From: pengdonglin <[email protected]> The existing documentation for funcgraph-retval is outdated and partially incorrect, as it describes limitations that have now been resolved.
Recent changes (e.g., using BTF to obtain function return types) have addressed key issues: 1. Return values are now printed only for non-void functions. 2. Values are trimmed to the correct width of the return type, avoiding garbage data from high bits. Cc: Steven Rostedt (Google) <[email protected]> Cc: Masami Hiramatsu <[email protected]> Cc: Xiaoqin Zhang <[email protected]> Signed-off-by: pengdonglin <[email protected]> --- Documentation/trace/ftrace.rst | 74 ++++++++++++++++++---------------- 1 file changed, 40 insertions(+), 34 deletions(-) diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst index d1f313a5f4ad..de97b65433cc 100644 --- a/Documentation/trace/ftrace.rst +++ b/Documentation/trace/ftrace.rst @@ -1454,6 +1454,10 @@ Options for function_graph tracer: printed in hexadecimal format. By default, this option is off. + funcgraph-retaddr + When set, the return address will always be printed. + By default, this option is off. + sleep-time When running function graph tracer, to include the time a task schedules out in its function. @@ -2800,7 +2804,7 @@ It is default disabled. 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 +an equal sign "ret =". When encountering system call failures, it can be very helpful to quickly locate the function that first returns an error code. @@ -2810,16 +2814,16 @@ returns an error code. Example with funcgraph-retval:: 1) | cgroup_migrate() { - 1) 0.651 us | cgroup_migrate_add_task(); /* = 0xffff93fcfd346c00 */ + 1) 0.651 us | cgroup_migrate_add_task(); /* ret=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 */ + 1) 0.732 us | cgroup_taskset_next(); /* ret=0xffff93fc8fb20000 */ + 1) 1.232 us | } /* cgroup_taskset_first ret=0xffff93fc8fb20000 */ + 1) 0.380 us | sched_rt_can_attach(); /* ret=0x0 */ + 1) 2.335 us | } /* cpu_cgroup_can_attach ret=-22 */ + 1) 4.369 us | } /* cgroup_migrate_execute ret=-22 */ + 1) 7.143 us | } /* cgroup_migrate ret=-22 */ The above example shows that the function cpu_cgroup_can_attach returned the error code -22 firstly, then we can read the code @@ -2836,37 +2840,39 @@ printed in hexadecimal format. Example with funcgraph-retval-hex:: 1) | cgroup_migrate() { - 1) 0.651 us | cgroup_migrate_add_task(); /* = 0xffff93fcfd346c00 */ + 1) 0.651 us | cgroup_migrate_add_task(); /* ret=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) 0.732 us | cgroup_taskset_next(); /* ret=0xffff93fc8fb20000 */ + 1) 1.232 us | } /* cgroup_taskset_first ret=0xffff93fc8fb20000 */ + 1) 0.380 us | sched_rt_can_attach(); /* ret=0x0 */ + 1) 2.335 us | } /* cpu_cgroup_can_attach ret=0xffffffea */ + 1) 4.369 us | } /* cgroup_migrate_execute ret=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. - -- In certain procedure call standards, such as arm64's AAPCS64, when a - type is smaller than a GPR, it is the responsibility of the consumer - to perform the narrowing, and the upper bits may contain UNKNOWN values. - Therefore, it is advisable to check the code for such cases. For instance, - when using a u8 in a 64-bit GPR, bits [63:8] may contain arbitrary values, - especially when larger types are truncated, whether explicitly or implicitly. - Here are some specific cases to illustrate this point: +Note that there are some limitations when using the funcgraph-retval +option: + +- If CONFIG_DEBUG_INFO_BTF is disabled (n), a return value is printed even for + functions with a void return type. When CONFIG_DEBUG_INFO_BTF is enabled (y), + the return value is printed only for non-void functions. + +- If a return value occupies multiple registers, only the value in the first + register is recorded and printed. For example, on the x86 architecture, a + 64-bit return value is stored across eax (lower 32 bits) and edx (upper 32 bits), + but only the contents of eax are captured. + +- Under certain procedure-call standards (e.g., arm64's AAPCS64), when the return + type is smaller than a general-purpose register (GPR), the caller is responsible + for narrowing the value; the upper bits of the register may contain undefined data. + For instance, when a u8 is returned in 64-bit GPR, bits [63:8] can hold arbitrary + values, especially when larger types are truncated (explicitly or implicitly). It + is therefore advisable to inspect the code in such cases. If CONFIG_DEBUG_INFO_BTF + is enabled (y), the return value is automatically trimmed to the width of the return + type. + + The following examples illustrate the behavior: **Case One**: -- 2.34.1
