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

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Date:	Fri, 21 Nov 2008 11:09:18 -0800
From:	"Yinghai Lu" <yinghai@...nel.org>
To:	"H. Peter Anvin" <hpa@...or.com>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"Ingo Molnar" <mingo@...e.hu>
Cc:	"Dimitri Sivanich" <sivanich@....com>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, "john stultz" <johnstul@...ibm.com>
Subject: Re: [PATCH 2/2 v3] SGI RTC: add generic timer system interrupt

On Fri, Nov 21, 2008 at 10:26 AM, H. Peter Anvin <hpa@...or.com> 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.

should just remove probe_nr_irqs.

YH
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