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

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Message-ID: <AANLkTim_O0CXMCv_qVDKOP72=VC63eoZuvpKWm=mme7k@mail.gmail.com>
Date:	Wed, 1 Sep 2010 10:00:42 +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, 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.
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