linux-kernel - Re: IRQ migration on CPU offline path

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Date:	Thu, 15 Dec 2011 16:28:24 +0000
From:	Will Deacon <will.deacon@....com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	"tglx@...utronix.de" <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: IRQ migration on CPU offline path

Hi Eric,

Cheers for the response.

On Thu, Dec 15, 2011 at 01:25:19AM +0000, Eric W. Biederman wrote:
> Will Deacon <will.deacon@....com> writes:
> > I've been looking at the IRQ migration code on x86 (fixup_irqs) for the CPU
> > hotplug path in order to try and fix a bug we have on ARM with the
> > desc->affinity mask not getting updated. Compared to irq_set_affinity, the code
> > is pretty whacky (I guess because it's called via stop_machine) so I wondered if
> > you could help me understand a few points:
> 
> There is a lot of craziness on that path because of poor hardware design
> on x86 we can't know when an irq has actually be migrated, and other
> nasties.
> 
> There is also the issue that I expect is still the case that we have the
> generic layer asking us to cpu migration and the associated irq
> migrations with the irqs disabled which at least for the bits of poorly
> designed hardware made the entire path a best effort beast.

Argh, ok. Does this mean that other architectures should just preserve the
interface that x86 gives (for example not triggering IRQ affinity
notifiers)?

> If x86 becomes a good clean example in this corner case I would be
> amazed.  Last I looked I almost marked it all as CONFIG_BROKEN because
> we were trying to do the impossible.  Unfortunately peoples laptops
> go through this path when they suspend and so it was more painful to
> disable hacky racy mess than to keep living with it.
> 
> There has been an increase in the number of cases where it is possible
> to actually perform the migration with irqs disabled so on a good day
> that code might even work.  

Right, so this stuff is fairly fragile. We can probably get a reasonable
version working on ARM (with the GIC present) but I'm not sure what to do
about the notifiers I mentioned earlier and proper migration of threaded
interrupt handlers.

I'll take a look at some other archs.

Thanks,

Will
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