linux-kernel - Re: [PATCH RFC clocksource 2/5] clocksource: Retry clock read if long delays detected

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Date:   Wed, 6 Jan 2021 11:53:21 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Rik van Riel <riel@...com>
Cc:     linux-kernel@...r.kernel.org, kernel-team@...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>
Subject: Re: [PATCH RFC clocksource 2/5] clocksource: Retry clock read if
 long delays detected

On Wed, Jan 06, 2021 at 11:28:00AM -0500, Rik van Riel wrote:
> On Tue, 2021-01-05 at 16:41 -0800, paulmck@...nel.org wrote:
> > 
> > @@ -203,7 +204,6 @@ static void
> > clocksource_watchdog_inject_delay(void)
> >  		injectfail = inject_delay_run;
> >  	if (!(++injectfail / inject_delay_run % inject_delay_freq)) {
> >  		printk("%s(): Injecting delay.\n", __func__);
> > -		injectfail = 0;
> >  		for (i = 0; i < 2 * WATCHDOG_THRESHOLD / NSEC_PER_MSEC;
> > i++)
> >  			udelay(1000);
> 
> Wait, patch 1 just added that line?
> 
> Should patch 1 not add it and this
> patch go without
> this removal? :)

Good catch, will fix.  ;-)

> +               wdagain_nsec = clocksource_cyc2ns(delta, watchdog-
> >mult, watchdog->shift);
> +               if (wdagain_nsec < 0 || wdagain_nsec >
> WATCHDOG_MAX_SKEW) {
> +                       wderr_nsec = wdagain_nsec;
> +                       if (nretries++ < max_read_retries)
> +                               goto retry;
> +               }
> 
> Given that clocksource_cyc2ns uses unsigned multiplication
> followed by a right shift, do we need to test for <0?

I am worried about the possibility of the "shift" argument to
clocksource_cyc2ns() being zero.  For example, unless I am missing
something, clocksource_tsc has a zero .shift field.

							Thanx, Paul