Re: [PATCH] tracing:Do not profile dumpstack utilities to fix stack trace output

[Steven Rostedt]

On Sat, 19 Apr 2014 15:45:46 +0800
Jiaxing Wang <wangjiaxing@xxxxxxxxxxxxxx> wrote:

> When function_graph tracer is enabled, we get stack trace like this:
> 
>          Depth    Size   Location    (88 entries)
>          -----    ----   --------
>    0)     3232       0   update_curr+0xe/0x1e0
>    1)     3232       0   dump_trace+0xb5/0x1b0
>    2)     3232       0   ftrace_ops_list_func+0xe6/0x100
>    3)     3232      68   ftrace_call+0x5/0xb
>    4)     3164      92   enqueue_entity+0x38/0xe20
>    5)     3072       0   return_to_handler+0x0/0xf
>    6)     3072      80   save_stack_trace+0x30/0x50
>    7)     2992       0   return_to_handler+0x0/0xf
>    8)     2992      28   stack_trace_call+0x119/0x340
>    9)     2964       0   return_to_handler+0x0/0xf
>   10)     2964      12   enqueue_task_fair+0x47/0x7e0
>   11)     2952       0   return_to_handler+0x0/0xf
>   12)     2952      16   enqueue_task+0x45/0x70
>   ... ...
> 
> The 'save_stack_trace+0x30/0x50' and 'stack_trace_call+0x119/0x340'
> entries are misplaced below the ftrace_ops_list_func and ftrace_call entries,
> which looks very confusing.
> 
> And dump_stack() is also affected when function_graph tracer is enabled:
> 
>   [  425.353391] Call Trace:
>   [  425.353449]  [<c16531c1>] ftrace_graph_caller+0x19/0x19
>   [  425.353496]  [<c1012546>] print_context_stack+0xc6/0xd0
>   [  425.353547]  [<c108ab84>] enqueue_entity+0x894/0xe60
>   [  425.353597]  [<c107b4c5>] ? enqueue_task+0x45/0x70
>   [  425.353649]  [<c108b159>] ? enqueue_task_fair+0x9/0x7e0
>   [  425.353694]  [<c107b4c5>] ? enqueue_task+0x45/0x70
>   [  425.353743]  [<c1041037>] ? prepare_ftrace_return+0x77/0xa0
>   [  425.353791]  [<c108b159>] ? enqueue_task_fair+0x9/0x7e0
>   [  425.353856]  [<c16531c1>] ftrace_graph_caller+0x19/0x19
>   [  425.353901]  [<c10116b5>] dump_trace+0xb5/0x1b0
>   [  425.353958]  [<c107b4dd>] ? enqueue_task+0x5d/0x70
>   [  425.354002]  [<c107b436>] ? update_rq_clock.part.80+0x6/0x50
>   [  425.354074]  [<c16531c1>] ftrace_graph_caller+0x19/0x19
>   [  425.354120]  [<c1012666>] show_trace_log_lvl+0x46/0x50
>   [  425.354178]  [<c16531c1>] ftrace_graph_caller+0x19/0x19
>   [  425.354222]  [<c1011802>] show_stack_log_lvl+0x52/0xe0
>   [  425.354272]  [<c16531c1>] ftrace_graph_caller+0x19/0x19
>   [  425.354317]  [<c10126c6>] show_stack+0x26/0x50
>   [  425.354369]  [<c16531c1>] ftrace_graph_caller+0x19/0x19
>   [  425.354415]  [<c1643be1>] dump_stack+0x41/0x52
>   [  425.354483]  [<c16531c1>] ftrace_graph_caller+0x19/0x19
>   [  425.354527]  [<c108b197>] enqueue_task_fair+0x47/0x7e0
>   [  425.354576]  [<c16531c1>] ftrace_graph_caller+0x19/0x19
>   [  425.354621]  [<c107b4c5>] enqueue_task+0x45/0x70
>   ... ...
> 
> This happens because the functions in the route of stack_trace and
> dump_stack() get function_graph traced, their return addresses on stack are
> replaced by 'return_to_handler' and their real return addresses are
> saved in current->ret_stack[].(Specifically, these functons include
> save_stack_trace,dump_trace,print_context_stack,print_ftrace_graph_addr
> in the case of stack_trace and show_stack,show_stack_log_lvl,show_trace_log_lvl
> ,dump_trace,print_context_stack,print_ftrace_graph_addr in the case of
> dump_stack()).
> 
> When dumping the stack, their 'return_to_handler' on stack are skipped
> but not their corresponding current->ret_stack entries which then are matched
> with other function's 'return_to_handler' on the stack and resulting the
> wrong output above.

This sounds like a separate bug that should be fixed.

> 
> To fix this, we can disable the profiling of dumpstack utilities.
> 
> Signed-off-by: Jiaxing Wang <wangjiaxing@xxxxxxxxxxxxxx>
> ---
>  arch/x86/kernel/Makefile | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
> index f4d9600..9dd8ce4 100644
> --- a/arch/x86/kernel/Makefile
> +++ b/arch/x86/kernel/Makefile
> @@ -14,6 +14,9 @@ CFLAGS_REMOVE_pvclock.o = -pg
>  CFLAGS_REMOVE_kvmclock.o = -pg
>  CFLAGS_REMOVE_ftrace.o = -pg
>  CFLAGS_REMOVE_early_printk.o = -pg
> +CFLAGS_REMOVE_dumpstack.o = -pg
> +CFLAGS_REMOVE_dumpstack_32.o = -pg
> +CFLAGS_REMOVE_stacktrace.o = -pg

I have no problem with this change, but it is 3.16 material. But if the
function graph tracer is causing the stack trace to be incorrect, then
that is a separate issues that we should try to rectify.

-- Steve

>  endif
>  
>  CFLAGS_irq.o := -I$(src)/../include/asm/trace

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