Re: [PATCH v5 0/3] Use BTF to trim return values

[Peng Donglin <dolinux.peng@gmail.com>]

On 9/2/26 01:57, Steven Rostedt wrote:
> On Tue,  1 Sep 2026 21:46:01 +0800
> Donglin Peng <dolinux.peng@gmail.com> wrote:
> 
>> From: pengdonglin <pengdonglin@xiaomi.com>
>>
>> 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 */