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

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Message-ID: <ZylaDu6NjJIq6rUf@google.com>
Date: Mon, 4 Nov 2024 15:34:38 -0800
From: Namhyung Kim <namhyung@...nel.org>
To: Ian Rogers <irogers@...gle.com>
Cc: Arnaldo Carvalho de Melo <acme@...nel.org>,
	Kan Liang <kan.liang@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
	linux-perf-users@...r.kernel.org
Subject: Re: [PATCH] perf test: Fix LBR test by adding indirect calls

On Sat, Nov 02, 2024 at 09:58:03PM -0700, Ian Rogers wrote:
> On Sat, Nov 2, 2024 at 5:24 PM Namhyung Kim <namhyung@...nel.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@...nel.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.

Ok, probably we can disbale optimizations in this code like others
in the test workloads.

Thanks for your review!
Namhyung