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

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

[Ian Rogers]

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@hirez.programming.kicks-ass.net/

Fixes: 9ae1e990f1ab ("perf tools: Remove broken __no_tail_call
       attribute")
---
 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 83638097c3bc..c8ce86bceea8 100644
--- a/tools/perf/tests/dwarf-unwind.c
+++ b/tools/perf/tests/dwarf-unwind.c
@@ -24,6 +24,23 @@
 /* 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,
@@ -95,7 +112,7 @@ 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;
@@ -126,7 +143,7 @@ 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;
@@ -145,7 +162,7 @@ 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;
@@ -164,14 +181,22 @@ 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)
-- 
2.29.2.299.gdc1121823c-goog

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

[Miguel Ojeda]

On Sat, Nov 14, 2020 at 1:08 AM 'Ian Rogers' via Clang Built Linux
<clang-built-linux@googlegroups.com> 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@hirez.programming.kicks-ass.net/
>
> Fixes: 9ae1e990f1ab ("perf tools: Remove broken __no_tail_call
>        attribute")
> ---
>  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 83638097c3bc..c8ce86bceea8 100644
> --- a/tools/perf/tests/dwarf-unwind.c
> +++ b/tools/perf/tests/dwarf-unwind.c
> @@ -24,6 +24,23 @@
>  /* 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.
> + */

It would be nice to put the GCC version rather than the date.

> +#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

I would try avoid this nest of conditions and instead do it like in
`compiler_attributes.h`, i.e. make use of `__has_attribute`
unconditional by making sure it works for all versions/compilers, and
then just:

    #if __has_attribute(disable_tail_calls)
    # define NO_TAIL_CALL_ATTRIBUTE __attribute__((disable_tail_calls))
    # define NO_TAIL_CALL_BARRIER
    #else
    # define NO_TAIL_CALL_ATTRIBUTE
    # define NO_TAIL_CALL_BARRIER __asm__ __volatile__("" : : : "memory");
    #endif

In fact, I think it would be best to simply have a mimic of
`compiler_attributes.h` suitable for `tools/`.

Cheers,
Miguel

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

[Miguel Ojeda]

On Sat, Nov 14, 2020 at 9:14 PM Ian Rogers <irogers@google.com> wrote:
>
> Unfortunately no GCC version actually has this fixed.

Then we can say GCC <= 11 does not support it yet or something like that.

> This seems overly complex and unnecessary.

How is 1 condition more complex than 3 different ones?

Cheers,
Miguel

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

[Miguel Ojeda]

On Mon, Nov 16, 2020 at 7:48 AM Ian Rogers <irogers@google.com> wrote:
>
> GCC [0-9]+ :-) Perhaps just a reference to the GCC bug rather than a date.

That would be very good.

> In linux/compiler_attributes.h add:
> #define __GCC4_has_attribute_disable_tail_calls 0
> to the #ifndef __has_attribute block. We can't do this locally here as after that #include __has_attribute will be defined.

As far as I know, `tools/` use their own `compiler*` files, which is
why I was suggesting creating the equivalent there.

> In terms of lines of code, there's not much difference. Arguably there is a bit more cognitive load from the #include and that disable_tail_call needs the funny handling that's here but won't obviously be hinted at by placing it in a shared header. I'm a little concerned that someone will come across this in shared code and then go and break this test again with well intentioned cleanup.

Fewer lines, fewer conditions :-) The `#include` is hardly important
given kernel developers already know and use compiler attributes in
many places (they are included in the majority of compilation units).

Actually, we can simplify further. The attribute itself should be
pulled from the `compiler_attributes.h` (a `tools/` one, if needed),
and the barrier should likely be the `barrier()` macro (ditto).

Then, we just need:

    #if __has_attribute(disable_tail_calls)
    # define NO_TAIL_CALL_BARRIER
    #else
    # define NO_TAIL_CALL_BARRIER barrier()
    #endif

because using the attribute directly just works -- the only special
thing here is the conditional barrier.

And this conditional barrier should probably be shared, too, defining
it wherever `barrier()` (or equivalent) is defined for `tools/`. And
the name could be `barrier_for_tail_call()` or something like that.

Of course, we don't need to do all this for this patch, but we should
always attempt to minimize/simplify the diffs later on -- that is why
I suggested using the unconditional `__has_attribute` as if it was
already properly defined (we had to untangle similar stuff when I
added `compiler_attributes.h`).

Cheers,
Miguel

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

[Jiri Olsa]

On Fri, Nov 13, 2020 at 04:08:03PM -0800, 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@hirez.programming.kicks-ass.net/
> 
> Fixes: 9ae1e990f1ab ("perf tools: Remove broken __no_tail_call
>        attribute")

missing SOB

LGTM and test is passing for me ;-)

Tested-by: Jiri Olsa <jolsa@redhat.com>

jirka

> ---
>  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 83638097c3bc..c8ce86bceea8 100644
> --- a/tools/perf/tests/dwarf-unwind.c
> +++ b/tools/perf/tests/dwarf-unwind.c
> @@ -24,6 +24,23 @@
>  /* 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,
> @@ -95,7 +112,7 @@ 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;
> @@ -126,7 +143,7 @@ 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;
> @@ -145,7 +162,7 @@ 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;
> @@ -164,14 +181,22 @@ 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)
> -- 
> 2.29.2.299.gdc1121823c-goog
>