linux-kernel - Re: hangs in verify_pmtmr

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Date:	Wed, 03 Jun 2015 14:11:40 -0700
From:	Dave Hansen <dave@...1.net>
To:	John Stultz <john.stultz@...aro.org>
CC:	LKML <linux-kernel@...r.kernel.org>,
	"Yu, Fenghua" <fenghua.yu@...el.com>,
	the arch/x86 maintainers <x86@...nel.org>
Subject: Re: hangs in verify_pmtmr_rate()

On 06/03/2015 01:46 PM, John Stultz wrote:
> On Wed, Jun 3, 2015 at 12:55 PM, Dave Hansen <dave@...1.net> wrote:
>> I'm seeing boot hangs when trying to boot a 32-bit 4.1.0-rc5 kernel on
>> some 64-bit CPUs (I'm not sure if it is a regression).  The NMI watchdog
>> shows init_acpi_pm_clocksource() as the last thing in the backtrace,
>> specifically verify_pmtmr_rate()'s I/O instructions.  It appears to be
>> mach_countup()'s while loop that gets stuck.
>>
>> Booting with "pmtmr=0" works around this for me, as would unsetting
>> CONFIG_X86_PM_TIMER I'd imagine.
>>
>> The hardware I'm doing this on is a bit wonky and I think the hpet is
>> broken on it.
>>
>> Does this look like *really* broken hardware, or something that we
>> should be detecting and able to recover from?
> 
> Hrm. Does this machine have a working PIT?

It _should_.  :)

> Does pit_calibrate_tsc() end up being used on this box to calibrate
> the TSC (its similar logic, so it should get stuck in the same way),
> or does it use a different method for tsc calibration?

I end up seeing:

> 	tsc: Fast TSC calibration failed
> 	tsc: Using PIT calibration value
> 	tsc: Detected 911.616 MHz processor
> 	Calibrating delay loop (skipped) , value calculated using timer frequency.. 1823.23 BogoMIPS... (lpj=3646464)

Which makes it look like native_calibrate_tsc() managed to successfully
do a pit_calibrate_tsc() and got to the code below (otherwise  we would
have hit the (tsc_pit_min == ULONG_MAX) case).

>         /* We don't have an alternative source, use the PIT calibration value */
>         if (!hpet && !ref1 && !ref2) {
>                 pr_info("Using PIT calibration value\n");
>                 return tsc_pit_min;
>         }

I later see:

> 	timekeeping watchdog: Marking clocksource 'tsc' as unstable, because skew is too large:
> 	 'refined-jiffies' wd_now: ffeef86 wd_last: fffeef09 mask: ffffffff


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