linux-kernel - Re: [PATCH v10 clocksource 1/7] clocksource: Provide module parameters to inject delays in watchdog

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Message-ID: <20210426152801.GY975577@paulmck-ThinkPad-P17-Gen-1>
Date:   Mon, 26 Apr 2021 08:28:01 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     tglx@...utronix.de, linux-kernel@...r.kernel.org,
        john.stultz@...aro.org, sboyd@...nel.org, corbet@....net,
        Mark.Rutland@....com, maz@...nel.org, kernel-team@...com,
        neeraju@...eaurora.org, feng.tang@...el.com,
        zhengjun.xing@...el.com, Chris Mason <clm@...com>
Subject: Re: [PATCH v10 clocksource 1/7] clocksource: Provide module
 parameters to inject delays in watchdog

On Sun, Apr 25, 2021 at 09:07:36PM -0700, Andi Kleen wrote:

[ . . . ]

> > +	clocksource.inject_delay_period= [KNL]
> > +			Number of calls to clocksource_watchdog() before
> > +			delays are injected between reads from the
> > +			two clocksources.  Values of zero disable this
> > +			delay injection.  These delays can cause clocks
> > +			to be marked unstable, so use of this parameter
> > +			should therefore be avoided on production systems.
> > +			Defaults to zero (disabled).
> > +
> > +	clocksource.inject_delay_repeat= [KNL]
> > +			Number of repeated clocksource_watchdog() delay
> > +			injections per period.	If inject_delay_period
> > +			is five and inject_delay_repeat is three, there
> > +			will be five delay-free reads followed by three
> > +			delayed reads.
> 
> I'm not sure command line options are the right way to do this.
> How about integrating it with the fault injection framework in debugfs.
> 
> This way syzkaller etc. can play with it, which long term would
> give much better test coverage.
> 
> This wouldn't allow boot time coverage, but presumably that's not
> too important here.

Boot-time coverage is important, as we saw in kbuild test robot testing
of v9 of this patchset, which triggered clocksource_tsc_early, but not
clocksource_tsc.  Note that v10 avoids this triggering.

							Thanx, Paul