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 09:26:55 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     "Paul E. McKenney" <paulmck@...nel.org>
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>, x86@...nel.org,
        linux-kernel@...r.kernel.org, rui.zhang@...el.com,
        andi.kleen@...el.com, 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 03:37:26PM -0800, Paul E. McKenney wrote:
> On Wed, Dec 01, 2021 at 12:19:43AM +0100, Thomas Gleixner wrote:
> > On Tue, Nov 30 2021 at 14:48, Paul E. McKenney wrote:
> > > On Tue, Nov 30, 2021 at 10:55:45PM +0100, Thomas Gleixner wrote:
> > >> > OK, HPET or nothing, then.
> > >> 
> > >> Older machines also have pm_timer. But those beasts seem to have lost
> > >> that too.
> > >
> > > I suppose that one way of avoiding clock-skew messages is to have only
> > > one clock.
> > 
> > Indeed. It's a complete mystery why it takes ages to implement reliable
> > clocks in hardware.
> 
> That one is easy.  It is because the previous clocksource watchdog was
> too lenient.  ;-)
> 
> (Sorry, couldn't resist...)
> 
> > >> >> We really need to remove the watchdog requirement for modern hardware.
> > >> >> Let me stare at those patches and get them merged.
> > >> >
> > >> > You are more trusting of modern hardware than I am, but for all I know,
> > >> > maybe rightfully so.  ;-)
> > >> 
> > >> Well, I rather put a bet on the hardware, which has become reasonable
> > >> over the last decade, than on trying to solve a circular dependency
> > >> problem with tons of heuristics which won't ever work correctly.
> > >
> > > Use of HPET to check the interval length would not be circular, right?
> > 
> > As long as the HPET works reliably :)
> 
> Is it also a complete mystery why clocksources previously deemed
> reliable no longer work reliably?  ;-)

For HPET, it's a long story :) Back in 2012 or so, the HPET on Baytrail
platform has a new feature that it will stop counting in PC10 (a cpuidle
state), which prevent it to be a clocksource, and we have to disable
HPET explicitly for that platform. Since then, some new platforms also
have the same feature, and their HPET got disabled too. 

Thanks,
Feng