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Date: Fri, 2 Apr 2010 22:44:46 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@osdl.org>, linux-kernel@vger.kernel.org,
       Stephane Eranian <eranian@google.com>
Subject: Re: [RFC,PATCH 2/2] perf, x86: Utilize the LBRs for machine/oops
 debugging
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* Peter Zijlstra <peterz@infradead.org> wrote:

> On Mon, 2010-03-29 at 19:02 +0200, Ingo Molnar wrote:
>
> > 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.
> 
> Right, could do, but once it crashed it clearly to late to enable anything 
> ;-)

No. What i mean is that with your patch, a debugging session would go like 
this:

  < kernel crashes >                            # reboot #1

  < admin logs in and scratches head >
  < admin consults kernel hackers and enables lbr_debug=1 in /etc/grub.conf >

  < admin reboots >                             # reboot #2

  < kernel crashes again >                      # reboot #3

With the sysctl we'd have one reboot less:

  < kernel crashes >                            # reboot #1

  < admin logs in and scratches head >
  < admin consults kernel hackers and tweaks /proc/sys/kernel/x86/lbr_debug >

  < kernel crashes again >                      # reboot #2

Thanks,

	Ingo