linux-kernel - Re: [PATCH 0/7] Boot IRQ quirks and rerouting

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Date:	Wed, 04 Jun 2008 14:44:00 -0400
From:	Jon Masters <jcm@...hat.com>
To:	Stefan Assmann <sassmann@...e.de>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Olaf Dabrunz <od@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/7] Boot IRQ quirks and rerouting

On Wed, 2008-06-04 at 11:49 +0200, Stefan Assmann wrote:

> On the chips (ICHx, ...) we saw, the interrupt lines on the PIC also go
> to the first IO-APIC. So the boot interrupts go to both the PIC and the
> first IO-APIC.

Yup, it's system dependent too, but it's a mess in many cases. I've seen
numerous systems falling over - typically, the classical case will be
some IO controller when under heavy load will stop doing interrupts.
Another gotcha is that a lot of the time, these legacy interrupt (boot
interrupts) happen to be shared between e.g. a disk controller and a USB
host controller. On a server system running -RT, it's quite common that
there won't be much going on with the other device, so the problem goes
unnoticed for a long time...but it still bites you in the end.

> When running in APIC mode all PIC IRQs are disabled, except for the
> timer maybe. Boot interrupts still arrive on the first IO-APIC and end
> up as being counted as spurious interrupts.

Yup. I think the good thing is that both SuSE and Red Hat have
identified the same problems, posited the same root cause, and
contemplated similar workarounds...so I think we understand it.

Jon.

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