• Milian Wolff's avatar
    perf report: Fix off-by-one for non-activation frames · 1982ad48
    Milian Wolff authored
    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
    ~~~~~~~~~~~~~~~
    
    With 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
    ~~~~~~~~~~~~~~~
    
    Similarly it works for signal frames:
    
    ~~~~~~~~~~~~~~~
      __noinline void bar(void)
      {
        volatile long cnt = 0;
    
        for (cnt = 0; cnt < 100000000; cnt++);
      }
    
      __noinline void foo(void)
      {
        bar();
      }
    
      void sig_handler(int sig)
      {
        foo();
      }
    
      int main(void)
      {
        signal(SIGUSR1, sig_handler);
        raise(SIGUSR1);
    
        foo();
        return 0;
      }
    ~~~~~~~~~~~~~~~~
    
    Before, the report wrongly points to `signal.c:29` after raise():
    
    ~~~~~~~~~~~~~~~~
      $ perf report --stdio --no-children -g srcline -s srcline
      ...
       100.00%  signal.c:11
                |
                ---bar signal.c:11
                   |
                   |--50.49%--main signal.c:29
                   |          __libc_start_main
                   |          _start
                   |
                    --49.51%--0x33a8f
                              raise .:0
                              main signal.c:29
                              __libc_start_main
                              _start
    ~~~~~~~~~~~~~~~~
    
    With this patch in, the issue is fixed and we instead get:
    
    ~~~~~~~~~~~~~~~~
       100.00%  signal   signal            [.] bar
                |
                ---bar signal.c:11
                   |
                   |--50.49%--main signal.c:29
                   |          __libc_start_main
                   |          _start
                   |
                    --49.51%--0x33a8f
                              raise .:0
                              main signal.c:27
                              __libc_start_main
                              _start
    ~~~~~~~~~~~~~~~~
    
    Note how this patch fixes this issue for both unwinding methods, i.e.
    both dwfl and libunwind. The former case is straight-forward thanks
    to dwfl_frame_pc(). For libunwind, we replace the functionality via
    unw_is_signal_frame() for any but the very first frame.
    Signed-off-by: default avatarMilian Wolff <milian.wolff@kdab.com>
    Signed-off-by: default avatarNamhyung Kim <namhyung@kernel.org>
    Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
    Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
    Cc: David Ahern <dsahern@gmail.com>
    Cc: Jiri Olsa <jolsa@kernel.org>
    Cc: Jiri Olsa <jolsa@redhat.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Yao Jin <yao.jin@linux.intel.com>
    Cc: kernel-team@lge.com
    Link: http://lkml.kernel.org/r/20170524062129.32529-4-namhyung@kernel.orgSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
    1982ad48
unwind-libunwind-local.c 17.3 KB