linux-kernel - Re: [PATCH] x86_64: remove never used nodenumer in pda

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Date:	Tue, 19 Feb 2008 21:59:38 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Mike Travis <travis@....com>
Cc:	Andi Kleen <andi@...stfloor.org>, Yinghai Lu <Yinghai.Lu@....COM>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	lameter@....com
Subject: Re: [PATCH] x86_64: remove never used nodenumer in pda

On Tue, Feb 19, 2008 at 10:32:38AM -0800, Mike Travis wrote:
> Andi Kleen wrote:
> > On Tue, Feb 19, 2008 at 07:48:54AM -0800, Mike Travis wrote:
> >> Andi Kleen wrote:
> >>> Yinghai Lu <Yinghai.Lu@....COM> writes:
> >>>
> >>>> we don't need copy too. already have x86_cpu_to_node_map
> >>> That's a regression (probably from Mike's patches?). Until recently it was 
> >>> used.
> >> Yes, I had removed it because I couldn't find any references to it.
> > 
> > Hmm, maybe it regressed earlier. Sorry if I blamed you incorrectly.
> > Anyways at some point this definitely worked. I remember writing
> > the code ;-) 
> > 
> >> And reading one's own percpu variables should be as efficient as
> >> reading one's own pda.
> > 
> > Sure, but the point is that getting the current node is a common
> > operation, so it should be a single reference and not go through a 
> > lookup table.
> > 
> > How it is actually implemented -- through PDA or through you new
> > equivalent per cpu variables -- does not really matter as long 
> > as the resulting code is only a single instruction. Using
> > the lookup array from the cpu number is wrong.
> > 
> > My patch just fixed it back to use the PDA in the old style for now,
> > but if all your per cpu stuff is merged (I admit I haven't tracked 
> > if it is or not) replacing that with a per cpu variable would work too
> > if it then generates the same code as with PDA.
> > 
> > -Andi
> 
> I'll look at it some more as I don't really have a preference either.
> One thing that also bothered me was other cpus read the per cpu
> variable to get the node number whilst the current cpu reads the pda
> variable.  I'll see about resolving that quirky-ness.  (All one or all
> the other.)

This information should always come from the same variable.

> (And of course the problem with cpus on nodes with no local memory
> needs to be resolved as well.)

All CPUs get assigned to some node at boot.

And there should be always per cpu variables or pda to use.

-Andi
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