linux-kernel - Re: Interrupt Affinity in SMP

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Message-ID: <AANLkTik9OzNu-jaE6qiJJznb0riOn5CsgPO6aBoDaYHJ@mail.gmail.com>
Date:	Mon, 19 Jul 2010 13:22:15 -0600
From:	Robert Hancock <hancockrwd@...il.com>
To:	Bryan Hundven <bryanhundven@...il.com>
Cc:	linux-kernel@...r.kernel.org, mchehab@...hat.com
Subject: Re: Interrupt Affinity in SMP

On Sat, Jul 17, 2010 at 2:02 PM, Bryan Hundven <bryanhundven@...il.com> wrote:
> Please see the two attached examples.
>
> Notice on the 5410 example how we start with the affinity set to 0xff,
> and change it to 0xf0.
> This should spread the interrupts over the last 4 cores of this quad
> core - dual processor system.
>
> Also notice on the 5645 example, with the same commands we start with
> 0xffffff and change to 0xfff000 to spread the interrupts over the last
> 12 cores, but only the first of the last twelve cores receive
> interrupts.
>
> This is the inconsistency I was trying to explain before.

As I mentioned before, I believe the behavior in this case is chipset
dependent, and potentially not something the kernel has control over.
In most cases distributing the same interrupt across multiple cores is
likely a bad idea in any case, unless the interrupt load is actually
so high that a single CPU can't handle it.
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