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 15:01:13 -0700
From:	Arthur Kepner <akepner@....com>
To:	Thomas Gleixner <tglx@...utronix.de>
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, Sep 27, 2010 at 10:46:02PM +0200, Thomas Gleixner wrote:
> ...
> Sigh. Why is this a x86 specific problem ?
>

It's obviously not. But we're particularly seeing it on x86 
systems, so an x86-specific fix would address our problem.

> If we setup an irq on a node then we should set the affinity to the
> target node in general. 

OK.

> .... 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
> 

No, I don't think that's true. 

The problem we're seeing is that one driver asks for a large 
number of interrupts (on no CPU in particular). And because of the 
way that the vectors are initially assigned to CPUs (in 
__assign_irq_vector()), a particular CPU can have all its vectors 
consumed. 

Now, a second driver comes along, and requests an interrupt on 
a specific CPU, N. But CPU N is out of interrupts, so that driver 
fails.

This all happens before a user-space irq balancer is available.

-- 
Arthur
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