linux-kernel - Re: [PATCH] X86: Change the default value of nr_irqs from 32 to NR

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Date:	Thu, 21 Aug 2008 16:21:57 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Yinghai Lu <yhlu.kernel@...il.com>
CC:	Alex Nixon <alex.nixon@...rix.com>, Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] X86: Change the default value of nr_irqs from 32 to NR_IRQs

Yinghai Lu wrote:
> On Wed, Aug 20, 2008 at 4:23 PM, Alex Nixon <alex.nixon@...rix.com> wrote:
>   
>> Alex Nixon wrote:
>>     
>>> Yinghai Lu wrote:
>>>       
>>>> when !CONFIG_HAVE_SPARSE_IRQ, with dyn_array, could allocate irq_desc
>>>> and etc as less as possible.
>>>> when CONFIG_HAVE_SPARESE_IRQ, no actually meaning for nr_irqs.
>>>>
>>>> YH
>>>>         
>>> So I believe the only case this affects is !CONFIG_HAVE_SPARSE_IRQ
>>>
>>> The worry is that with CONFIG_HAVE_DYN_ARRAY we may waste memory by
>>> pre-allocating more irq_descs than may be necessary (NR_IRQs vs 32)?
>>>
>>> With !CONFIG_HAVE_DYN_ARRAY however, a static array of size NR_IRQS is
>>> allocated instead - so doesn't defaulting nr_irqs back to NR_IRQS just
>>> revert to the old behaviour (with the exception of the irq_descs being
>>> allocated in pre_alloc_dyn_array instead)?
>>>
>>> - Alex
>>>
>>>       
>> Sorry to pester you Yinghai, but I'd like to get a patch for this out one
>> way or another as Xen is _completely_ unusable with 5 or more VCPUs.
>>
>> Can you explain more clearly what the problem with the patch is?
>>     
>
> small real system doesn't have MSI ioapic will have nr_irqs == 32.
> your patch will increase that to 224 again.
>
> sth like ?
>   

No.  A Xen-capable kernel can also run native, so it must do whatever a
normal kernel would do when booting native.

At what point in the boot does nr_irqs need to be set?  Could we just
override it at some point?

> #ifdef CONFIG_XEN
>   

I assume you mean ifndef here?

> int __init probe_nr_irqs(void)
> {
>         int idx;
>         int nr = 0;
>
>         for (idx = 0; idx < nr_ioapics; idx++)
>                 nr += io_apic_get_redir_entries(idx);
>
>         /* double it for hotplug and msi and nmi */
>         nr <<= 1;
>
>         /* something wrong ? */
>         if (nr < 32)
>                 nr = 32;
>
>         return nr;
> }
> #else
> int __init probe_nr_irqs(void)
> {
>        return NR_IRQS;
> }
> #endif
>   

    J
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