Re: [PATCH v10 2/8] tracing: Add documentation for funcgraph-retval and funcgraph-retval-hex
From: Mark Rutland
Date: Mon Apr 03 2023 - 04:30:29 EST
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@xxxxxxxxxxxxxx>
> ---
> 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.