linux-kernel - Re: 4KSTACKS + DEBUG

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Message-ID: <483D76AF.8000402@redhat.com>
Date:	Wed, 28 May 2008 10:13:51 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Mike Snitzer <snitzer@...il.com>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jesper Juhl <jesper.juhl@...il.com>
Subject: Re: 4KSTACKS + DEBUG_STACKOVERFLOW harmful

Mike Snitzer wrote:

> Hi Eric,
> 
> Did you happen to get a patch together that reduces the stack usage of
> dump_stack?

Nope... but Andi Kleen sent a patch to put the warning on the irq stack
rather than the main process stack, so it avoids the original problem
with DEBUG_STACKOVERFLOW at least.

> Also, what did you use to print your (above) indented callchain stack
> usage of dump_stack?

That was just hand-edited... :)

> I'd like to be able to audit the worst case stack usage of _all_ call
> chains that originate from a given thread. This would effectively be
> like DEBUG_STACK_USAGE except with finer grained (per call-chain)
> statistics.  One crude way of doing this is to dump_stack() whenever a
> task's call-chain is the new "winner" as the biggest stack hog.

When will you test for the new winner?

> To do this safely it would seem to me that a leaner dump_stack() is needed...

It depends, I guess; if you have 8k stacks it'd probably fit ok in
almost all cases, I think.

> Lastly, would it be reasonable to utilize systemtap to implement what
> I described above?  I'm actually looking to debug 4KSTACKS as
> unobtrusively as possible so as to not alter the underlying kernel (in
> this case it happens to be a RHEL5 kernel but this could apply to any
> kernel).

I don't actually know if systemtap can do what you want (not saying it
can't; just saying I don't know...)

-Eric
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