linux-kernel - Re: [PATCH] x86_64: resize NR

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Message-ID: <alpine.LFD.1.00.0803260920020.2775@woody.linux-foundation.org>
Date:	Wed, 26 Mar 2008 09:29:58 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Pavel Machek <pavel@....cz>
cc:	Alan Mayer <ajm@....com>, mingo@...e.hu,
	linux-kernel list <linux-kernel@...r.kernel.org>,
	Robin Holt <holt@....com>, Jack Steiner <steiner@....com>,
	Russ Anderson <rja@....com>
Subject: Re: [PATCH] x86_64: resize NR_IRQS for large machines

On Wed, 26 Mar 2008, Pavel Machek wrote:
> 
> This is very ugly. Why not include it unconditionally -- with guard in
> apicdef.h?

I do agree that it's ugly, but I think the ugliness is more serious than 
that.

What I think we should do is to make NR_IRQS no longer be a compile-time 
constant, but instead just do something like

	unsigned int NR_IRQS __read_mostly;

and then just set it early in the boot sequence depending on the real CPU 
numbers etc.

I realize that this will require some changes to a few arrays that are 
statically allocated and depend on NR_IRQ's (notably "irq_desc"), but 
don't you guys think that this would be a cleaner thing?

[ I suspect that irq_desc[] itself could quite reasonably be a rather much 
  smaller __read_mostly hash-table of dynamically allocated entries - the 
  thing would be only modified at boot, so it should cache beautifully 
  even across hundreds of CPU's ]

Whatever. I'm not opposed to this whole static thing, but I do wonder if 
it's worth doing that way. There *may* be performance reasons for doing it 
the way we're doing it, but quite frankly, I think the #define is mostly 
purely historical, from when it was just a fixed number (originally 16!) 
and it made sense to think of it as a small static array.

			Linus
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