linux-kernel - Re: [PATCH] x86: 64bit support more than 256 irq v2

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Date:	Fri, 01 Aug 2008 20:37:34 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	"Yinghai Lu" <yhlu.kernel@...il.com>
Cc:	"Ingo Molnar" <mingo@...e.hu>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Dhaval Giani" <dhaval@...ux.vnet.ibm.com>,
	"Mike Travis" <travis@....com>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: 64bit support more than 256 irq v2

"Yinghai Lu" <yhlu.kernel@...il.com> writes:

> On Fri, Aug 1, 2008 at 7:58 PM, Eric W. Biederman <ebiederm@...ssion.com> wrote:
>> "Yinghai Lu" <yhlu.kernel@...il.com> writes:
>>
>>> On Fri, Aug 1, 2008 at 7:02 PM, Yinghai Lu <yhlu.kernel@...il.com> wrote:
>>>> Dhaval Giani got:
>>>> kernel BUG at arch/x86/kernel/io_apic_64.c:357!
>>>> invalid opcode: 0000 [1] SMP
>>>> CPU 24
>>>> ...
>>>>
>>>> his system (x3950) has 8 ioapic, irq > 256
> ..
>>
>> There was a patch that came out a while ago that set NR_IRQS as NR_IOAPICS*32
> from
>> SGI.  Where that got to I don't recall.
>
> #ifdef CONFIG_X86_32
> # define MAX_IO_APICS 64
> #else
> # define MAX_IO_APICS 128
> # define MAX_LOCAL_APIC 32768
> #endif
>
> so they can take 128*32 = 4096?

Yes.  The real problem was just the complete explosion of irqs on SGI
machines that have paltry I/O to the outside world compared to their cpu
count.  Have 131072 irqs seemed a little much for them.

It is all a game of tuning heuristics right now, and if we can totally
kill NR_IRQS it all goes away.  But that is an issue to solve before
the next merge window not to kill the regression.

Eric

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