From 8c08cdf9fe911489d4cfc7b61dd8ff75f7cb08e5 Mon Sep 17 00:00:00 2001
From: Edward Hervey <edward@centricular.com>
Date: Thu, 28 May 2020 08:06:59 +0200
Subject: [PATCH] gstinfo: Improve usage of libdw for backtraces

When getting backtraces, we were always creating a new Dwfl context and then
discarding it. The problem with that is that it resulted in having to re-scan a
lot of information for every single backtrace.

In order to fix that issue, use a global on-demand Dwfl context and use it with
a lock.

Furthermore, we were scanning the mappings of the
process (dwfl_linux_proc_report) for *every single step* in the backtrace, and
that function is horrendously expensive (does sscanf on /proc/PID/maps
...). While there is a possibility that new mappings might be available (new
plugins being loaded for example), we can limit ourselves to just do it once per
backtrace.

These two modifications speed up the elements_leaks unit test (which traces all
pads with full backtraces) by a factor of 6.

Partially fixes #567

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/504>
---
 gst/gstinfo.c | 50 +++++++++++++++++++++++++++++++++++++-------------
 1 file changed, 37 insertions(+), 13 deletions(-)

diff --git a/gst/gstinfo.c b/gst/gstinfo.c
index 4349c82a22..913fe421ab 100644
--- a/gst/gstinfo.c
+++ b/gst/gstinfo.c
@@ -147,6 +147,28 @@ static char *gst_info_printf_pointer_extension_func (const char *format,
 
 #ifdef HAVE_DW
 #include <elfutils/libdwfl.h>
+static Dwfl *_global_dwfl = NULL;
+static GMutex _dwfl_mutex;
+
+#define GST_DWFL_LOCK() g_mutex_lock(&_dwfl_mutex);
+#define GST_DWFL_UNLOCK() g_mutex_unlock(&_dwfl_mutex);
+
+static Dwfl *
+get_global_dwfl (void)
+{
+  if (g_once_init_enter (&_global_dwfl)) {
+    static Dwfl_Callbacks callbacks = {
+      .find_elf = dwfl_linux_proc_find_elf,
+      .find_debuginfo = dwfl_standard_find_debuginfo,
+    };
+    Dwfl *_dwfl = dwfl_begin (&callbacks);
+    g_mutex_init (&_dwfl_mutex);
+    g_once_init_leave (&_global_dwfl, _dwfl);
+  }
+
+  return _global_dwfl;
+}
+
 #endif /* HAVE_DW */
 #endif /* HAVE_UNWIND */
 
@@ -2800,12 +2822,6 @@ append_debug_info (GString * trace, Dwfl * dwfl, const void *ip)
   Dwfl_Module *module;
   const gchar *function_name;
 
-  if (dwfl_linux_proc_report (dwfl, getpid ()) != 0)
-    return FALSE;
-
-  if (dwfl_report_end (dwfl, NULL, NULL))
-    return FALSE;
-
   addr = (uintptr_t) ip;
   module = dwfl_addrmodule (dwfl, addr);
   function_name = dwfl_module_addrname (module, addr);
@@ -2843,13 +2859,15 @@ generate_unwind_trace (GstStackTraceFlags flags)
 
 #ifdef HAVE_DW
   Dwfl *dwfl = NULL;
-  Dwfl_Callbacks callbacks = {
-    .find_elf = dwfl_linux_proc_find_elf,
-    .find_debuginfo = dwfl_standard_find_debuginfo,
-  };
 
-  if ((flags & GST_STACK_TRACE_SHOW_FULL))
-    dwfl = dwfl_begin (&callbacks);
+  if ((flags & GST_STACK_TRACE_SHOW_FULL)) {
+    dwfl = get_global_dwfl ();
+    if (G_UNLIKELY (dwfl == NULL)) {
+      GST_WARNING ("Failed to initialize dwlf");
+      goto done;
+    }
+    GST_DWFL_LOCK ();
+  }
 #endif /* HAVE_DW */
 
   unret = unw_getcontext (&uc);
@@ -2864,6 +2882,12 @@ generate_unwind_trace (GstStackTraceFlags flags)
 
     goto done;
   }
+#ifdef HAVE_DW
+  /* Due to plugins being loaded, mapping of process might have changed,
+   * so always scan it. */
+  if (dwfl_linux_proc_report (dwfl, getpid ()) != 0)
+    goto done;
+#endif
 
   while (unw_step (&cursor) > 0) {
 #ifdef HAVE_DW
@@ -2897,7 +2921,7 @@ generate_unwind_trace (GstStackTraceFlags flags)
 done:
 #ifdef HAVE_DW
   if (dwfl)
-    dwfl_end (dwfl);
+    GST_DWFL_UNLOCK ();
 #endif
 
   return g_string_free (trace, FALSE);