RE: [PATCH] x86_64: Make NR_IRQS configurable in Kconfig

[Protasevich, Natalie]

> > Currrently 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 Moores 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.
> > 
> > Signed-off-by: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
> > ---
> >  arch/x86_64/Kconfig      |   13 +++++++++++++
> >  include/asm-x86_64/irq.h |    3 ++-
> >  2 files changed, 15 insertions(+), 1 deletions(-)
> > 
> > diff --git a/arch/x86_64/Kconfig b/arch/x86_64/Kconfig index 
> > 7598d99..d744e5b 100644
> > --- a/arch/x86_64/Kconfig
> > +++ b/arch/x86_64/Kconfig
> > @@ -384,6 +384,19 @@ 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 (224-4096)"
> > +	range 256 4096
> > +	depends on SMP
> > +	default "4096"
> > +	help
> > +	  This allows you to specify the maximum number of IRQs 
> which this
> > +	  kernel will support. Current maximum is 4096 IRQs as that
> > +	  is slightly larger than has observed in the field.
> > +
> > +	  This is purely to save memory - each supported IRQ requires
> > +	  memory in the static kernel configuration.
> 
> If (a) "nor does hardware typically provide that many irq sources"
> and (b) "This is purely to save memory", why is the default
> 4096 instead of something smaller?
> 

4k being a humble maximum is definitely a relative term here, but on the
system with "only" 64 or 128 processors the cpu*224 would be much higher
:) However, maybe CONFIG_TINY that Andi suggested would leverage this
number also. What do you think, Eric?

--Natalie
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