> From: Thomas Gleixner [mailto:t...@linutronix.de]
> Sent: Thursday, November 27, 2014 10:23 PM
> To: Berthier, Emmanuel
> Cc: H. Peter Anvin; x...@kernel.org; Jarzmik, Robert; LKML; Andy Lutomirski
> Subject: Re: [PATCH v2] [LBR] Dump LBRs on Exception
>
> On Thu, 27 Nov 2014, Emmanuel Berthier wrote:
> > diff --git a/arch/x86/kernel/cpu/perf_event_intel_lbr.c
> > b/arch/x86/kernel/cpu/perf_event_intel_lbr.c
> > index 45fa730..0a69365 100644
> > --- a/arch/x86/kernel/cpu/perf_event_intel_lbr.c
> > +++ b/arch/x86/kernel/cpu/perf_event_intel_lbr.c
> > @@ -4,7 +4,7 @@
> > #include <asm/perf_event.h>
> > #include <asm/msr.h>
> > #include <asm/insn.h>
> > -
>
> This newline is intentional to seperate asm includes from the local one.
Got it.
> > static void __intel_pmu_lbr_enable(void) {
> > u64 debugctl;
> > struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
> >
> > + lbr_set_used_by_perf(true);
>
> This cannot work.
>
> CPU0 CPU1
>
> __intel_pmu_lbr_enable()
> lbr_set_used_by_perf(true);
> __intel_pmu_lbr_disable()
> lbr_set_used_by_perf(false);
>
> This is a per cpu property.
>
> And there is more to that. Let's look at a single CPU.
>
> lbr for oops is enabled
>
> context switch()
> __intel_pmu_lbr_enable() -> LBR used by perf, oops dumper disabled
>
> context switch()
> __intel_pmu_lbr_disable() -> LBR not longer used by perf, oops
> dumper enabled
>
> So after that context switch we crash in the kernel and LBR is empty because
> we did disable it at the context switch.
>
> So you need per cpu state, which handles the LBR dumper state:
>
> #define LBR_OOPS_DISABLED 0x01
> #define LBR_PERF_USAGE 0x02
>
> DEFINE_PER_CPU(unsigned long, lbr_dump_state) = LBR_OOPS_DISABLED;
>
> lbr_perf_enable()
> this_cpu_add(lbr_dump_state, LBR_PERF_USAGE);
>
> lbr_perf_disable()
> if (!this_cpu_sub_return(lbr_dump_state, LBR_PERF_USAGE))
> enable_lbr_oops();
>
> Now of course you need to handle this in the exception path per cpu as well.
Agree, I will do that.
Thx.
Emmanuel.
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