linux-kernel - Re: [PATCH RFC] time: validate watchdog clocksource using second best candidate

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Date:   Mon, 20 May 2019 18:04:58 +0300
From:   Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Stephen Boyd <sboyd@...nel.org>,
        John Stultz <john.stultz@...aro.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC] time: validate watchdog clocksource using second best
 candidate

On 18.05.2019 21:26, Thomas Gleixner wrote:
> On Sat, 18 May 2019, Konstantin Khlebnikov wrote:
> 
>> On 18.05.2019 18:17, Thomas Gleixner wrote:
>>> On Wed, 15 May 2019, Konstantin Khlebnikov wrote:
>>>
>>>> Timekeeping watchdog verifies doubtful clocksources using more reliable
>>>> candidates. For x86 it likely verifies 'tsc' using 'hpet'. But 'hpet'
>>>> is far from perfect too. It's better to have second opinion if possible.
>>>>
>>>> We're seeing sudden jumps of hpet counter to 0xffffffff:
>>>
>>> On which kind of hardware? A particular type of CPU or random ones?
>>
>> In general this is very rare event.
>>
>> This exact pattern have been seen ten times or so on several servers with
>> Intel(R) Xeon(R) CPU E5-2660 v4 @ 2.00GHz
>> (this custom built platform with chipset Intel C610)
>>
>> and haven't seen for previous generation
>> Intel(R) Xeon(R) CPU E5-2650 v2 @ 2.60GHz
>> (this is another custom built platform)
> 
> Same chipset? Note the HPET is part of the chipset not of the CPU.

Almost the same. Intel C600.

> 
>> Link was in patch: https://lore.kernel.org/patchwork/patch/667413/
> 
> Hmm. Not really helpful either.
> 
>>>> This patch uses second reliable clocksource as backup for validation.
>>>> For x86 this is usually 'acpi_pm'. If watchdog and backup are not consent
>>>> then other clocksources will not be marked as unstable at this iteration.
>>>
>>> The mess you add to the watchdog code is unholy and that's broken as there
>>> is no guarantee for acpi_pm (or any other secondary watchdog) being
>>> available.
>>
>> ACPI power management timer is a pretty standard x86 hardware.
> 
> Used to be.
> 
>> But my patch should work for any platform with any second reliable
>> clocksource.
> 
> Which is close to zero if PM timer is not exposed.
> 
>> If there is no second clocksource my patch does noting:
>> watchdog_backup stays NULL and backup_consent always true.
> 
> That still does not justify the extra complexity for a few custom built
> systems.

 >
 > Aside of that this leaves the HPET in a half broken state. HPET is not only
 > used as a clock event device it's also exposed by HPET device. So no, if we
 > figure out that HPET is broken on some platforms we have to blacklist and
 > disable it completely and not just duct tape the place which exposes the
 > wreckage.
 >

If re-reading helps then HPET is fine.
This is temporary failure, probably bus issue.


I'll add re-reading with debug logging and try to collect more information this year.