linux-kernel - Re: [RFC/PATCH] cpuset: cpuset irq affinities

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Message-ID: <20080229213207.GN27212@elte.hu>
Date:	Fri, 29 Feb 2008 22:32:07 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Paul Jackson <pj@....com>, tglx@...utronix.de, oleg@...sign.ru,
	rostedt@...dmis.org, maxk@...lcomm.com,
	linux-kernel@...r.kernel.org, rientjes@...gle.com
Subject: Re: [RFC/PATCH] cpuset: cpuset irq affinities


* Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:

> > Could you educate me a little, Peter, on what these irqs are and on 
> > the sorts of ways people might want to place them across CPUs?
> 
> I'm not sure I know what you're asking. IRQ are hardware notifiers and 
> do all kinds of things depending on the hardware. Network cards 
> typically use them to notify the CPU of incoming packets. Video cards 
> can do vsync notifiers, empty dma buffers, whatnot.

irq affinity masks can basically be thought of as: "these are the CPUs 
where external hardware events will trigger certain kernel functions and 
cause overhead on those CPUs". An IRQ can have followup effects: softirq 
execution, workqueue execution, etc.

so managing the IRQ masks is very meaningful and just as meaningful as 
managing the affinity masks of tasks. You can think of "IRQ# 123" as 
"special kernel task # 123".

	Ingo
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