linux-kernel - Re: [RFC,PATCH 2/2] perf, x86: Utilize the LBRs for machine/oops debugging

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Date:	Mon, 29 Mar 2010 19:02:59 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Linus Torvalds <torvalds@...l.org>, linux-kernel@...r.kernel.org,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [RFC,PATCH 2/2] perf, x86: Utilize the LBRs for machine/oops
 debugging

* Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:

> The LBRs are relatively cheap to keep enabled and provide some history to 
> OOPSen, also some CPUs are reported to keep them over soft-reset, which 
> allows us to use them to debug things like tripple faults.
> 
> Therefore introduce a boot option: lbr_debug=on, which always enable the 
> LBRs and will print the LBRs on CPU init and die().

Yummie!

Have you got some sample lbr_debug=1 output as well by any chance, with a 
crash provoked somewhere? How good is the output in practice? (i.e. how many 
artificial entries do we have at the end of the buffer, filled with crash 
related addresses?)

Also, i think we should use something more descriptive than lbr_debug=y. 
Perhaps crash_trace=1 or so?

Plus, it would be nice to have a sysctl entry for this as well - so that 
production systems can enable this if they want to enrich the output of some 
difficult-to-analyze kernel crash, without yet another reboot.

	Ingo
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