Messages in this thread | | | Subject | Re: [RFC,PATCH 2/2] perf, x86: Utilize the LBRs for machine/oops debugging | From | Peter Zijlstra <> | Date | Mon, 29 Mar 2010 19:12:48 +0200 |
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On Mon, 2010-03-29 at 19:02 +0200, Ingo Molnar wrote: > * Peter Zijlstra <a.p.zijlstra@chello.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?)
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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