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

[Ingo Molnar]

* Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:

> On Mon, 25 Aug 2008, Linus Torvalds wrote:
> > 
> > checkstack.pl 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.

yeah, that's OK i guess - distros can still enable 4K support if they 
wish to. Someone interested in improving the stack footprint situation 
should dust off the max-stack-footprint tracer so that we can catch 
these things in a more structured way.

And i guess the next generation of 4K CPUs support should just get away 
from cpumask_t-on-kernel-stack model altogether, as the current model is 
not maintainable. We tried the on-kernel-stack variant, and it really 
does not work reliably. We can fix this in v2.6.28.

	Ingo
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