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.
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 */
Changelog:
v5:
- Add an inline fallback for btf_trim_retval() when
CONFIG_DEBUG_INFO_BTF is disabled.
- Release the BTF reference after return type lookup.
v4:
- Link:
https://lore.kernel.org/all/[email protected]/
- Build trace_btf.c when CONFIG_DEBUG_INFO_BTF is enabled.
- Remove the redundant BPF_SYSCALL dependency from
CONFIG_PROBE_EVENTS_BTF_ARGS.
- Update the funcgraph-retval documentation and cover letter.
v3:
- Link:
https://lore.kernel.org/all/[email protected]/
- Print the return value based on its type for human readability,
thanks Masami.
- Update the documentation and cover letter.
v2:
- Link:
https://lore.kernel.org/all/[email protected]/
- Update the funcgraph-retval documentation.
- Revise the cover letter.
v1:
- Link:
https://lore.kernel.org/all/[email protected]/
pengdonglin (3):
ftrace: Build trace_btf.c when CONFIG_DEBUG_INFO_BTF is enabled
fgraph: Enhance funcgraph-retval with BTF-based type-aware output
tracing: Update funcgraph-retval documentation
Documentation/trace/ftrace.rst | 88 ++++++++++++++++------------
kernel/trace/Kconfig | 2 +-
kernel/trace/Makefile | 2 +-
kernel/trace/trace_btf.c | 68 +++++++++++++++++++++
kernel/trace/trace_btf.h | 16 +++++
kernel/trace/trace_functions_graph.c | 50 ++++++++++++----
6 files changed, 173 insertions(+), 53 deletions(-)
--
2.34.1
