linux-kernel - Re: [PATCH] x86_64: resize NR_IRQS for large machines (re-submit)

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Date:	Thu, 27 Mar 2008 17:33:15 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Alan Mayer <ajm@....com>
Cc:	torvalds@...ux-foundation.org,
	linux-kernel list <linux-kernel@...r.kernel.org>,
	Robin Holt <holt@....com>, Jack Steiner <steiner@....com>,
	Russ Anderson <rja@....com>
Subject: Re: [PATCH] x86_64: resize NR_IRQS for large machines (re-submit)


* Alan Mayer <ajm@....com> wrote:

> > well, i dont it has to be (or it should be) an all or nothing patch, 
> > given the complexity and risks involved.
> > 
> > - we should first introduce a nr_irqs variable and a Kconfig switch 
> >   (say CONFIG_ARCH_HAS_DYNAMIC_NR_IRQS) for architectures to toggle. If 
> >   the switch is toggled, nr_irqs is a variable, otherwise it's a carbon 
> >   copy of NR_IRQS. Some array-definition, declaration and initialization 
> >   wrappers are provided as well.
> > 
> > - then the core code, x86 and most drivers can be converted to nr_irqs.
> >   The switch might initially even be user-selectable if
> >   CONFIG_DEBUG_KERNEL, to ease regression testing.
> > 
> > - other architectures will follow one by one, fixing their
> >   arch-dependent drivers as well in the process
> > 
> > - finally we get rid of the wrappers.
> > 
> > 	Ingo
> > 
> 
> Okay, let's see if I understand this.
> 
> First patch introduces a config switch and a variable, nr_irqs that is
> set to NR_IRQS.  It also dynamically allocates the currently staticly
> allocated arrays that are dimensioned by NR_IRQS.  It also initializes
> these dynamically allocated data structures.  This is all done under
> the config switch, initially off by default.  
> 
> Second patch changes core code, x86 and most drivers to use nr_irqs.
> This patch will also introduce a calculation of nr_irqs, based on
> interrupt sources, that is a better estimate of the number of irqs
> in the running system than just picking a guaranteed not-to-exceed
> value that may be too big.
> Is there a way to identify which drivers need to be addressed?
> 
> Then, test the crap out of it.  
> 
> Other architectures will follow, with the work being done by people
> familiar with those architectures.
> 
> Clean up anything that's left over that's now been made unnecessary by
> the conversion by everyone.  Including the config option?
> 
> Do I have the gist of it?

i think you got it right, yes. But ... this is just a quick first-look 
suggestion from me, YMMV. Maybe you find a way to do it much easier to 
just convert everything at once. I tend to do things more gradually, in 
my experience it's very hard and time-consuming to change the world all 
at once - it's hard both to you the developer (you dont know whether it 
works until you have a very substantial amount of code written - while 
in a more gradual approach it can be converted one by one perhaps) - and 
it's hard for users and fellow kernel hackers.

	Ingo
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