Subject: Re: [PATCH 2/2] x86: get more exact nr_irqs

From: [email-masked] (Eric W. Biederman)
Date: Mon, 04 Jan 2010 08:55:56 -0800

Yinghai Lu <[email-masked]> writes:

> first check with NR_VECTORS - FIRST_EXTERNAL_VECTOR - 0x20
> aka minus exceptions and system vectors.
>
> NR_CPUS = 512, and nr_cpu_ids = 128
> will have NR_IRQS = 256 + 512 * 64 = 33024
>
> assume we have 20 intel ixgbe 6 port cards (with sriov and ixgbevf)
> 	20 * 6 * 64 * 3 = 23040
>
> first will get:
> 	128 * (256 - 64) = 24576
> then with nr_irqs_gsi will get
> 	(120 + 8 * 128 + 120 * 256) = 31864
>
> so 24576 will be used for nr_irqs.
>
> 24576 * 8 = 196608 bytes will be used for irq_desc_ptrs[]
>
> before this patch:
>     have nr_irqs = 120 + 8 * 128 + 120 * 64 = 8824
> 	and irq_desc_ptrs[] is 70592
>
> Signed-off-by: Yinghai Lu <[email-masked]>

I am lost.    arch_probe_nr_irqs appears to be total nonsense.

We have three concepts. 
- The number of irq sources we can talk about.  ( nr_irqs)
- The number of irqs we can possibly service.   ((NR_VECTORS - 0x30) *nr_cpu_ids)
- The number of irqs we actually connected up to cards in the
  system that we need to do something with.

Why do we need to allocate arrays at all?

arch_probe_nr_irqs looks like a pile of magic numbers (even more magic
with the addition of 0x20), that is always going to be a little bit
wrong.

We should be able to remove the arrays all together and allocate
irq_desc dynamically.

> ---
>  arch/x86/kernel/apic/io_apic.c |   11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
>
> Index: linux-2.6/arch/x86/kernel/apic/io_apic.c
> ===================================================================
> --- linux-2.6.orig/arch/x86/kernel/apic/io_apic.c
> +++ linux-2.6/arch/x86/kernel/apic/io_apic.c
> @@ -3833,15 +3833,20 @@ int __init arch_probe_nr_irqs(void)
>  {
>  	int nr;
>  
> -	if (nr_irqs > (NR_VECTORS * nr_cpu_ids))
> -		nr_irqs = NR_VECTORS * nr_cpu_ids;
> +	/* 0x20 for ipi etc system vectors */
> +	nr = NR_VECTORS - FIRST_EXTERNAL_VECTOR - 0x20;

If you are going to subtract of the number of ipis please
put appropriate defines in irq_vectors.h.  A raw 0x20 is
wrong.

> +
> +	nr *= nr_cpu_ids;
> +
> +	if (nr < nr_irqs)
> +		nr_irqs = nr;
>  	nr = nr_irqs_gsi + 8 * nr_cpu_ids;
>  #if defined(CONFIG_PCI_MSI) || defined(CONFIG_HT_IRQ)
>  	/*
>  	 * for MSI and HT dyn irq
>  	 */
> -	nr += nr_irqs_gsi * 64;
> +	nr += nr_irqs_gsi * 256;

This part seems like magic voodoo.  Why should their
be a correlation between the number of gsis and the number
of msis?

Eric