linux-kernel - Re: [PATCH clocksource 5/6] clocksource: Suspend the watchdog temporarily when high read latency detected

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Date:   Wed, 11 Jan 2023 12:26:58 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     linux-kernel@...r.kernel.org, john.stultz@...aro.org,
        sboyd@...nel.org, corbet@....net, Mark.Rutland@....com,
        maz@...nel.org, kernel-team@...a.com, neeraju@...eaurora.org,
        ak@...ux.intel.com, feng.tang@...el.com, zhengjun.xing@...el.com,
        Waiman Long <longman@...hat.com>,
        John Stultz <jstultz@...gle.com>,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH clocksource 5/6] clocksource: Suspend the watchdog
 temporarily when high read latency detected

On Wed, Jan 04 2023 at 17:07, Paul E. McKenney wrote:
> This can be reproduced by running memory intensive 'stream' tests,
> or some of the stress-ng subcases such as 'ioport'.
>
> The reason for these issues is the when system is under heavy load, the
> read latency of the clocksources can be very high.  Even lightweight TSC
> reads can show high latencies, and latencies are much worse for external
> clocksources such as HPET or the APIC PM timer.  These latencies can
> result in false-positive clocksource-unstable determinations.
>
> Given that the clocksource watchdog is a continual diagnostic check with
> frequency of twice a second, there is no need to rush it when the system
> is under heavy load.  Therefore, when high clocksource read latencies
> are detected, suspend the watchdog timer for 5 minutes.

We should have enough heuristics in place by now to qualify the output of
the clocksource watchdog as a random number generator, right?