0/*
> > >   /sys/devices/system/clocksource/clocksource0/available_clocksource:tsc hpet acpi_pm
> > >   /sys/devices/system/clocksource/clocksource0/current_clocksource:tsc
> > >   
> > > The lscpu info is:
> > >   
> > >   Architecture:                    x86_64
> > >   CPU op-mode(s):                  32-bit, 64-bit
> > >   Address sizes:                   39 bits physical, 48 bits virtual
> > >   Byte Order:                      Little Endian
> > >   CPU(s):                          12
> > >   On-line CPU(s) list:             0-11
> > >   Vendor ID:                       GenuineIntel
> > >   BIOS Vendor ID:                  Intel(R) Corporation
> > >   Model name:                      Intel(R) Atom(TM) CPU C3850 @ 2.10GHz
> > >   BIOS Model name:                 Intel(R) Atom(TM) CPU C3850 @ 2.10GHz  CPU @ 2.1GHz
> > >   BIOS CPU family:                 43
> > >   CPU family:                      6
> > >   Model:                           95
> > >   Thread(s) per core:              1
> > >   Core(s) per socket:              12
> > >   Socket(s):                       1
> > >   Stepping:                        1
> > > 
> > > Maybe this cpu model (0x5F) has been used by some type of platforms
> > > which has met the false alarm watchdog issue.
> > 
> > It has them; but they are not *reliable*.
>  
> Yes, that's possible. I tried to CC the author Bin in case he can
> provide more background or information for his statement, but his
> email address is unreachable now.

IIRC HPET stops in C10 or something stupid like that, I forgot what the
problem with ACPI_PM is.