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    SubjectRe: [PATCH 0/3] smp: reduce stack requirements for smp_call_function_mask

    * Mike Travis <> wrote:

    > * Cleanup cpumask_t usages in smp_call_function_mask function chain
    > to prevent stack overflow problem when NR_CPUS=4096.
    > * Reduce the number of passed cpumask_t variables in the following
    > call chain for x86_64:
    > smp_call_function_mask -->
    > arch_send_call_function_ipi->
    > smp_ops.send_call_func_ipi -->
    > genapic->send_IPI_mask
    > Since the smp_call_function_mask() is an EXPORTED function, we
    > cannot change it's calling interface for a patch to 2.6.27.
    > The smp_ops.send_call_func_ipi interface is internal only and
    > has two arch provided functions:
    > arch/x86/kernel/smp.c: .send_call_func_ipi = native_send_call_func_ipi
    > arch/x86/xen/smp.c: .send_call_func_ipi = xen_smp_send_call_function_ipi
    > arch/x86/mach-voyager/voyager_smp.c: (uses native_send_call_func_ipi)
    > Therefore modifying the internal interface to use a cpumask_t pointer
    > is straight-forward.
    > The changes to genapic are much more extensive and are affected by the
    > recent additions of the x2apic modes, so they will be done for 2.6.28 only.
    > Based on 2.6.27-rc5-git6.
    > Applies to linux-2.6.tip/master (with FUZZ).

    applied to tip/cpus4096, thanks Mike.

    I'm still wondering whether we should get rid of non-reference based
    cpumask_t altogether ...

    Did you have a chance to look at the ftrace/stacktrace tracer in latest
    tip/master, which will show the maximum stack footprint that can occur?

    Also, i've applied the patch below as well to restore MAXSMP in a muted
    form - with big warning signs added as well.


    From 363a5e3d7b4b69371f21bcafd7fc76e68c73733a Mon Sep 17 00:00:00 2001
    From: Ingo Molnar <>
    Date: Sat, 6 Sep 2008 15:24:52 +0200
    Subject: [PATCH] x86: add MAXSMP

    restore MAXSMP, it's a nice debugging helper to trigger various crashes
    and problems with maximum sized x86 systems.

    Make it depend on EXPERIMENTAL and DEBUG_KERNEL, and inform the user
    about the effects (stacksize, overhead, memory usage) of this flag.

    Signed-off-by: Ingo Molnar <>
    arch/x86/Kconfig | 11 ++++++++---
    1 files changed, 8 insertions(+), 3 deletions(-)

    diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
    index ed97f2b..91212c1 100644
    --- a/arch/x86/Kconfig
    +++ b/arch/x86/Kconfig
    @@ -580,10 +580,15 @@ config IOMMU_HELPER

    config MAXSMP
    bool "Configure Maximum number of SMP Processors and NUMA Nodes"
    - depends on X86_64 && SMP && BROKEN
    - default n
    + depends on X86_64 && SMP && DEBUG_KERNEL && EXPERIMENTAL
    - Configure maximum number of CPUS and NUMA Nodes for this architecture.
    + Configure maximum number of CPUS and NUMA Nodes for this
    + architecture (up to 4096!).
    + This can increase memory usage, bigger stack footprint and can
    + add some runtime overhead as well so unless you want a generic
    + distro kernel you likely want to say N.
    If unsure, say N.

    config NR_CPUS

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