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    SubjectRe: [PATCH] x86_64: Make NR_IRQS configurable in Kconfig
    On Mon, 07 Aug 2006 12:53:35 -0600 Eric W. Biederman wrote:

    >
    > Currently on a SMP system we can theoretically support
    > NR_CPUS*224 irqs. Unfortunately our data structures
    > don't cope will with that many irqs, nor does hardware
    > typically provide that many irq sources.
    >
    > With the number of cores starting to follow Moore's Law,
    > and the apicid limits being raised beyond an 8bit
    > number trying to track our current maximum with our
    > current data structures would be fatal and wasteful.
    >
    > So this patch decouples the number of irqs we support
    > from the number of cpus. We can revisit this decision
    > once someone reworks the current data structures.
    >
    > This version has my stupid typos fix and the true maximum
    > exposed to make it clear that I have a low default. The
    > worst that I can see happening is there won't be any
    > per_cpu space left for modules if someone sets this
    > too high, but the system should still boot.
    >
    > For non-SMP systems the default is set to 224 IRQs.
    >
    > Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
    > ---
    > arch/x86_64/Kconfig | 14 ++++++++++++++
    > include/asm-x86_64/irq.h | 3 ++-
    > 2 files changed, 16 insertions(+), 1 deletions(-)
    >
    > diff --git a/arch/x86_64/Kconfig b/arch/x86_64/Kconfig
    > index 7598d99..c87b0bc 100644
    > --- a/arch/x86_64/Kconfig
    > +++ b/arch/x86_64/Kconfig
    > @@ -384,6 +384,20 @@ config NR_CPUS
    > This is purely to save memory - each supported CPU requires
    > memory in the static kernel configuration.
    >
    > +config NR_IRQS
    > + int "Maximum number of IRQs (224-57344)"
    > + range 224 57344
    > + default "4096" if SMP
    > + default "224" if !SMP
    > + help
    > + This allows you to specify the maximum number of IRQs which this
    > + kernel will support. Current default is 4096 IRQs as that
    > + is slightly larger than has observed in the field. Setting
    > + a noticeably larger value will exhaust your per cpu memory,
    > + and waste memory in the per irq arrays.

    If you'll fix this text for the non-SMP case too, I think
    you'll be done. :)


    > + If unsure leave this at the default.
    > +
    > config HOTPLUG_CPU
    > bool "Support for hot-pluggable CPUs (EXPERIMENTAL)"
    > depends on SMP && HOTPLUG && EXPERIMENTAL
    > diff --git a/include/asm-x86_64/irq.h b/include/asm-x86_64/irq.h
    > index 5006c6e..34b264a 100644
    > --- a/include/asm-x86_64/irq.h
    > +++ b/include/asm-x86_64/irq.h
    > @@ -31,7 +31,8 @@ #define NR_VECTORS 256
    >
    > #define FIRST_SYSTEM_VECTOR 0xef /* duplicated in hw_irq.h */
    >
    > -#define NR_IRQS (NR_VECTORS + (32 *NR_CPUS))
    > +/* We can use at most NR_CPUS*224 irqs at one time */
    > +#define NR_IRQS (CONFIG_NR_IRQS)
    > #define NR_IRQ_VECTORS NR_IRQS
    >
    > static __inline__ int irq_canonicalize(int irq)
    > --

    ---
    ~Randy
    -
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