Re: [RFC,PATCH 2/2] perf, x86: Utilize the LBRs for machine/oopsdebugging

[Peter Zijlstra]

On Mon, 2010-03-29 at 19:02 +0200, Ingo Molnar wrote:
> * Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> 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?)

lbr_debug=on actually, =1 doesn't parse. I had some output, but I was
still looking at finding out why my %pF formats for CPU0 didn't have
symbols in, probably a init sequence issue.

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

Well, I would like to keep LBR in the name, since that is the mechanism
used, we could maybe do lbr_trace or somesuch?

> 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 ;-)

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