Re: [PATCH v2] perf test: Fix dwarf unwind for optimized builds.

From: Arnaldo Carvalho de Melo
Date: Thu Sep 23 2021 - 09:32:30 EST


Em Thu, Sep 23, 2021 at 12:00:11AM +0200, Jiri Olsa escreveu:
> On Wed, Sep 22, 2021 at 10:38:12AM -0700, Ian Rogers wrote:
> > To ensure the stack frames are on the stack tail calls optimizations
> > need to be inhibited. If your compiler supports an attribute use it,
> > otherwise use an asm volatile barrier.
> >
> > The barrier fix was suggested here:
> > https://lore.kernel.org/lkml/20201028081123.GT2628@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
> > Tested with an optimized clang build and by forcing the asm barrier
> > route with an optimized clang build.
> >
> > A GCC bug tracking a proper disable_tail_calls is:
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97831
> >
> > Fixes: 9ae1e990f1ab ("perf tools: Remove broken __no_tail_call
> > attribute")
> >
> > v2. is a rebase. The original fix patch generated quite a lot of
> > discussion over the right place for the fix:
> > https://lore.kernel.org/lkml/20201114000803.909530-1-irogers@xxxxxxxxxx/
> > The patch reflects my preference of it being near the use, so that
> > future code cleanups don't break this somewhat special usage.
>
> Acked-by: Jiri Olsa <jolsa@xxxxxxxxxx>

Thanks, applied.

- Arnaldo


> thanks,
> jirka
>
> >
> > Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
> > ---
> > tools/perf/tests/dwarf-unwind.c | 39 +++++++++++++++++++++++++++------
> > 1 file changed, 32 insertions(+), 7 deletions(-)
> >
> > diff --git a/tools/perf/tests/dwarf-unwind.c b/tools/perf/tests/dwarf-unwind.c
> > index a288035eb362..c756284b3b13 100644
> > --- a/tools/perf/tests/dwarf-unwind.c
> > +++ b/tools/perf/tests/dwarf-unwind.c
> > @@ -20,6 +20,23 @@
> > /* For bsearch. We try to unwind functions in shared object. */
> > #include <stdlib.h>
> >
> > +/*
> > + * The test will assert frames are on the stack but tail call optimizations lose
> > + * the frame of the caller. Clang can disable this optimization on a called
> > + * function but GCC currently (11/2020) lacks this attribute. The barrier is
> > + * used to inhibit tail calls in these cases.
> > + */
> > +#ifdef __has_attribute
> > +#if __has_attribute(disable_tail_calls)
> > +#define NO_TAIL_CALL_ATTRIBUTE __attribute__((disable_tail_calls))
> > +#define NO_TAIL_CALL_BARRIER
> > +#endif
> > +#endif
> > +#ifndef NO_TAIL_CALL_ATTRIBUTE
> > +#define NO_TAIL_CALL_ATTRIBUTE
> > +#define NO_TAIL_CALL_BARRIER __asm__ __volatile__("" : : : "memory");
> > +#endif
> > +
> > static int mmap_handler(struct perf_tool *tool __maybe_unused,
> > union perf_event *event,
> > struct perf_sample *sample,
> > @@ -91,7 +108,7 @@ static int unwind_entry(struct unwind_entry *entry, void *arg)
> > return strcmp((const char *) symbol, funcs[idx]);
> > }
> >
> > -noinline int test_dwarf_unwind__thread(struct thread *thread)
> > +NO_TAIL_CALL_ATTRIBUTE noinline int test_dwarf_unwind__thread(struct thread *thread)
> > {
> > struct perf_sample sample;
> > unsigned long cnt = 0;
> > @@ -122,7 +139,7 @@ noinline int test_dwarf_unwind__thread(struct thread *thread)
> >
> > static int global_unwind_retval = -INT_MAX;
> >
> > -noinline int test_dwarf_unwind__compare(void *p1, void *p2)
> > +NO_TAIL_CALL_ATTRIBUTE noinline int test_dwarf_unwind__compare(void *p1, void *p2)
> > {
> > /* Any possible value should be 'thread' */
> > struct thread *thread = *(struct thread **)p1;
> > @@ -141,7 +158,7 @@ noinline int test_dwarf_unwind__compare(void *p1, void *p2)
> > return p1 - p2;
> > }
> >
> > -noinline int test_dwarf_unwind__krava_3(struct thread *thread)
> > +NO_TAIL_CALL_ATTRIBUTE noinline int test_dwarf_unwind__krava_3(struct thread *thread)
> > {
> > struct thread *array[2] = {thread, thread};
> > void *fp = &bsearch;
> > @@ -160,14 +177,22 @@ noinline int test_dwarf_unwind__krava_3(struct thread *thread)
> > return global_unwind_retval;
> > }
> >
> > -noinline int test_dwarf_unwind__krava_2(struct thread *thread)
> > +NO_TAIL_CALL_ATTRIBUTE noinline int test_dwarf_unwind__krava_2(struct thread *thread)
> > {
> > - return test_dwarf_unwind__krava_3(thread);
> > + int ret;
> > +
> > + ret = test_dwarf_unwind__krava_3(thread);
> > + NO_TAIL_CALL_BARRIER;
> > + return ret;
> > }
> >
> > -noinline int test_dwarf_unwind__krava_1(struct thread *thread)
> > +NO_TAIL_CALL_ATTRIBUTE noinline int test_dwarf_unwind__krava_1(struct thread *thread)
> > {
> > - return test_dwarf_unwind__krava_2(thread);
> > + int ret;
> > +
> > + ret = test_dwarf_unwind__krava_2(thread);
> > + NO_TAIL_CALL_BARRIER;
> > + return ret;
> > }
> >
> > int test__dwarf_unwind(struct test *test __maybe_unused, int subtest __maybe_unused)
> > --
> > 2.33.0.464.g1972c5931b-goog
> >

--

- Arnaldo