linux-kernel - Re: [PATCH] sparse_irq aka dyn

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Message-Id: <20081113.151435.146617325.davem@davemloft.net>
Date:	Thu, 13 Nov 2008 15:14:35 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	travis@....com
Cc:	paulus@...ba.org, akpm@...ux-foundation.org, yinghai@...nel.org,
	mingo@...e.hu, tglx@...utronix.de, hpa@...or.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sparse_irq aka dyn_irq v13

From: Mike Travis <travis@....com>
Date: Thu, 13 Nov 2008 15:11:29 -0800

> David Miller wrote:
> > From: Paul Mackerras <paulus@...ba.org>
> > Date: Fri, 14 Nov 2008 09:19:13 +1100
> > 
> >> Andrew Morton writes:
> >>
> >>> Other architectures want (or have) sparse interrupts.  Are those guys
> >>> paying attention here?
> >> On powerpc we have a mapping from virtual irq numbers (in the range 0
> >> to NR_IRQS-1) to physical irq numbers (which can be anything) and back
> >> again.  I think our approach is simpler than what's being proposed
> >> here, though we don't try to keep the irqdescs node-local as this
> >> patch seems to (fortunately our big systems aren't so NUMA-ish as to
> >> make that necessary).
> > 
> > This is exactly what sparc64 does as well, same as powerpc, and
> > as Paul said it's so much incredibly simpler than the dyn_irq stuff.
> 
> One problem is that pre-defining a static NR_IRQ count is almost always
> wrong when the NR_CPUS count is large, and should be adjusted as resources
> require.

We use a value of 256 and I've been booting linux on 128 cpu sparc64
systems with lots of PCI-E host controllers (and others have booted it
on even larger ones).  All of which have several NUMA domains.

It's not an issue.

> Large UV systems will take a performance hit from off-node accesses
> when the CPU count (or more likely the NODE count) reaches some
> threshold.  So keeping as much interrupt context close to the
> interrupting source is a good thing.

Just because the same piece of information is repeated over and
over again doesn't mean it really matters.
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