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Date: Wed, 1 Sep 2010 09:20:16 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>, peterz@infradead.org, fweisbec@gmail.com,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/3] [lockup detector] sync touch_*_watchdog back to old
 semantics
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* Cyrill Gorcunov <gorcunov@gmail.com> wrote:

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

Ok - can you cite the old watchdog code, did it really do a nr_cpus 
loop?

Thanks,

	Ingo