As the documentation for dwfl_frame_pc says, frames that are no activation frames need to have their program counter decremented by one to properly find the function of the caller.
This fixes many cases where perf report currently attributes the cost to the next line. I.e. I have code like this: ~~~~~~~~~~~~~~~ #include <thread> #include <chrono> using namespace std; int main() { this_thread::sleep_for(chrono::milliseconds(1000)); this_thread::sleep_for(chrono::milliseconds(100)); this_thread::sleep_for(chrono::milliseconds(10)); return 0; } ~~~~~~~~~~~~~~~ Now compile and record it: ~~~~~~~~~~~~~~~ g++ -std=c++11 -g -O2 test.cpp echo 1 | sudo tee /proc/sys/kernel/sched_schedstats perf record \ --event sched:sched_stat_sleep \ --event sched:sched_process_exit \ --event sched:sched_switch --call-graph=dwarf \ --output perf.data.raw \ ./a.out echo 0 | sudo tee /proc/sys/kernel/sched_schedstats perf inject --sched-stat --input perf.data.raw --output perf.data ~~~~~~~~~~~~~~~ Before this patch, the report clearly shows the off-by-one issue. Most notably, the last sleep invocation is incorrectly attributed to the "return 0;" line: ~~~~~~~~~~~~~~~ Overhead Source:Line ........ ........... 100.00% core.c:0 | ---__schedule core.c:0 schedule do_nanosleep hrtimer.c:0 hrtimer_nanosleep sys_nanosleep entry_SYSCALL_64_fastpath .tmp_entry_64.o:0 __nanosleep_nocancel .:0 std::this_thread::sleep_for<long, std::ratio<1l, 1000l> > thread:323 | |--90.08%--main test.cpp:9 | __libc_start_main | _start | |--9.01%--main test.cpp:10 | __libc_start_main | _start | --0.91%--main test.cpp:13 __libc_start_main _start ~~~~~~~~~~~~~~~ When compiling perf using libdwfl for unwinding instead of libunwind and having this patch here applied, the issue is fixed. The report becomes much more usable: ~~~~~~~~~~~~~~~ Overhead Source:Line ........ ........... 100.00% core.c:0 | ---__schedule core.c:0 schedule do_nanosleep hrtimer.c:0 hrtimer_nanosleep sys_nanosleep entry_SYSCALL_64_fastpath .tmp_entry_64.o:0 __nanosleep_nocancel .:0 std::this_thread::sleep_for<long, std::ratio<1l, 1000l> > thread:323 | |--90.08%--main test.cpp:8 | __libc_start_main | _start | |--9.01%--main test.cpp:9 | __libc_start_main | _start | --0.91%--main test.cpp:10 __libc_start_main _start ~~~~~~~~~~~~~~~ Cc: Arnaldo Carvalho de Melo <a...@redhat.com> Cc: David Ahern <dsah...@gmail.com> Cc: Namhyung Kim <namhy...@kernel.org> Cc: Peter Zijlstra <a.p.zijls...@chello.nl> Cc: Yao Jin <yao....@linux.intel.com> Signed-off-by: Milian Wolff <milian.wo...@kdab.com> --- tools/perf/util/unwind-libdw.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) diff --git a/tools/perf/util/unwind-libdw.c b/tools/perf/util/unwind-libdw.c index f90e11a555b2..943a06291587 100644 --- a/tools/perf/util/unwind-libdw.c +++ b/tools/perf/util/unwind-libdw.c @@ -168,12 +168,16 @@ frame_callback(Dwfl_Frame *state, void *arg) { struct unwind_info *ui = arg; Dwarf_Addr pc; + bool isactivation; - if (!dwfl_frame_pc(state, &pc, NULL)) { + if (!dwfl_frame_pc(state, &pc, &isactivation)) { pr_err("%s", dwfl_errmsg(-1)); return DWARF_CB_ABORT; } + if (!isactivation) + --pc; + return entry(pc, ui) || !(--ui->max_stack) ? DWARF_CB_ABORT : DWARF_CB_OK; } -- 2.13.0