• Steven Rostedt (Red Hat)'s avatar
    tracing: Fix stack tracer with fentry use · d4ecbfc4
    Steven Rostedt (Red Hat) authored
    When gcc 4.6 on x86 is used, the function tracer will use the new
    option -mfentry which does a call to "fentry" at every function
    instead of "mcount". The significance of this is that fentry is
    called as the first operation of the function instead of the mcount
    usage of being called after the stack.
    This causes the stack tracer to show some bogus results for the size
    of the last function traced, as well as showing "ftrace_call" instead
    of the function. This is due to the stack frame not being set up
    by the function that is about to be traced.
     # cat stack_trace
            Depth    Size   Location    (48 entries)
            -----    ----   --------
      0)     4824     216   ftrace_call+0x5/0x2f
      1)     4608     112   ____cache_alloc+0xb7/0x22d
      2)     4496      80   kmem_cache_alloc+0x63/0x12f
    The 216 size for ftrace_call includes both the ftrace_call stack
    (which includes the saving of registers it does), as well as the
    stack size of the parent.
    To fix this, if CC_USING_FENTRY is defined, then the stack_tracer
    will reserve the first item in stack_dump_trace[] array when
    calling save_stack_trace(), and it will fill it in with the parent ip.
    Then the code will look for the parent pointer on the stack and
    give the real size of the parent's stack pointer:
     # cat stack_trace
            Depth    Size   Location    (14 entries)
            -----    ----   --------
      0)     2640      48   update_group_power+0x26/0x187
      1)     2592     224   update_sd_lb_stats+0x2a5/0x4ac
      2)     2368     160   find_busiest_group+0x31/0x1f1
      3)     2208     256   load_balance+0xd9/0x662
    I'm Cc'ing stable, although it's not urgent, as it only shows bogus
    size for item #0, the rest of the trace is legit. It should still be
    corrected in previous stable releases.
    Cc: stable@vger.kernel.org
    Signed-off-by: default avatarSteven Rostedt <rostedt@goodmis.org>
trace_stack.c 9.11 KB