Re: [PATCH] tracing:Do not profile dumpstack utilities to fix stack trace output
From: Jiaxing Wang
Date: Sat Apr 19 2014 - 19:48:14 EST
On 04/19/2014 11:18 PM, Steven Rostedt wrote:
> 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.
>
Return addresses are skipped as stack_trace dump stack from the return
address of dump_trace() and dump_stack() dump stack from the return address
of show_stack().
Here is the stack_trace case:
stack current->ret_stack[]
----- --------------------
return_to_handler print_context_stack+0xc6/0xd0
return_to_handler dump_trace+0xb5/0x1b0
return_to_handler save_stack_trace+0x30/0x50
return_to_handler stack_trace_call+0x119/0x340
ftrace_ops_listfunc+0xe6/0x100
ftrace_call+0x5/0xb
So when dump, the first two 'return_to_handler' entries are skipped but
not the current->ret_stack[] entries.
And the following is the dump_stack() case:
stack current->ret_stack[]
----- --------------------
return_to_handler print_context_stack+0xc6/0xd0
return_to_handler dump_trace+0xb5/0x1b0
return_to_handler show_trace_log_lvl+0x46/0x50
return_to_handler show_stack_log_lvl+0x52/0xe0
return_to_handler show_stack+0x26/0x50
return_to_handler dump_stack+0x41/0x52
In this case the first five 'return_to_handler' entries on stack are
skipped but not the current->ret_stack[] entries.
So the problem is caused by dumpstack utilities get traced by function_graph
tracer, and disable the profiling of them fixes this.
Thanks.
>>
>> 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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