linux-kernel - Re: [PATCH] x86_64: Make NR_IRQS configurable in Kconfig

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Message-Id: <20060807120454.79b6e1dc.rdunlap@xenotime.net>
Date:	Mon, 7 Aug 2006 12:04:54 -0700
From:	"Randy.Dunlap" <rdunlap@...otime.net>
To:	ebiederm@...ssion.com (Eric W. Biederman)
Cc:	Andrew Morton <akpm@...l.org>, Andi Kleen <ak@...e.de>,
	"Protasevich, Natalie" <Natalie.Protasevich@...SYS.com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [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@...ssion.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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