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SubjectRe: [PATCH v2 0/4] clocksource: Avoid incorrect hpet fallback
From
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.

>
> 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.
>
> 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.


>
> 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.
> 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.

Cheers,
Longman

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