From: Waiman Long <longman@redhat.com>
To: paulmck@kernel.org
Cc: John Stultz <john.stultz@linaro.org>,
Thomas Gleixner <tglx@linutronix.de>,
Stephen Boyd <sboyd@kernel.org>, Feng Tang <feng.tang@intel.com>,
linux-kernel@vger.kernel.org,
Peter Zijlstra <peterz@infradead.org>,
Cassio Neri <cassio.neri@gmail.com>,
Linus Walleij <linus.walleij@linaro.org>,
Frederic Weisbecker <frederic@kernel.org>
Subject: Re: [PATCH v2 0/4] clocksource: Avoid incorrect hpet fallback
Date: Wed, 17 Nov 2021 13:51:51 -0500 [thread overview]
Message-ID: <7cb5ab62-21ea-f649-2009-38b8c1ff283a@redhat.com> (raw)
In-Reply-To: <20211117165426.GG641268@paulmck-ThinkPad-P17-Gen-1>
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
next prev parent reply other threads:[~2021-11-17 18:52 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-11-16 23:44 [PATCH v2 0/4] clocksource: Avoid incorrect hpet fallback Waiman Long
2021-11-16 23:44 ` [PATCH v2 1/4] clocksource: Avoid accidental unstable marking of clocksources Waiman Long
2021-11-16 23:44 ` [PATCH v2 2/4] clocksource: Reduce the default clocksource_watchdog() retries to 2 Waiman Long
2021-11-16 23:44 ` [PATCH v2 3/4] clocksource: Dynamically increase watchdog_max_skew Waiman Long
2021-11-16 23:44 ` [PATCH v2 4/4] clocksource: Add a Kconfig option for WATCHDOG_MAX_SKEW Waiman Long
2021-11-17 16:54 ` [PATCH v2 0/4] clocksource: Avoid incorrect hpet fallback Paul E. McKenney
2021-11-17 18:51 ` Waiman Long [this message]
2021-11-17 21:25 ` Paul E. McKenney
2021-11-17 21:55 ` Waiman Long
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