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SubjectRe: [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

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