Re: [PATCH 1/3] [lockup detector] sync touch_*_watchdog back to old semantics

[Cyrill Gorcunov]

On 9/1/10, Ingo Molnar <mingo@xxxxxxx> wrote:
>
> * Don Zickus <dzickus@xxxxxxxxxx> wrote:
>
>>  void touch_nmi_watchdog(void)
>>  {
>> -	__get_cpu_var(watchdog_nmi_touch) = true;
>> +	if (watchdog_enabled) {
>> +		unsigned cpu;
>> +
>> +		for_each_present_cpu(cpu) {
>> +			if (per_cpu(watchdog_nmi_touch, cpu) != true)
>> +				per_cpu(watchdog_nmi_touch, cpu) = true;
>> +		}
>
> Hm, this is going to be a scalability nightmare with lots of CPUs. Not
> only do we have a nr_cpus loop, but we touch per-cpu areas of _other_
> CPUs - a big scalability nono.
>
> Why do we need to do this? We never needed to touch other CPU's NMI
> lockup accounting data areas - why has this changed? The changelog does
> not explain this.
>
> Thanks,
>
> 	Ingo
>
I believe this came from old nmi watchdog code where it might be
useful when nmi watchdog activated via io-apic. I'm trying to figure
out if we really need it still.
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