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Date: Fri, 8 Jan 2010 12:06:21 -0800
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Subject: Re: [PATCH 5/5] x86: update nr_irqs according cpu num
From: Yinghai Lu <yinghai@kernel.org>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>, Ingo Molnar <mingo@elte.hu>,
       Thomas Gleixner <tglx@linutronix.de>,
       Andrew Morton <akpm@linux-foundation.org>,
       Jesse Brandeburg <jesse.brandeburg@intel.com>,
       linux-kernel@vger.kernel.org
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On Fri, Jan 8, 2010 at 11:21 AM, H. Peter Anvin <hpa@zytor.com> wrote:
> On 01/08/2010 11:11 AM, Eric W. Biederman wrote:
>> Yinghai Lu <yinghai@kernel.org> writes:
>>
>>> that is max number on run time.
>>
>> Ouch!  Unless I misread this code this will leave nr_irqs at
>> NR_IRQS_LEGACY. aka 16.
>>
>> Let's do something stupid and simple.
>> nr_irqs = nr_cpus_ids * 256;  /* Semi-arbitrary number */
>
> This would be 1048576 on the biggest machines we currently support.
> Now, the number of IRQ *vectors* is limited to
> (224-system vectors)*(cpu count), so one could argue that if there is
> anything that is not semi-arbitrary it would be that number, but that
> doesn't account for vector sharing.

(256 - 32 - 16 - system_vectors) * cpu_count + 16

the 16 is legacy from IRQ0 to 15 has domain with all cpus set.

later could move arch_probe_nr_irqs out of early_irq_init,
call it after init_IRQ(), so could use first_system_vector exactly
instead of 0x10

>
> Do we have any place which requires nr_irqs to be *stable*, or can we
> simply treat it as a high water mark for IRQ numbers used?
>
>> Ideally we would set "nr_irqs = 0x7fffffff;" but we have just enough
>> places using nr_irqs that I think those loops would get painful if we
>> were to do that.
yes, those loops
>
> Ideally we should presumably get rid of nr_irqs completely?

some interface in /proc/interrupts need it to make sure sth in sequence.

YH