Subject: Re: + i386-time-avoid-pit-smp-lockups.patch added to -mm tree
From: Daniel Walker <dwalker@mvista.com>
To: Andi Kleen <ak@suse.de>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>, linux-kernel@vger.kernel.org,
       johnstul@us.ibm.com, mingo@elte.hu, tglx@linutronix.de
In-Reply-To: <200610191650.25678.ak@suse.de>
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	 <200610191626.10662.ak@suse.de> <45379031.601@yahoo.com.au>
	 <200610191650.25678.ak@suse.de>
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Date: Thu, 19 Oct 2006 08:01:53 -0700
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On Thu, 2006-10-19 at 16:50 +0200, Andi Kleen wrote:
> On Thursday 19 October 2006 16:48, Nick Piggin wrote:
> > Andi Kleen wrote:
> > >>An SMP kernel can boot on UP hardware, in which case I think
> > >>num_possible_cpus() will be 1, won't it?
> > > 
> > > 
> > > 0 was a typo, i meant 1 for UP of course. 0 would be nonsensical.
> > 
> > Sure, I realised that. For a UP kernel, the test will compile away.
> > 
> > But Daniel seems to say there is dead code that could be compiled
> > out for SMP kernels. I just don't think that is possible because the
> > SMP kernel can boot a UP system where num_possible_cpus() is 1.
> 
> I thought he meant !CONFIG_SMP kernels.

definitely CONFIG_SMP=y . The code block I quoted would disable the PIT
clocksource w/ more than one cpu. So the pit clocksource is just dead
weight on SMP systems. However, like Nick was saying it's possible to
boot CONFIG_SMP on a UP system, but removing the pit in that situation
may not hurt anything.

Daniel 

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