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SubjectRe: [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@sgi.com
WORK: 651-683-3131
HOME: 651-407-0134
--



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