linux-kernel - Re: [clocksource] 8901ecc231: stress-ng.lockbus.ops_per

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Message-ID: <20210805153310.GF4397@paulmck-ThinkPad-P17-Gen-1>
Date:   Thu, 5 Aug 2021 08:33:10 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     Chao Gao <chao.gao@...el.com>, Feng Tang <feng.tang@...el.com>,
        kernel test robot <oliver.sang@...el.com>,
        John Stultz <john.stultz@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Stephen Boyd <sboyd@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Mark Rutland <Mark.Rutland@....com>,
        Marc Zyngier <maz@...nel.org>,
        Xing Zhengjun <zhengjun.xing@...ux.intel.com>,
        Chris Mason <clm@...com>, LKML <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        lkp@...ts.01.org, lkp@...el.com, ying.huang@...el.com,
        zhengjun.xing@...el.com
Subject: Re: [clocksource] 8901ecc231: stress-ng.lockbus.ops_per_sec -9.5%
 regression

On Wed, Aug 04, 2021 at 09:34:13PM -0700, Andi Kleen wrote:
> 
> > My current thought is that if more than (say) 100 consecutive attempts
> > to read the clocksource get hit with excessive delays, it is time to at
> > least do a WARN_ON(), and maybe also time to disable the clocksource
> > due to skew.  The reason is that if reading the clocksource -always-
> > sees excessive delays, perhaps the clock driver or hardware is to blame.
> > 
> > Thoughts?
> 
> On TDX this would be fatal because we don't have a usable fallback source
> 
> (just jiffies). Better try as hard as possible.

At some point, won't the system's suffering in silence become quite the
disservice to its users?

One alternative would be to give a warning splat, but avoid reporting
skew.  Unless there is the traditional 62.5ms of skew, of course.

							Thanx, Paul