From: pengdonglin <[email protected]>
This patch series addresses two limitations of the funcgraph-retval feature:
1. Void-returning functions still print a return value, creating misleading
noise in the trace output.
2. For functions returning narrower types (e.g., char, short), the displayed
value can be incorrect because high bits of the register may contain
undefined data.
By leveraging BTF to obtain precise return type information, we now:
1. Void function filtering: Functions with void return type no longer
display any return value in the trace output, eliminating unnecessary
clutter.
2. Type-aware value formatting: The return value is now properly truncated to
match the actual width of the return type before being displayed.
Additionally, the value is formatted according to its type for better human
readability.
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 */
Note: enabling funcgraph-retval now adds overhead due to repeated
btf_find_by_name_kind()
calls during trace output. A separate series [1] optimizes this function with
binary search (O(log n) vs current O(n)), which will greatly reduce the impact.
Here is a performance comparison:
1. Original funcgraph-retval:
# time cat trace | wc -l
101024
real 0m0.682s
user 0m0.000s
sys 0m0.695s
2. Enhanced funcgraph-retval:
# time cat trace | wc -l
99326
real 0m12.886s
user 0m0.010s
sys 0m12.680s
3. Enhanced funcgraph-retval + optimizined btf_find_by_name_kind:
# time cat trace | wc -l
102922
real 0m0.794s
user 0m0.000s
sys 0m0.810s
Changelog:
v3:
- Print the return value based on its type for human readability, thanks Masami
- Update 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]/
[1] https://lore.kernel.org/all/[email protected]/
pengdonglin (2):
fgraph: Enhance funcgraph-retval with BTF-based type-aware output
tracing: Update funcgraph-retval documentation
Documentation/trace/ftrace.rst | 78 ++++++++++-------
kernel/trace/trace_functions_graph.c | 124 ++++++++++++++++++++++++---
2 files changed, 156 insertions(+), 46 deletions(-)
--
2.34.1
