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Message-Id: <20230402071713.4f278b65ba9bb67c72ab2fb5@kernel.org>
Date: Sun, 2 Apr 2023 07:17:13 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Donglin Peng <pengdonglin@...gfor.com.cn>
Cc: rostedt@...dmis.org, linux@...linux.org.uk, mark.rutland@....com,
will@...nel.org, catalin.marinas@....com,
rmk+kernel@...linux.org.uk, palmer@...belt.com,
paul.walmsley@...ive.com, aou@...s.berkeley.edu,
tglx@...utronix.de, dave.hansen@...ux.intel.com, x86@...nel.org,
bp@...en8.de, hpa@...or.com, chenhuacai@...nel.org,
zhangqing@...ngson.cn, kernel@...0n.name, mingo@...hat.com,
peterz@...radead.org, xiehuan09@...il.com, dinghui@...gfor.com.cn,
huangcun@...gfor.com.cn, dolinux.peng@...il.com,
linux-trace-kernel@...r.kernel.org, loongarch@...ts.linux.dev,
linux-riscv@...ts.infradead.org,
linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v10 2/8] tracing: Add documentation for funcgraph-retval
and funcgraph-retval-hex
On Fri, 31 Mar 2023 05:47:38 -0700
Donglin Peng <pengdonglin@...gfor.com.cn> wrote:
> Add documentation for the two newly introduced options for the
> function_graph tracer. The funcgraph-retval option is used to
> control whether or not to display the return value, while the
> funcgraph-retval-hex option is used to control the display
> format of the return value.
>
> Signed-off-by: Donglin Peng <pengdonglin@...gfor.com.cn>
Looks good to me.
Acked-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
Thank you!
> ---
> v9:
> - Update limitation description
>
> v7:
> - Rename trace option 'graph_retval_hex' to 'funcgraph-retval-hex'
> - Update documentation description
>
> v6:
> - Modify the limitations for funcgraph-retval
> - Optimize the English expression
>
> v5:
> - Describe the limitations of funcgraph-retval
> ---
> Documentation/trace/ftrace.rst | 74 ++++++++++++++++++++++++++++++++++
> 1 file changed, 74 insertions(+)
>
> diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst
> index b927fb2b94dc..f572ae419219 100644
> --- a/Documentation/trace/ftrace.rst
> +++ b/Documentation/trace/ftrace.rst
> @@ -1328,6 +1328,19 @@ Options for function_graph tracer:
> only a closing curly bracket "}" is displayed for
> the return of a function.
>
> + funcgraph-retval
> + When set, the return value of each traced function
> + will be printed after an equal sign "=". By default
> + this is off.
> +
> + funcgraph-retval-hex
> + When set, the return value will always be printed
> + in hexadecimal format. If the option is not set and
> + the return value is an error code, it will be printed
> + in signed decimal format; otherwise it will also be
> + printed in hexadecimal format. By default, this option
> + is off.
> +
> sleep-time
> When running function graph tracer, to include
> the time a task schedules out in its function.
> @@ -2673,6 +2686,67 @@ It is default disabled.
> 0) 1.757 us | } /* kmem_cache_free() */
> 0) 2.861 us | } /* putname() */
>
> +The return value of each traced function can be displayed after
> +an equal sign "=". When encountering system call failures, it
> +can be verfy helpful to quickly locate the function that first
> +returns an error code.
> +
> + - hide: echo nofuncgraph-retval > trace_options
> + - show: echo funcgraph-retval > trace_options
> +
> + Example with funcgraph-retval::
> +
> + 1) | cgroup_migrate() {
> + 1) 0.651 us | cgroup_migrate_add_task(); /* = 0xffff93fcfd346c00 */
> + 1) | cgroup_migrate_execute() {
> + 1) | cpu_cgroup_can_attach() {
> + 1) | cgroup_taskset_first() {
> + 1) 0.732 us | cgroup_taskset_next(); /* = 0xffff93fc8fb20000 */
> + 1) 1.232 us | } /* cgroup_taskset_first = 0xffff93fc8fb20000 */
> + 1) 0.380 us | sched_rt_can_attach(); /* = 0x0 */
> + 1) 2.335 us | } /* cpu_cgroup_can_attach = -22 */
> + 1) 4.369 us | } /* cgroup_migrate_execute = -22 */
> + 1) 7.143 us | } /* cgroup_migrate = -22 */
> +
> +The above example shows that the function cpu_cgroup_can_attach
> +returned the error code -22 firstly, then we can read the code
> +of this function to get the root cause.
> +
> +When the option funcgraph-retval-hex is not set, the return value can
> +be displayed in a smart way. Specifically, if it is an error code,
> +it will be printed in signed decimal format, otherwise it will
> +printed in hexadecimal format.
> +
> + - smart: echo nofuncgraph-retval-hex > trace_options
> + - hexadecimal always: echo funcgraph-retval-hex > trace_options
> +
> + Example with funcgraph-retval-hex::
> +
> + 1) | cgroup_migrate() {
> + 1) 0.651 us | cgroup_migrate_add_task(); /* = 0xffff93fcfd346c00 */
> + 1) | cgroup_migrate_execute() {
> + 1) | cpu_cgroup_can_attach() {
> + 1) | cgroup_taskset_first() {
> + 1) 0.732 us | cgroup_taskset_next(); /* = 0xffff93fc8fb20000 */
> + 1) 1.232 us | } /* cgroup_taskset_first = 0xffff93fc8fb20000 */
> + 1) 0.380 us | sched_rt_can_attach(); /* = 0x0 */
> + 1) 2.335 us | } /* cpu_cgroup_can_attach = 0xffffffea */
> + 1) 4.369 us | } /* cgroup_migrate_execute = 0xffffffea */
> + 1) 7.143 us | } /* cgroup_migrate = 0xffffffea */
> +
> +At present, there are some limitations when using the funcgraph-retval
> +option, and these limitations will be eliminated in the future:
> +
> +- Even if the function return type is void, a return value will still
> + be printed, and you can just ignore it.
> +
> +- Even if return values are stored in multiple registers, only the
> + value contained in the first register will be recorded and printed.
> + To illustrate, in the x86 architecture, eax and edx are used to store
> + a 64-bit return value, with the lower 32 bits saved in eax and the
> + upper 32 bits saved in edx. However, only the value stored in eax
> + will be recorded and printed.
> +
> You can put some comments on specific functions by using
> trace_printk() For example, if you want to put a comment inside
> the __might_sleep() function, you just have to include
> --
> 2.25.1
>
--
Masami Hiramatsu (Google) <mhiramat@...nel.org>
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