On 9/2/26 01:57, Steven Rostedt wrote:
On Tue, 1 Sep 2026 21:46:01 +0800
Donglin Peng <[email protected]> wrote:
From: pengdonglin <[email protected]>
The funcgraph-retval option currently records one machine-word return
value without considering the function's declared return type. As a
result, void-returning functions can produce meaningless output, and
return values narrower than a general-purpose register can include
undefined high bits.
This series uses kernel BTF to identify the return type of traced
functions and to improve funcgraph-retval output:
- Do not print a return value for functions with a void return type.
- Trim integer, enum, struct, and union values to the available return
value width when the BTF type provides suitable size information.
- Format integer, enum, and boolean values according to their BTF
encoding.
- Mark values as "(trunc)" when the return type is wider than the
value captured by the function graph tracer.
Hmm, I really don't like the "(trunc)" If it's the real return type, then
it should just print what the real size is. No need to state it was truncated.
Thanks, I agree. We can remove the "(trunc)" suffix and print the actual
type information instead, for example:
ret=0x5[struct:0x10]
This would indicate that the return type is a struct with a size of 0x10
bytes.
Would this be acceptable?
-- Steve
Here is an output comparison:
Before:
# perf ftrace -G vfs_read --graph-opts retval
...
1) | touch_atime() {
1) | atime_needs_update() {
1) 0.069 us | make_vfsuid(); /* ret=0x0 */
1) 0.067 us | make_vfsgid(); /* ret=0x0 */
1) | current_time() {
1) 0.197 us | ktime_get_coarse_real_ts64_mg(); /*
ret=0x187f886aec3ed6f5 */
1) 0.352 us | } /* current_time ret=0x69380753 */
1) 0.792 us | } /* atime_needs_update ret=0x0 */
1) 0.937 us | } /* touch_atime ret=0x0 */
After:
# perf ftrace -G vfs_read --graph-opts retval
...
2) | touch_atime() {
2) | atime_needs_update() {
2) 0.070 us | make_vfsuid(); /* ret=0x0 */
2) 0.070 us | make_vfsgid(); /* ret=0x0 */
2) | current_time() {
2) 0.162 us | ktime_get_coarse_real_ts64_mg();
2) 0.312 us | } /* current_time ret=0x69380649(trunc) */
2) 0.753 us | } /* atime_needs_update ret=false */
2) 0.899 us | } /* touch_atime */
