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From:   Thomas Gleixner <tglx@linutronix.de>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     linux-kernel@vger.kernel.org, john.stultz@linaro.org,
        sboyd@kernel.org, corbet@lwn.net, Mark.Rutland@arm.com,
        maz@kernel.org, kernel-team@meta.com, neeraju@codeaurora.org,
        ak@linux.intel.com, feng.tang@intel.com, zhengjun.xing@intel.com,
        Waiman Long <longman@redhat.com>,
        John Stultz <jstultz@google.com>,
        "Paul E . McKenney" <paulmck@kernel.org>
Subject: Re: [PATCH clocksource 5/6] clocksource: Suspend the watchdog
 temporarily when high read latency detected
In-Reply-To: <20230105010701.1773895-5-paulmck@kernel.org>
References: <20230105010429.GA1773522@paulmck-ThinkPad-P17-Gen-1>
 <20230105010701.1773895-5-paulmck@kernel.org>
Date:   Wed, 11 Jan 2023 12:26:58 +0100
Message-ID: <87r0w1ia65.ffs@tglx>
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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?