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SubjectRe: [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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