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

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Message-ID: <491CC310.3050807@sgi.com>
Date:	Thu, 13 Nov 2008 16:15:12 -0800
From:	Mike Travis <travis@....com>
To:	David Miller <davem@...emloft.net>
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

David Miller wrote:
> 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.

Are you saying that having a fixed count of IRQ's is not an issue?  With
NR_CPUS=4096 what would you fix it to?  (Currently it's NR_CPUS * 32
but that might not be sufficient.)  Would NR_CPUS=16384 make it 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.

Which information is repeated over and over?  I was under the
impression that each and every interrupt writes to the irq_desc
entry for that irq?  If this is in a big list on node 0, that is
data passing over the system bus.

Or am I missing what you're getting at?

Thanks,
Mike



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