[PATCH v1 2/2] perf libdw: Optimize fallback logic for missing DWARF

[Ian Rogers <irogers@google.com>]

When libdw encounters a DWARF definition it cannot accurately
un-inline or an address block natively missing structural line maps,
dwfl_module_getsrc() falls back immediately with 0. This causes perf's
addr2line_style loop to fall back to launching the cmd__addr2line()
sub-process wrapper. In environments generating massive numbers of
unmapped traces, this forces thousands of redundant `addr2line -e`
fork/execs in a polling loop.

This patch intercepts failure scenarios:
1. Returns -1 when dwfl_module_getsrc() returns NULL (meaning the DWARF
   is parsed but the line mapping is natively missing).
2. Evaluates `ret < 0` in addr2line() to dynamically bypass the subprocess
   fallback and return 0. This precisely acts as a failure condition,
   incrementing the dso->a2l_fails counter and accurately triggering the
   A2L_FAIL_LIMIT throttling without shelling out.
3. Incorporates exact error string checking for "no DWARF" to safely
   abort alternative fallbacks directly within both libdw resolving and
   the unwinder logic (unwind-libdw).

This preserves fallbacks if unwinder or DWARF parsing genuinely
aborts, but avoids falling back when mappings are absent.

Assisted-by: Antigravity:gemini-3.6-flash
Signed-off-by: Ian Rogers <irogers@google.com>
---
 tools/perf/util/libdw.c        | 11 +++++++++--
 tools/perf/util/srcline.c      |  2 ++
 tools/perf/util/unwind-libdw.c | 15 ++++++++++++---
 3 files changed, 23 insertions(+), 5 deletions(-)

diff --git a/tools/perf/util/libdw.c b/tools/perf/util/libdw.c
index f53caee980b0..7c681b03c654 100644
--- a/tools/perf/util/libdw.c
+++ b/tools/perf/util/libdw.c
@@ -186,8 +186,15 @@ int libdw__addr2line(const char *dso_name, u64 addr, char **file, unsigned int *
 	 * between the regular ELF addr2line addresses and those to use with
 	 * libdw.
 	 */
-	if (!dwfl_module_getdwarf(mod, &bias))
-		return 0;
+	if (!dwfl_module_getdwarf(mod, &bias)) {
+		const char *err = dwfl_errmsg(-1);
+
+		/*
+		 * Abort fallbacks specifically when DWARF is completely missing,
+		 * but allow alternative backends to try if parsing failed.
+		 */
+		return (err && strstr(err, "no DWARF")) ? -1 : 0;
+	}
 
 	/* Find source line information for the address. */
 	dwline = dwfl_module_getsrc(mod, addr + bias);
diff --git a/tools/perf/util/srcline.c b/tools/perf/util/srcline.c
index 8e954f0c860a..8283a741c018 100644
--- a/tools/perf/util/srcline.c
+++ b/tools/perf/util/srcline.c
@@ -176,6 +176,8 @@ static int addr2line(const char *dso_name, u64 addr, char **file, unsigned int *
 		}
 		if (ret > 0)
 			return ret;
+		if (ret < 0)
+			return 0;
 	}
 
 	return 0;
diff --git a/tools/perf/util/unwind-libdw.c b/tools/perf/util/unwind-libdw.c
index 63a5c2253174..69d4585c870c 100644
--- a/tools/perf/util/unwind-libdw.c
+++ b/tools/perf/util/unwind-libdw.c
@@ -441,8 +441,17 @@ int libdw__get_entries(unwind_entry_cb_t cb, void *arg,
 	}
 
  out:
-	if (err)
-		pr_debug("unwind: failed with '%s'\n", dwfl_errmsg(-1));
+	if (err) {
+		const char *msg = dwfl_errmsg(-1);
+
+		pr_debug("unwind: failed with '%s'\n", msg);
+		/*
+		 * Abort fallbacks specifically when unwinding information is completely
+		 * missing, but allow fallback unwinders to try if parsing simply failed.
+		 */
+		if (msg && strstr(msg, "no DWARF"))
+			err = -EINVAL;
+	}
 
 	for (i = 0; i < ui->idx; i++)
 		map_symbol__exit(&ui->entries[i].ms);
@@ -460,6 +469,6 @@ int libdw__get_entries(unwind_entry_cb_t cb, void *arg,
 	 *  < 0 : fatal error (e.g. -ENOMEM). Aborts unwinding entirely.
 	 */
 	if (err)
-		return (err == -ENOMEM) ? -ENOMEM : (entries > 0 ? 1 : 0);
+		return (err == -ENOMEM || err == -EINVAL) ? err : (entries > 0 ? 1 : 0);
 	return entries;
 }
-- 
2.55.0.766.g2966f0265a-goog