Re: [PATCH v2] [LBR] Dump LBRs on Exception
From: Thomas Gleixner
Date: Thu Nov 27 2014 - 16:23:08 EST
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
> 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.
This is a per cpu property.
And there is more to that. Let's look at a single CPU.
lbr for oops is enabled
__intel_pmu_lbr_enable() -> LBR used by perf, oops dumper disabled
__intel_pmu_lbr_disable() -> LBR not longer used by perf, oops
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;
if (!this_cpu_sub_return(lbr_dump_state, LBR_PERF_USAGE))
Now of course you need to handle this in the exception path per cpu as
> * Exception entry points.
> @@ -1063,6 +1103,8 @@ ENTRY(\sym)
> subq $ORIG_RAX-R15, %rsp
> CFI_ADJUST_CFA_OFFSET ORIG_RAX-R15
> + STOP_LBR
We really cannot do this unconditionally for every exception. This
wants to be conditional, i.e.
So we can select which exceptions actually get that treatment.
do_page_fault is probably the only one which is interesting
Now looking at your macro maze, I really wonder whether we can do it a
little bit less convoluted. We need to push/pop registers. error_entry
saves the registers already and has a (admitedly convoluted)
kernel/user space check. But we might be able to do something sane
there. Cc'ing Andy as he is the master of that universe.
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