linux-kernel - Re: [PATCH v3] lockdep: Make MAX_STACK_TRACE

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Date:	Wed, 21 Apr 2010 14:03:00 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Gregory Haskins <ghaskins@...ell.com>
Cc:	John Kacur <jkacur@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Luis Claudio R. Goncalves" <lgoncalv@...hat.com>,
	Clark Williams <williams@...hat.com>,
	linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org
Subject: Re: [PATCH v3] lockdep: Make MAX_STACK_TRACE_ENTRIES configurable.

On Wed, 2010-04-21 at 05:47 -0600, Gregory Haskins wrote:
> 
> I am not sure if Johns solution is the right/best one per se, but I can attest
> that I used to hit this problem _all_ the time and it was somewhat annoying
> to need to patch the kernel on all of my machines to fix it.  I realize that I
> perhaps do not represent the average user, but it was a pain-point for me.
> FWIW, John's patch would indeed make my life easier since I tend to share the
> .config between builds. 

Right, so all I'm wanting to know if its a symptom of some funny or an
actual depletion, in the latter case I think the best solution is to
simply increase the number. In the former case we should of course fix
the real issue instead of making it disappear.

So one case I remember is where some code managed to create 1k classes
where 1 would have sufficed, this resulted in at least 1k extra stack
traces to be stored, consuming vast amounts of stack_entries.

So please, if you can reproduce, look at where these entries are going,
lots of classes with the same name are a good hint that something is
fishy. Classes with more than 13 (4*nr_states + 1) stacks should also
never happen, etc..

Is this specific to -RT, or do we see it without as well? If so, what in
-RT grows this? Are we creating a class per rt_mutex spinlock or
something silly like that?

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