Messages in this thread | | | Date | Fri, 21 Nov 2008 10:26:32 -0800 | From | "H. Peter Anvin" <> | Subject | Re: [PATCH 2/2 v3] SGI RTC: add generic timer system interrupt |
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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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