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    SubjectRe: [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@citrix.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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