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

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Date:	Tue, 26 Aug 2008 09:59:37 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	"Alan D. Brunelle" <Alan.Brunelle@...com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kernel Testers List <kernel-testers@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Mike Travis <travis@....com>
Subject: Re: [Bug #11342] Linux 2.6.27-rc3: kernel BUG at mm/vmalloc.c -
	bisected

* Christoph Lameter <cl@...ux-foundation.org> wrote:

> Alan D. Brunelle wrote:
> 
> > I think you're right: the kernel as a whole may not be ready for 4,096
> > CPUs apparently...
> 
> Mike has been working diligently on getting all these cpumasks off the 
> stack for the last months and has created an infrastructure to do 
> this. So I think we are close. It might just be a matter of merging 
> some more patches that are still left in Ingo's tree.

hm, there are no such patches left that i know of - the only bits in 
-tip are the zero-based percpu, which was found to be a bit fragile in 
testing:

 earth4:~/tip> git-log-line --author=Travis linus..
 d379497: Zero based percpu: infrastructure to rebase the per cpu area to zero
 b3a0cb4: x86: extend percpu ops to 64 bit

[and it has no relevance to stack footprint.]

So i dont think the current cpumask_t approach will work. We simply 
should not get into an endless fight against the windmills that 
introduce on-stack cpumask_t again and again. We should just take the 
plunge once and do a clean alloc/free cpumask model. Most of the hotpath 
cpumasks are constant or pre-constructed, so they are not a real issue.

Plus, on the general question of stack footprint problems and the 
difficulty of debugging them, the worst-case stack footprint tracer i 
wrote for -rt some time ago should be dusted off as well and put into 
ftrace. David has something quite close to that for Sparc64 already.

	Ingo
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