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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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