Re: [Xen-devel] [PATCH] x86/nmi: lower initial watchdog frequency to avoid boot hangs

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 06.02.18 at 17:14, <igor.druzhinin@xxxxxxxxxx> wrote:
> On 06/02/18 16:07, Jan Beulich wrote:
>>>>> On 05.02.18 at 22:18, <igor.druzhinin@xxxxxxxxxx> wrote:
>>> --- a/xen/arch/x86/nmi.c
>>> +++ b/xen/arch/x86/nmi.c
>>> @@ -34,7 +34,8 @@
>>>  #include <asm/apic.h>
>>>  
>>>  unsigned int nmi_watchdog = NMI_NONE;
>>> -static unsigned int nmi_hz = HZ;
>>> +/* initial watchdog frequency - shouldn't be too high to avoid boot hangs 
> */
>>> +static unsigned int nmi_hz = HZ / 10;
>> 
>> For one - the comment should explain what "too high" means.
>> Further - what if on another system 10Hz is still too high? I also hope
>> you realize that you slow down boot a little for everyone just
>> because of this one machine model. Can the lower frequency perhaps
>> be set via DMI quirk, or otherwise obtain a command line override
>> (perhaps something like "watchdog=probe:10Hz")?
>>
> 
> I can improve the comment message.
> Why does this change slow down anything while I'm lowering the frequency
> - not making it higher?

We wait for two occurrences of the NMI in wait_for_nmis().

> The alternative approach would be to reshuffle
> the code and take the reason before programming the next interrupt as
> suggested before. In that case the actual frequency would be adjusted
> naturally I think.

Thinking about this, reading the reason early seems like a good idea
to me irrespective of the issue here.

Jan


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