From: pengdonglin <[email protected]>
This patch series addresses two limitations of the funcgraph-retval feature:
1. Previously, return values were printed even for void functions.
2. For return types narrower than a register, high-bit corruption could
lead to incorrect values.
By leveraging BTF to obtain precise return type information, we now:
1. Suppress output for void functions (eliminating noise)
2. Properly truncate values to match the actual return type width (ensuring
correctness)
These improvements make funcgraph-retval more accurate and useful.
Before:
# perf ftrace -G vfs_read --graph-opts retval
...
1) | _raw_spin_unlock() {
1) 0.074 us | do_raw_spin_unlock(); /* ret=0x1 */
1) 0.072 us | preempt_count_sub(); /* ret=0x0 */
1) 0.373 us | } /* _raw_spin_unlock ret=0x80000000 */
1) | down_read() {
1) 0.070 us | __cond_resched(); /* ret=0x0 */
1) 0.069 us | preempt_count_add(); /* ret=0x0 */
1) 0.071 us | preempt_count_sub(); /* ret=0x0 */
1) 0.612 us | } /* down_read ret=0x80000000 */
1) | down_write() {
1) 0.070 us | __cond_resched(); /* ret=0x0 */
1) 0.071 us | preempt_count_add(); /* ret=0x0 */
1) 0.070 us | preempt_count_sub(); /* ret=0x0 */
1) 0.605 us | } /* down_write ret=0x80000000 */
After:
# perf ftrace -G vfs_read --graph-opts retval
...
0) | _raw_spin_unlock() {
0) 0.075 us | do_raw_spin_unlock();
0) 0.075 us | preempt_count_sub();
0) 0.380 us | } /* _raw_spin_unlock */
0) | down_read() {
0) 0.070 us | __cond_resched(); /* ret=0x0 */
0) 0.073 us | preempt_count_add();
0) 0.072 us | preempt_count_sub();
0) 0.586 us | } /* down_read */
0) | down_write() {
0) 0.070 us | __cond_resched(); /* ret=0x0 */
0) 0.072 us | preempt_count_add();
0) 0.072 us | preempt_count_sub();
0) 0.676 us | } /* down_write */
Changelog:
v2:
- Update the funcgraph-retval documentation
- Revise the cover letter
v1:
- Link:
https://lore.kernel.org/all/[email protected]/
pengdonglin (2):
fgraph: Use BTF to trim and filter return values
tracing: Update funcgraph-retval documentation
Documentation/trace/ftrace.rst | 76 ++++++++++++++++------------
kernel/trace/trace_functions_graph.c | 64 +++++++++++++++++++----
2 files changed, 97 insertions(+), 43 deletions(-)
--
2.34.1
