Message-ID: <4926FD58.7000406@zytor.com>
Date: Fri, 21 Nov 2008 10:26:32 -0800
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Dimitri Sivanich <sivanich@sgi.com>
CC: Andrew Morton <akpm@linux-foundation.org>, mingo@elte.hu,
       tglx@linutronix.de, linux-kernel@vger.kernel.org,
       john stultz <johnstul@us.ibm.com>
Subject: Re: [PATCH 2/2 v3] SGI RTC: add generic timer system interrupt
References: <20081023163041.GA14574@sgi.com> <20081119212202.GA3377@sgi.com> <20081119212350.GB3377@sgi.com> <20081119212631.GC3377@sgi.com> <20081120151208.f7892050.akpm@linux-foundation.org> <4925F095.40107@zytor.com> <20081121171537.GA12370@sgi.com>
In-Reply-To: <20081121171537.GA12370@sgi.com>
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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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