[PATCH v5 1/2] function_graph: Support recording and printing the return value of function

[Donglin Peng]

On 2023/3/21 23:11, Florian Kauer wrote:
> On 21.03.23 15:44, Steven Rostedt wrote:
>> On Tue, 21 Mar 2023 15:09:40 +0100
>> Florian Kauer <florian.kauer at linutronix.de> wrote:
>>
>>> On 20.03.23 14:16, Donglin Peng wrote:
>>>> When using the function_graph tracer to analyze system call failures,
>>>> it can be time-consuming to analyze the trace logs and locate the kernel
>>>> function that first returns an error. This change aims to simplify the
>>>> process by recording the function return value to the 'retval' member of
>>>> 'ftrace_graph_ent' and printing it when outputing the trace log.
>>>
>>> I just came across your patch by pure luck and it helped me a lot
>>> to trace down a problem I had, thanks!
>>>
>>> So you can have my
>>> Tested-by: Florian Kauer <florian.kauer at linutronix.de>
>>>   
>>>> New trace options are introduced: funcgraph-retval and graph_retval_hex.
>>>
>>> I would personally prefer to have the second option scoped better, so for example
>>> "funcgraph-retval-hex".
>>
>> That could be an ftrace option.
> 
> What do you mean? In the current implementation both funcgraph-retval and graph_retval_hex
> are options for the function_graph tracer, but one is prefixed with "funcgraph-" as nearly
> all other options for the function_graph tracer and one is not (and is even snake_case, while
> the others are kebab-case). So it just looks inconsistent for me, but there might be a reason?

Agree, I also think "funcgraph-retval-hex" may look better.

Hi Hiramatsu, what do you think?

> 
> By the way: The documentation patch also references "function-retval" instead of
> "funcgraph-retval" in the documentation of the graph_retval_hex option.

Thanks, it has been removed in v6.

> 
>> Anyway, could you tell us your use case, and that could go into the change
>> log of this patch as "one use case that this helped with".
> 
> Nothing spectacular. I just wanted to find out why ICMP port unreachable messages
> sporadically lead to -111 (Connection Refused) for __sys_sendto() when IP_RECVERR is set
> and the call never fails if IP_RECVERR is not set. (I am still unsure if this is
> REALLY intended behavior, but at least it makes sense why this occurs when reading
> the sources).
> 
> And with this patch, the -111 is directly popping up in the trace, but I do not think
> that my missing knowledge about details of the kernel network stack
> really qualifies as a good argument ;-)
> 
> Greetings,
> Florian