linux-kernel - Re: [PATCH v2 0/4] clocksource: Avoid incorrect hpet fallback

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Date:   Wed, 17 Nov 2021 13:25:47 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Waiman Long <longman@...hat.com>
Cc:     John Stultz <john.stultz@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Stephen Boyd <sboyd@...nel.org>,
        Feng Tang <feng.tang@...el.com>, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Cassio Neri <cassio.neri@...il.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Frederic Weisbecker <frederic@...nel.org>
Subject: Re: [PATCH v2 0/4] clocksource: Avoid incorrect hpet fallback

On Wed, Nov 17, 2021 at 01:51:51PM -0500, Waiman Long wrote:
> On 11/17/21 11:54, Paul E. McKenney wrote:
> > On Tue, Nov 16, 2021 at 06:44:22PM -0500, Waiman Long wrote:
> > A few questions:
> > 
> > 1.	Once you have all the patches in place, is the increase in
> > 	WATCHDOG_MAX_SKEW from 50us to 100us necessary?
> 
> I think so. Using Feng's reproducer, I was able to cause a hpet-hpet delay
> of more than 90us on a 1-socket system. With a default 50us
> WATCHDOG_MAX_SKEW, the chance of a warning showing up will be much higher.
> Trying to minimize the chance that a warning may appear is my primary reason
> to increase WATCHDOG_MAX_SKEW.

Should we downgrade the "had to retry read" complain to pr_info(),
and make the only real warning be the case where a large number of
consecutive read attempts fail?  I believe that Heiner Kallweit was
looking for something like this.

> > 2.	The reason for having cs->uncertainty_margin set to
> > 	2*WATCHDOG_MAX_SKEW was to allow for worst-case skew from both
> > 	the previous and the current reading.  Are you sure that
> > 	dropping back to WATCHDOG_MAX_SKEW avoids false positives?
> 
> I can remove the hunk of changing cs->uncertainty_margin. It is critical for
> this patch.

Assuming "not critical", good!

> > 3.	In patch 3/4, shouldn't clock_skew_skip be a field in the
> > 	clocksource structure rather than a global?  If a system had
> > 	multiple clocks being checked, wouldn't having this as a field
> > 	make things more predictable?  Or am I missing something subtle
> > 	here?
> 
> Yes, you are right. I should have put it into clocksource structure. I will
> make the change in v3.

Sounds good!  Looking forward to v3!

> > 4.	These are intended to replace this commit in -rcu, correct?
> > 
> > 	9d5739316f36 ("clocksource: Forgive repeated long-latency watchdog clocksource reads")
> > 
> > 	But not this commit, correct?
> > 
> > 	5444fb39fd49 ("torture: Test splatting for delay-ridden clocksources")
>
> Yes, that is my intention.

Very good, thank you!

> > And would you like me to queue these, or would you rather send them
> > separately?  (Either way works for me, just please let me know.)
> 
> I don't have a preference either way. If you are willing to queue these, it
> will be great too.

Happy to do so!

							Thanx, Paul