Re: [PATCH v10 2/8] tracing: Add documentation for funcgraph-retval and funcgraph-retval-hex

[Mark Rutland <mark.rutland@arm.com>]

On Fri, Mar 31, 2023 at 05:47:38AM -0700, Donglin Peng 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@sangfor.com.cn>
> ---
> 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.

With some procedure call standards (e.g. arm64's AAPCS64), when a type is
smaller than a GPR it's up to the consumer to perform the narrowing, and the
upport bits may contain UNKNOWN values. For example, with a u8 in a 64-bit GPR,
bits [3:8] may contain arbitrary values.

It's probably worth noting that this means *some* manual processing will always
be necessary for such cases.

That's mostly visible around where largelr types get truncated (whether
explciitly or implicitly), e.g.

	u8 narrow_to_u8(u64 val)
	{
		// implicitly truncated
		return val;
	}

... could be compiled to:

	narrow_to_u8:
		< ... ftrace instrumentation ... >
		RET

... and so:
	
	narrow_to_u8(0x123456789abcdef);

... might be recorded as returning 0x123456789abcdef rather than 0xef.


That can happen in surprising ways, e.g.

	int error_if_not_4g_aligned(u64 val)
	{
		if (val & GENMASK(63, 32))
			return -EINVAL;

		return 0;
	}

... could be compiled to:

	error_if_not_4g_aligned:
		CBNZ	w0, .Lnot_aligned
		RET				// bits [31:0] are zero, bits
						// [63:32] are UNKNOWN
	.Lnot_aligned:
		MOV	x0, #-EINVAL
		RET

.... and so:

	error_if_not_4g_aligned(SZ_8G)

... could return with bits [63:32] non-zero

Thanks,
Mark.

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