linux-kernel - Re: [PATCH RFC] Simplify the Linux kernel by reducing its state space

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Date:	Sat, 31 Mar 2012 18:22:12 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	linux-kernel@...r.kernel.org, linux-alpha@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	uclinux-dist-devel@...ckfin.uclinux.org,
	linux-hexagon@...r.kernel.org, linux-ia64@...r.kernel.org,
	linux-m32r@...linux-m32r.org, linux-m32r-ja@...linux-m32r.org,
	linux-mips@...ux-mips.org, linux-am33-list@...hat.com,
	linux-parisc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	linux-s390@...r.kernel.org, linux-sh@...r.kernel.org,
	sparclinux@...r.kernel.org, tglx@...utronix.de,
	dhowells@...hat.com, jejb@...isc-linux.org, linux390@...ibm.com,
	x86@...nel.org, cmetcalf@...era.com
Subject: Re: [PATCH RFC] Simplify the Linux kernel by reducing its state	space

On Sat, Mar 31, 2012 at 11:32:00PM +0100, Russell King - ARM Linux wrote:
> On Sun, Apr 01, 2012 at 12:33:21AM +0800, Paul E. McKenney wrote:
> > Although there have been numerous complaints about the complexity of
> > parallel programming (especially over the past 5-10 years), the plain
> > truth is that the incremental complexity of parallel programming over
> > that of sequential programming is not as large as is commonly believed.
> > Despite that you might have heard, the mind-numbing complexity of modern
> > computer systems is not due so much to there being multiple CPUs, but
> > rather to there being any CPUs at all.  In short, for the ultimate in
> > computer-system simplicity, the optimal choice is NR_CPUS=0.
> > 
> > This commit therefore limits kernel builds to zero CPUs.  This change
> > has the beneficial side effect of rendering all kernel bugs harmless.
> > Furthermore, this commit enables additional beneficial changes, for
> > example, the removal of those parts of the kernel that are not needed
> > when there are zero CPUs.
> > 
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > Reviewed-by: Thomas Gleixner <tglx@...utronix.de>
> 
> Great work, but I don't think you've gone far enough with this.
> 
> What would really help is if you could consolidate all these NR_CPUS
> definitions into one place so we don't have essentially the same thing
> scattered across all these architectures.  We're already doing this on
> ARM across our platforms, and its about time such an approach was taken
> across the entire kernel tree.
> 
> It looks like the MIPS solution would be the best one to pick.
> Could you rework your patch to do this please?
> 
> While you're at it, you might like to consider that having zero CPUs
> makes all this architecture support redundant, so maybe you've missed
> a trick there - according to my count, we could get rid of almost 3
> million lines of code from arch.  We could replace all that with a
> single standard implementation.
> 
> Bah, maybe I shouldn't have pushed that bpf_jit code for ARM after all...

;-) ;-) ;-)

							Thanx, Paul

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