linux-kernel - Re: [PATCH v3 2/2] x86/tsc: skip tsc watchdog checking for qualified platforms

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Date:   Wed, 1 Dec 2021 13:19:35 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     Luming Yu <luming.yu@...il.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>,
        Peter Zijlstra <peterz@...radead.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, paulmck@...nel.org,
        rui.zhang@...el.com, andi.kleen@...el.com,
        Len Brown <len.brown@...el.com>, tim.c.chen@...el.com
Subject: Re: [PATCH v3 2/2] x86/tsc: skip tsc watchdog checking for qualified
 platforms

On Tue, Nov 30, 2021 at 11:45:32PM -0500, Luming Yu wrote:
> On Tue, Nov 16, 2021 at 11:18 PM Feng Tang <feng.tang@...el.com> wrote:
> >
> > There are cases that tsc clocksources are wrongly judged as unstable by
> > clocksource watchdogs like hpet, acpi_pm or 'refined-jiffies'. While
> > there is hardly a general reliable way to check the validity of a
> > watchdog, and to protect the innocent tsc, Thomas Gleixner proposed [1]:
> >
> > "I'm inclined to lift that requirement when the CPU has:
> >
> >     1) X86_FEATURE_CONSTANT_TSC
> >     2) X86_FEATURE_NONSTOP_TSC
> >     3) X86_FEATURE_NONSTOP_TSC_S3
> >     4) X86_FEATURE_TSC_ADJUST
> >     5) At max. 4 sockets
> >
> 
> Hi Feng,
> 
> We do  need to decouple  tsc from HPET as the current HPET as a
> clocksource watchdog for tsc
> is only useful to find HPET read skews in some circumstances and the
> variations of HPET read come from many different sources. But
> none of which really came from the tsc quality, AFAICT.
> 
> so this patch is in line with my understanding of the problem.
> So , please use  reviewed-by :  luming.yu@...el.com , if it can help
> the merge of the patch. : -)

Thanks for the review and sharing the real world cases you've met!

- Feng