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

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Date:	Wed, 26 Mar 2008 13:55:23 -0500 (CDT)
From:	Alan Mayer <ajm@....com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Pavel Machek <pavel@....cz>, 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, Linus Torvalds wrote:

> 
> 
> 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
> 

Well, I was looking at it from that point of view.  But, when I found myself
looking at code, particularly in drivers, that indexed into the irq_desc array
and started modifying the descriptor in place and then calling setup_irq(),
I realized that what was needed was a redesign of the whole mess from first
principals.  I still think that's what needs to be done, but by some one with
more experience and credibility than me.  Maybe in a year I'd be willing
to attempt it, but not today.

		--ajm


--
Alan J. Mayer
SGI
ajm@....com
WORK: 651-683-3131
HOME: 651-407-0134
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