linux-kernel - Re: [LKP] Re: [clocksource] 6c52b5f3cf: stress-ng.opcode.ops_per

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Message-ID: <87tunwvmmu.ffs@nanos.tec.linutronix.de>
Date:   Fri, 23 Apr 2021 21:14:49 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     paulmck@...nel.org, Feng Tang <feng.tang@...el.com>
Cc:     Xing Zhengjun <zhengjun.xing@...ux.intel.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>,
        Chris Mason <clm@...com>, LKML <linux-kernel@...r.kernel.org>,
        lkp@...ts.01.org, lkp@...el.com
Subject: Re: [LKP] Re: [clocksource] 6c52b5f3cf: stress-ng.opcode.ops_per_sec -14.4% regression

On Thu, Apr 22 2021 at 07:24, Paul E. McKenney wrote:
> On Thu, Apr 22, 2021 at 03:41:26PM +0800, Feng Tang wrote:
> So what are our options?
>
> 1.	Clear CLOCK_SOURCE_MUST_VERIFY from tsc-early.
>
> 2.	#1, but add tsc-early into the watchdog list and set
> 	CLOCK_SOURCE_MUST_VERIFY once it is better calibrated.
>
> 3.	Add a field to struct clocksource that, if non-zero, gives
> 	the maximum drift in nanoseconds per half second (AKA
> 	WATCHDOG_INTERVAL).  If zero, the WATCHDOG_MAX_SKEW value
> 	is used.  Set this to (say) 150,000ns for tsc-early.
>
> 4.	As noted earlier, increase WATCHDOG_MAX_SKEW to 150 microseconds,
> 	which again is not a good approach given the real-world needs
> 	of real-world applications.
>
> 5.	Your ideas here.

#3 or add a flag to the clocksource which says 'frequency is guesswork' and
increase the threshold based on that.

If that flag is still set max_drift is != 0 after 20 seconds yell.

Thanks,

        tglx