perf: problem of 32bit mmap call graph recording

Hi all,

On 32 bit machine, we have problem to get the usersapce call graph of mmap syscall.

You can reproduce it with below command on 32bit machine.

sudo perf record -f -g -e raw_syscalls:sys_enter --filter "id==192" ls

sudo perf report |less

# Events: 26
#

# Overhead  Command      Shared Object  Symbol
# ........  .......  .................  ......
#
   100.00%       ls  [kernel.kallsyms]  [k] syscall_trace_enter
                 |
                 --- syscall_trace_enter
                     syscall_trace_entry
                    |          
                    |--88.46%-- __mmap  <--- only __mmap, no more userspace call stacks
                    |          
                     --11.54%-- 0xffffe424

Kernel relies on userspace bp register to get call graph information. But for mmap syscall, the bp register is used to pass in the sixth parameter(offset).

void *mmap(void *addr, size_t length, int prot, int flags,

                  int fd, off_t offset);

Anyway to fix this issue on 32bit machine?

Thanks,
Lin Ming

Ah... Well we can't do much about that I fear :)
The only way would be to rely on dwarf informations to do the unwind. The upside is that it would work about everywhere even on places that don't have frame pointers, the downside is some overhead on sample records (dump a bunch of stack every sample) and a lot of work :)