Re: [PATCH 2/2 v3] SGI RTC: add generic timer system interrupt

From: H. Peter Anvin
Date: Fri Nov 21 2008 - 13:27:36 EST


Dimitri Sivanich wrote:
>
> There are basically two issues with using 'normal IRQs' in cases like this:
>
> - Using normal IRQs would mean we would have an IRQ per cpu. The current
> hard coded maximum, NR_IRQS, is 4352 (NR_VECTORS + (32 * MAX_IO_APICS)).
> On machines with large numbers of cpus and an irq per cpu for each desired
> interrupt, that's a lot of IRQs. In addition, the GRU, will need 2 such
> IRQs per cpu. On 4096 cpu systems, you've already used up more than the
> limit just for that. Until some sort of infrastructure change takes place
> that would potentially allow this to be dynamically increased, such as
> Yinghai Lu's "sparse_irq aka dyn_irq v14" patch, this problem will exist.
>
> Furthermore, the actual runtime limit, nr_irqs, is set to 96 by
> probe_nr_irqs for our configuration. This is because that code assumes all
> vectors are io-apic vectors, not cpu centric vectors like the ones I'm
> talking about. With the current, scheme, even on a 128 cpu system, I'm out
> of IRQs immediately.
>
> - The current infrastructure for requesting vector/IRQ combinations doesn't
> allow one to request an interrupt priority higher than i/o device interrupt
> priorities. Clock event (high resolution timer) code should run at higher
> interrupt priority.

Okay, so it is a hack pending us taking care of issues in the current
code. #1 we're obviously working on, #2 I need to think some more about
but shouldn't be too hard to fix -- if real, it also affects other
interrupt-driven clock sources.

I'm OK with this being a temporary hack, but I want it to be recognized
as such and cleaned up as soon as possible.

-hpa



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