linux-kernel - Re: [PATCH 1/3] [lockup detector] sync touch_*

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Date:	Wed, 1 Sep 2010 11:01:06 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Don Zickus <dzickus@...hat.com>, peterz@...radead.org,
	fweisbec@...il.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] [lockup detector] sync touch_*_watchdog back to old semantics

On 9/1/10, Cyrill Gorcunov <gorcunov@...il.com> wrote:
> On 9/1/10, Ingo Molnar <mingo@...e.hu> wrote:
>>
>> * Don Zickus <dzickus@...hat.com> 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...
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