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

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Date:   Wed, 22 Mar 2023 13:01:09 +0800
From:   Donglin Peng <pengdonglin@...gfor.com.cn>
To:     Florian Kauer <florian.kauer@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>, mhiramat@...nel.org
Cc:     linux@...linux.org.uk, mark.rutland@....com, will@...nel.org,
        catalin.marinas@....com, palmer@...belt.com,
        paul.walmsley@...ive.com, tglx@...utronix.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, mingo@...hat.com,
        xiehuan09@...il.com, dinghui@...gfor.com.cn,
        huangcun@...gfor.com.cn, dolinux.peng@...il.com,
        linux-trace-kernel@...r.kernel.org,
        linux-riscv@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 1/2] function_graph: Support recording and printing the
 return value of function

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@...utronix.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@...utronix.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