linux-kernel - Re: [clocksource] 8c30ace35d: WARNING:at_kernel/time/clocksource.c:#clocksource

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Date:   Wed, 28 Apr 2021 11:31:41 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Feng Tang <feng.tang@...el.com>,
        kernel test robot <oliver.sang@...el.com>,
        0day robot <lkp@...el.com>,
        John Stultz <john.stultz@...aro.org>,
        Stephen Boyd <sboyd@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Mark Rutland <Mark.Rutland@....com>,
        Marc Zyngier <maz@...nel.org>, Andi Kleen <ak@...ux.intel.com>,
        Xing Zhengjun <zhengjun.xing@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>, lkp@...ts.01.org,
        kernel-team@...com, neeraju@...eaurora.org, zhengjun.xing@...el.com
Subject: Re: [clocksource]  8c30ace35d:
 WARNING:at_kernel/time/clocksource.c:#clocksource_watchdog

On Wed, Apr 28, 2021 at 12:14:49PM +0200, Thomas Gleixner wrote:
> On Tue, Apr 27 2021 at 18:48, Paul E. McKenney wrote:
> > On Tue, Apr 27, 2021 at 11:09:49PM +0200, Thomas Gleixner wrote:
> >> Paul,
> >> 
> >> On Tue, Apr 27 2021 at 10:50, Paul E. McKenney wrote:
> >> > On Tue, Apr 27, 2021 at 06:37:46AM -0700, Paul E. McKenney wrote:
> >> >> I suppose that I give it (say) 120 seconds instead of the current 60,
> >> >> which might be the right thing to do, but it does feel like papering
> >> >> over a very real initramfs problem.  Alternatively, I could provide a
> >> >> boot parameter allowing those with slow systems to adjust as needed.
> >> >
> >> > OK, it turns out that there are systems for which boot times in excess
> >> > of one minute are expected behavior.  They are a bit rare, though.
> >> > So what I will do is keep the 60-second default, add a boot parameter,
> >> > and also add a comment by the warning pointing out the boot parameter.
> >> 
> >> Oh, no. This starts to become yet another duct tape horror show.
> >> 
> >> I'm not at all against a more robust and resilent watchdog mechanism,
> >> but having a dozen knobs to tune and heuristics which are doomed to fail
> >> is not a solution at all.
> >
> > One problem is that I did the .max_drift patch backwards.  I tightened
> > the skew requirements on all clocks except those specially marked, and
> > I should have done the reverse.  With that change, all of the clocks
> > except for clocksource_tsc would work (or as the case might be, fail to
> > work) in exactly the same way that they do today, but still rejecting
> > false-positive skew events due to NMIs, SMIs, vCPU preemption, and so on.
> >
> > Then patch v10 7/7 can go away completely, and patch 6/7 becomes much
> > smaller (and gets renamed), for example, as shown below.
> >
> > Does that help?
> 
> No. Because the problem is on both ends. We have TSC early which has
> inaccurate frequency and we have watchdogs which are inaccurate,
> i.e. refined jiffies.
> 
> So the threshold has to take both into account.

Got it, and will fix.

							Thanx, Paul