linux-kernel - Re: [PATCH v7 clocksource] Do not mark clocks unstable due to delays for v5.13

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Message-ID: <87im4u4lft.ffs@nanos.tec.linutronix.de>
Date:   Sat, 10 Apr 2021 10:01:58 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     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@...com, neeraju@...eaurora.org,
        ak@...ux.intel.com
Subject: Re: [PATCH v7 clocksource] Do not mark clocks unstable due to delays for v5.13

On Fri, Apr 02 2021 at 15:48, Paul E. McKenney wrote:
> Hello!
>
> If there is a sufficient delay between reading the watchdog clock and the
> clock under test, the clock under test will be marked unstable through no
> fault of its own.  This series checks for this, doing limited retries
> to get a good set of clock reads.  If the clock is marked unstable
> and is marked as being per-CPU, cross-CPU synchronization is checked.
> This series also provides delay injection, which may be enabled via
> kernel boot parameters to test the checking for delays.
>
> Note that "sufficient delay" can be provided by SMIs, NMIs, and of course
> vCPU preemption.

I buy the vCPU preemption part and TBH guests should not have that
watchdog thing active at all for exactly this reason.

SMI, NMI injecting 62.5ms delay? If that happens then the performance of
the clocksource is the least of your worries.

Thanks,

        tglx