linux-kernel - Re: [RFC/PATCHv2] x86/irq: round-robin distribution of irqs to cpus w/in node

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Date:	Tue, 28 Sep 2010 10:08:52 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
cc:	Arthur Kepner <akepner@....com>, linux-kernel@...r.kernel.org,
	x86@...nel.org
Subject: Re: [RFC/PATCHv2] x86/irq: round-robin distribution of irqs to cpus
 w/in node

On Mon, 27 Sep 2010, Eric W. Biederman wrote:
> > On Mon, 27 Sep 2010, Arthur Kepner wrote:
> The deep bug is that create_irq_nr allocates a vector (which it does
> because at the time there was no better way to mark an irq in use on
> x86).  In the case of msi-x we really don't know the node that irq is
> going to be used on until we get a request irq.  We simply know which
> node the device is on.

Bah. So the whole per node allocation business is completely useless
at this point.

> If you want to see what is going follow the call trace looks like.
> pci_enable_msix 
>   arch_setup_msi_irqs
>     create_irq_nr
> 
> After pci_enable_msix is finished then the driver goes and makes all
> of the irqs per cpu irqs.
> 
> There are goofy things that happen when hardware asks for 1 irq per cpu.
> But since msi can ask for up to 4096 irqs (assuming the hardware
> supports it) I can totally see putting all 256 of those irqs on a single
> cpu, before you go to user space and let user space or something
> reassign all of those irqs in a per cpu way.
> 
> My gut feel says that the real answer is to delay assigning a vector
> to an irq until request_irq().  At which point we will know that someone
> at least wants to use the irq.

Right. So the solution would be:

create_irq allocates an irq number + irq descriptor, nothing else

chip->startup() will setup the vector and chip->shutdown releases
it. That requires to change the return value of chip->startup to int,
so we can return an error code, but that can be done in course of the
overhaul I'm working on. 

Right now I prefer not to add more crap to io_apic.c, it's horrible
enough already. I'll fix that with the cleanup.

Thanks

	tglx


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