linux-kernel - Re: [RFC/PATCHv2] x86/irq: round-robin distribution of irqs to cpus w/in node

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Date:	Mon, 27 Sep 2010 22:46:02 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Arthur Kepner <akepner@....com>
cc:	linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [RFC/PATCHv2] x86/irq: round-robin distribution of irqs to cpus
 w/in node

On Mon, 27 Sep 2010, Arthur Kepner wrote:

> 
> (Fixed a small error with yesterday's version, and add x86@...nel.org 
> to cc list.)
> 
> SGI has encountered situations where particular CPUs run out of
> interrupt vectors on systems with many (several hundred or more)
> CPUs. This happens because some drivers (particularly the mlx4_core
> driver) select the number of interrupts they allocate based on the
> number of CPUs, and because of how the default irq affinity is used.
> 
> Do  psuedo round-robin distribution of irqs to CPUs within a node 
> to avoid (or at least delay) running out of vectors on any particular 
> CPU.

Sigh. Why is this a x86 specific problem ?

If we setup an irq on a node then we should set the affinity to the
target node in general. The round robin inside the node is really not
a problem unless you hit:

   nr_irqs_per_node * nr_cpus_per_node > max_vectors_per_cpu

If that's the case then we probably have some more severe problems.

Again, I agree that we should target the irq to the node, but the fine
grained details can be done in user space.

Thanks,

	tglx
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