perf unwind: Don't show unwind error messages when augmenting frame pointer stack

Commit Fixes: b9f6fbb3b2c29736 ("perf arm64: Inject missing frames when
using 'perf record --call-graph=fp'") intended to add a 'best effort'
DWARF unwind that improved the frame pointer stack in most scenarios.

It's expected that the unwind will fail sometimes, but this shouldn't be
reported as an error. It only works when the return address can be
determined from the contents of the link register alone.

Fix the error shown when the unwinder requires extra registers by adding
a new flag that suppresses error messages. This flag is not set in the
normal --call-graph=dwarf unwind mode so that behavior is not changed.

Fixes: b9f6fbb3b2c29736 ("perf arm64: Inject missing frames when using 'perf record --call-graph=fp'")
Reported-by: John Garry <john.garry@huawei.com>
Signed-off-by: James Clark <james.clark@arm.com>
Tested-by: John Garry <john.garry@huawei.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alexandre Truong <alexandre.truong@arm.com>
Cc: German Gomez <german.gomez@arm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20220406145651.1392529-1-james.clark@arm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

1 files changed, 7 insertions, 3 deletions

diff --git a/tools/perf/util/unwind-libdw.c b/tools/perf/util/unwind-libdw.c
index a74b517f7497..94aa40f6e348 100644
--- a/tools/perf/util/unwind-libdw.c
+++ b/tools/perf/util/unwind-libdw.c
@@ -200,7 +200,8 @@ frame_callback(Dwfl_Frame *state, void *arg)
 	bool isactivation;
 
 	if (!dwfl_frame_pc(state, &pc, NULL)) {
-		pr_err("%s", dwfl_errmsg(-1));
+		if (!ui->best_effort)
+			pr_err("%s", dwfl_errmsg(-1));
 		return DWARF_CB_ABORT;
 	}
 
@@ -208,7 +209,8 @@ frame_callback(Dwfl_Frame *state, void *arg)
 	report_module(pc, ui);
 
 	if (!dwfl_frame_pc(state, &pc, &isactivation)) {
-		pr_err("%s", dwfl_errmsg(-1));
+		if (!ui->best_effort)
+			pr_err("%s", dwfl_errmsg(-1));
 		return DWARF_CB_ABORT;
 	}
 
@@ -222,7 +224,8 @@ frame_callback(Dwfl_Frame *state, void *arg)
 int unwind__get_entries(unwind_entry_cb_t cb, void *arg,
 			struct thread *thread,
 			struct perf_sample *data,
-			int max_stack)
+			int max_stack,
+			bool best_effort)
 {
 	struct unwind_info *ui, ui_buf = {
 		.sample		= data,
@@ -231,6 +234,7 @@ int unwind__get_entries(unwind_entry_cb_t cb, void *arg,
 		.cb		= cb,
 		.arg		= arg,
 		.max_stack	= max_stack,
+		.best_effort    = best_effort
 	};
 	Dwarf_Word ip;
 	int err = -EINVAL, i;