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

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Date:	Wed, 1 Sep 2010 07:30:55 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Don Zickus <dzickus@...hat.com>
Cc:	peterz@...radead.org, gorcunov@...il.com, fweisbec@...il.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] [lockup detector] sync touch_*_watchdog back to old
 semantics


* 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
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