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

[Cyrill Gorcunov]

On 9/1/10, Cyrill Gorcunov <gorcunov@xxxxxxxxx> wrote:
> 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.
>
Well, we can't drop it or make per-cpu specific, for example we need
it in case of panic with watchdog enabled and panic timeout set, or
boot delay set and etc. Seems same applies to printk_delay. Hmm...
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/