Re: [PATCH 3/3] ARM: perf: allow tracing with kernel tracepoints events

[Jean Pihet]

Will,

On 3 July 2014 19:54, Will Deacon <will.deacon@xxxxxxx> wrote:
> On Fri, Jun 27, 2014 at 03:57:47PM +0100, Jean Pihet wrote:
>> When tracing with tracepoints events the IP and CPSR are set to 0,
>> preventing the perf code to resolve the symbols:
>>
>> ./perf record -e kmem:kmalloc cal
>> [ perf record: Woken up 1 times to write data ]
>> [ perf record: Captured and wrote 0.007 MB perf.data (~321 samples) ]
>>
>> ./perf report
>> Overhead Command Shared Object Symbol
>> ........ ....... ............. ...........
>> 40.78%   cal     [unknown]     [.]00000000
>> 31.6%    cal     [unknown]     [.]00000000
>>
>> The examination of the gathered samples (perf report -D) shows the IP
>> is set to 0 and that the samples are considered as user space samples,
>> while the IP should be set from the registers and the samples should be
>> considered as kernel samples.
>>
>> The fix is to implement perf_arch_fetch_caller_regs for ARM, which
>> fills the necessary registers used for the callchain unwinding and
>> to determine the user/kernel space property of the samples: ip, sp, fp
>> and cpsr.
>>
>> Tested with perf record and tracepoints triggering (-e <tracepoint>), with
>> unwinding using fp (--call-graph fp) and dwarf info (--call-graph dwarf).
>>
>> Reported by Sneha Priya on linaro-dev, cf.
>> http://lists.linaro.org/pipermail/linaro-dev/2014-May/017151.html
>>
>> Signed-off-by: Jean Pihet <jean.pihet@xxxxxxxxxx>
>> Cc: Will Deacon <will.deacon@xxxxxxx>
>> Reported-by: Sneha Priya <sneha.cse@xxxxxxxxxxx>
>> ---
>>  arch/arm/include/asm/perf_event.h | 19 +++++++++++++++++++
>>  1 file changed, 19 insertions(+)
>>
>> diff --git a/arch/arm/include/asm/perf_event.h b/arch/arm/include/asm/perf_event.h
>> index 7558775..b02b5d3 100644
>> --- a/arch/arm/include/asm/perf_event.h
>> +++ b/arch/arm/include/asm/perf_event.h
>> @@ -26,6 +26,25 @@ struct pt_regs;
>>  extern unsigned long perf_instruction_pointer(struct pt_regs *regs);
>>  extern unsigned long perf_misc_flags(struct pt_regs *regs);
>>  #define perf_misc_flags(regs)        perf_misc_flags(regs)
>> +
>> +/*
>> + * Take a snapshot of the regs.
>> + * We only need a few of the regs:
>> + * - ip for PERF_SAMPLE_IP,
>> + * - sp & fp for fp & dwarf based callchain unwinding,
>> + * - cpsr for user_mode() tests.
>> + */
>> +#define perf_arch_fetch_caller_regs(regs, __ip) {    \
>> +     instruction_pointer(regs)= (__ip);              \
>> +     __asm__ (                                       \
>> +             "str    sp, %[_ARM_sp]          \n\t"   \
>> +             "str    fp, %[_ARM_fp]          \n\t"   \
>> +             "mrs    %[_ARM_cpsr], cpsr      \n\t"   \
>> +             : [_ARM_sp]   "=m" (regs->ARM_sp),      \
>> +               [_ARM_fp]   "=m" (regs->ARM_fp),      \
>> +               [_ARM_cpsr] "=r" (regs->ARM_cpsr)     \
>> +     );                                              \
>> +}
>
> You don't appear to have addressed my comments from last time. What changed?
A new patch set it on its way, with the commit description and the
comments in the code updated.

Jean

>
> Will
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