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 12:34:09 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Jeremy Fitzhardinge <jeremy@...p.org>
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?

Jeremy Fitzhardinge <jeremy@...p.org> writes:

> 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.

kstat_irqs.  Is arguably part of the irq structure.
And kstat_irqs is a major pain in my book.

And for a rare event you have a cacheline read.
I don't think we are quite there yet but we really want to allocate
irq_desc on the right NUMA node in a multi socket system, to reduce
the cache miss times.

Is it a big deal?  Probably not.  But I think it would be a bad
idea to increasingly use infrastructure that will make it hard
to optimize the code.

Especially since the common case in high performance drivers
is going to be, individually routable irq sources.  Having
one queue per cpu and one irq per queue.  Which sounds like
the same case you have.

Eric

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