linux-kernel - Re: [PATCH v2 clocksource 6/7] clocksource: Verify HPET and PMTMR when TSC unverified

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Date:   Wed, 1 Feb 2023 14:26:29 -0500
From:   Waiman Long <longman@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        "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,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>, x86@...nel.org
Subject: Re: [PATCH v2 clocksource 6/7] clocksource: Verify HPET and PMTMR
 when TSC unverified

On 2/1/23 05:24, Thomas Gleixner wrote:
> Paul!
>
> On Tue, Jan 24 2023 at 16:27, Paul E. McKenney wrote:
>> On systems with two or fewer sockets, when the boot CPU has CONSTANT_TSC,
>> NONSTOP_TSC, and TSC_ADJUST, clocksource watchdog verification of the
>> TSC is disabled.  This works well much of the time, but there is the
>> occasional production-level system that meets all of these criteria, but
>> which still has a TSC that skews significantly from atomic-clock time.
>> This is usually attributed to a firmware or hardware fault.  Yes, the
>> various NTP daemons do express their opinions of userspace-to-atomic-clock
>> time skew, but they put them in various places, depending on the daemon
>> and distro in question.  It would therefore be good for the kernel to
>> have some clue that there is a problem.
>>
>> The old behavior of marking the TSC unstable is a non-starter because a
>> great many workloads simply cannot tolerate the overheads and latencies
>> of the various non-TSC clocksources.  In addition, NTP-corrected systems
>> sometimes can tolerate significant kernel-space time skew as long as
>> the userspace time sources are within epsilon of atomic-clock time.
>>
>> Therefore, when watchdog verification of TSC is disabled, enable it for
>> HPET and PMTMR (AKA ACPI PM timer).  This provides the needed in-kernel
>> time-skew diagnostic without degrading the system's performance.
> I'm more than unhappy about this. We finally have a point where the TSC
> watchdog overhead can go away without adding TSC=reliable to the kernel
> commandline.
>
> Now you add an unconditionally enforce the watchdog again in a way which
> even cannot be disabled on the kernel command line.
>
> Patently bad idea, no cookies for you!

I have a similar concern about this patch as well. That is why I was 
suggesting to have this enabled for a limited time after boot for sanity 
checking purpose only.

The previous "[PATCH clocksource 5/6] clocksource: Suspend the watchdog 
temporarily when high read latency detected" patch, however, should be 
fine. Right?

Cheers,
Longman