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

    * H. Peter Anvin <> wrote:

    > 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.


    Dimitri, looks like there are no blocker issues - John's clocksource
    comments need to be addressed and then we should be green to go for
    having this applied.


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