linux-kernel - Re: Should irq_chip->mask disable percpu interrupts to all cpus, or just to this cpu?

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Date:	Wed, 24 Sep 2008 11:33:42 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Should irq_chip->mask disable percpu interrupts to all cpus,
 or just to this cpu?

Eric W. Biederman wrote:
> I really hate the notion of sharing a single irq_desc across
> multiple cpus as a preferred mode of operation.  As NUMA comes
> into play it guarantees we will have cross cpu memory fetches
> on a fast path for irq handling.
>
> Other than the beautiful way we print things in /proc/interrupts
> IRQ_PER_CPU feels like a really bad idea.  Especially in that
> it enshrines the nasty per cpu irq counters that scale horribly.
>   

I found handle_percpu_irq() which addresses my concerns.  It doesn't
attempt to mask the interrupt, takes no locks, and doesn't set or test
IRQ_INPROGRESS in desc->status, so it will scale perfectly across
multiple cpus.  It makes no changes to the desc structure, so there
isn't even any cacheline bouncing.

    J
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