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SubjectRe: [Bug #11342] Linux 2.6.27-rc3: kernel BUG at mm/vmalloc.c - bisected

On Mon, 25 Aug 2008, Linus Torvalds wrote:
> shows these things as the top problems:
> 0xffffffff80266234 smp_call_function_mask [vmlinux]: 2736
> 0xffffffff80234747 __build_sched_domains [vmlinux]: 2232
> 0xffffffff8023523f __build_sched_domains [vmlinux]: 2232
> Anyway, the reason smp_call_function_mask and friends have such _huge_
> stack usages for you is that they contain a 'cpumask_t' on the stack.

In fact, they contain multiple CPU-masks, each 4k-bits - 512 bytes - in
size. And they tend to call each other.

Quite frankly, I don't think we were really ready for 4k CPU's. I'm going
to commit this patch to make sure others don't do that many CPU's by
mistake. It marks MAXCPU's as being 'broken' so you cannot select it, and
also limits the number of CPU's that you _can_ select to "just" 512.

Right now, 4k cpu's is known broken because of the stack usage. I'm not
willing to debug more of these kinds of stack smashers, they're really
nasty to work with. I wonder how many other random failures these have
been involved with?

This patch also makes the ifdef mess in Kconfig much cleaner and avoids
duplicate definitions by just conditionally suppressing the question and
giving higher defaults.

We can enable MAXSMP and raise the CPU limits some time in the future. But
that future is not going to be before 2.6.27 - the code simply isn't ready
for it.

The reason I picked 512 CPU's as the limit is that we _used_ to limit
things to 255. So it's higher than it used to be, but low enough to still
feel safe. Considering that a 4k-bit CPU mask (512 bytes) _almost_ worked,
the 512-bit (64 bytes) masks are almost certainly fine.

Still, sane people should limit their NR_CPUS to 8 or 16 or something like
that. Very very few people really need the pain of big NR_CPUS. Not even
"just" 512 CPU's.

Travis, Ingo and Thomas cc'd, since they were involved in the original
commit (1184dc2ffe2c8fb9afb766d870850f2c3165ef25) that raised the limit.


arch/x86/Kconfig | 30 ++++++++----------------------
1 files changed, 8 insertions(+), 22 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 68d91c8..ed92864 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -577,35 +577,29 @@ config SWIOTLB

config MAXSMP
bool "Configure Maximum number of SMP Processors and NUMA Nodes"
- depends on X86_64 && SMP
+ depends on X86_64 && SMP && BROKEN
default n
Configure maximum number of CPUS and NUMA Nodes for this architecture.
If unsure, say N.

-config NR_CPUS
- int
- default "4096"
config NR_CPUS
- int "Maximum number of CPUs (2-4096)"
- range 2 4096
+ int "Maximum number of CPUs (2-512)" if !MAXSMP
+ range 2 512
depends on SMP
+ default "4096" if MAXSMP
default "32" if X86_NUMAQ || X86_SUMMIT || X86_BIGSMP || X86_ES7000
default "8"
This allows you to specify the maximum number of CPUs which this
- kernel will support. The maximum supported value is 4096 and the
+ kernel will support. The maximum supported value is 512 and the
minimum value which makes sense is 2.

This is purely to save memory - each supported CPU adds
approximately eight kilobytes to the kernel image.

config SCHED_SMT
bool "SMT (Hyperthreading) scheduler support"
@@ -996,17 +990,10 @@ config NUMA_EMU
into virtual nodes when booted with "numa=fake=N", where N is the
number of nodes. This is only useful for debugging.

- int
- default "9"
- int "Maximum NUMA Nodes (as a power of 2)"
+ int "Maximum NUMA Nodes (as a power of 2)" if !MAXSMP
range 1 9 if X86_64
+ default "9" if MAXSMP
default "6" if X86_64
default "4" if X86_NUMAQ
default "3"
@@ -1014,7 +1001,6 @@ config NODES_SHIFT
Specify the maximum number of NUMA Nodes available on the target
system. Increases memory reserved to accomodate various tables.

def_bool y

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