Re: [RFC PATCH 17/23] watchdog/hardlockup/hpet: Convert the timer's interrupt to NMI

[Randy Dunlap <rdunlap-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>]

On 06/19/2018 05:15 PM, Ricardo Neri wrote:
> On Sat, Jun 16, 2018 at 03:24:49PM +0200, Thomas Gleixner wrote:
>> On Fri, 15 Jun 2018, Ricardo Neri wrote:
>>> On Fri, Jun 15, 2018 at 11:19:09AM +0200, Thomas Gleixner wrote:
>>>> On Thu, 14 Jun 2018, Ricardo Neri wrote:
>>>>> Alternatively, there could be a counter that skips reading the HPET status
>>>>> register (and the detection of hardlockups) for every X NMIs. This would
>>>>> reduce the overall frequency of HPET register reads.
>>>>
>>>> Great plan. So if the watchdog is the only NMI (because perf is off) then
>>>> you delay the watchdog detection by that count.
>>>
>>> OK. This was a bad idea. Then, is it acceptable to have an read to an HPET
>>> register per NMI just to check in the status register if the HPET timer
>>> caused the NMI?
>>
>> The status register is useless in case of MSI. MSI is edge triggered ....
>>
>> The only register which gives you proper information is the counter
>> register itself. That adds an massive overhead to each NMI, because the
>> counter register access is synchronized to the HPET clock with hardware
>> magic. Plus on larger systems, the HPET access is cross node and even
>> slower.
> 
> It starts to sound that the HPET is too slow to drive the hardlockup detector.
> 
> Would it be possible to envision a variant of this implementation? In this
> variant, the HPET only targets a single CPU. The actual hardlockup detector
> is implemented by this single CPU sending interprocessor interrupts to the
> rest of the CPUs.
> 
> In this manner only one CPU has to deal with the slowness of the HPET; the
> rest of the CPUs don't have to read or write any HPET registers. A sysfs
> entry could be added to configure which CPU will have to deal with the HPET
> timer. However, profiling could not be done accurately on such CPU.

Please forgive my simple question:

What happens when this one CPU is the one that locks up?

thnx,
-- 
~Randy