Re: [PATCH] perf test: Fix LBR test by adding indirect calls

[Ian Rogers <irogers@google.com>]

On Sat, Nov 2, 2024 at 5:24 PM Namhyung Kim <namhyung@kernel.org> wrote:
>
> I've noticed sometimes perf record LBR tests failed on indirect call
> test because it has empty branch stacks more than expected.
>
> The test workload (thloop) spawns a thread and calls a loop function for
> 1 second both from the main thread and the new thread.  However neither
> of them has indirect calls in the body so it ended up with empty branch
> stacks.
>
>   LBR any indirect call test
>   [ perf record: Woken up 21 times to write data ]
>   [ perf record: Captured and wrote 5.607 MB /tmp/__perf_test.perf.data.pujKd (7924 samples) ]
>   LBR any indirect call test: 7924 samples
>   LBR any indirect call test [Failed empty br stack ratio exceed 2%: 3%]
>
> Refactor the test workload to call the test_loop() both directly and
> indirectly.  Now expectation of indirect call is 50% but let's add some
> margin for startup and finish routines.
>
> Signed-off-by: Namhyung Kim <namhyung@kernel.org>
> ---
>  tools/perf/tests/shell/record_lbr.sh | 2 +-
>  tools/perf/tests/workloads/thloop.c  | 9 ++++++---
>  2 files changed, 7 insertions(+), 4 deletions(-)
>
> diff --git a/tools/perf/tests/shell/record_lbr.sh b/tools/perf/tests/shell/record_lbr.sh
> index 8d750ee631f877fd..7a23b2095be8acba 100755
> --- a/tools/perf/tests/shell/record_lbr.sh
> +++ b/tools/perf/tests/shell/record_lbr.sh
> @@ -121,7 +121,7 @@ lbr_test "-j any_ret" "any ret" 2
>  lbr_test "-j ind_call" "any indirect call" 2
>  lbr_test "-j ind_jmp" "any indirect jump" 100
>  lbr_test "-j call" "direct calls" 2
> -lbr_test "-j ind_call,u" "any indirect user call" 100
> +lbr_test "-j ind_call,u" "any indirect user call" 52
>  lbr_test "-a -b" "system wide any branch" 2
>  lbr_test "-a -j any_call" "system wide any call" 2
>
> diff --git a/tools/perf/tests/workloads/thloop.c b/tools/perf/tests/workloads/thloop.c
> index 457b29f91c3ee277..fa5547939882cf6c 100644
> --- a/tools/perf/tests/workloads/thloop.c
> +++ b/tools/perf/tests/workloads/thloop.c
> @@ -18,14 +18,16 @@ static void sighandler(int sig __maybe_unused)
>
>  noinline void test_loop(void)
>  {
> -       while (!done);
> +       for (volatile int i = 0; i < 10000; i++)

I don't think the volatile here will stop a sufficiently eager
optimizing compiler. I think it may need to be static as well.

Thanks,
Ian

> +               continue;
>  }
>
>  static void *thfunc(void *arg)
>  {
>         void (*loop_fn)(void) = arg;
>
> -       loop_fn();
> +       while (!done)
> +               loop_fn();
>         return NULL;
>  }
>
> @@ -42,7 +44,8 @@ static int thloop(int argc, const char **argv)
>         alarm(sec);
>
>         pthread_create(&th, NULL, thfunc, test_loop);
> -       test_loop();
> +       while (!done)
> +               test_loop();
>         pthread_join(th, NULL);
>
>         return 0;
> --
> 2.47.0.163.g1226f6d8fa-goog
>