[PATCH v2 3/3] perf/sdt: Documentation

Hemant Kumar
2014-07-18T10:20:45Z

This patch adds the required documentation for perf support to SDT markers.

---
tools/perf/Documentation/SDT-markers.txt | 123 ++++++++++++++++++++++++++++++
tools/perf/Documentation/perf-list.txt | 7 +-
2 files changed, 129 insertions(+), 1 deletion(-)
create mode 100644 tools/perf/Documentation/SDT-markers.txt

diff --git a/tools/perf/Documentation/SDT-markers.txt b/tools/perf/Documentation/SDT-markers.txt
new file mode 100644
index 0000000..420ba2e
--- /dev/null
+++ b/tools/perf/Documentation/SDT-markers.txt
@@ -0,0 +1,123 @@
+Support to perf for listing the SDT markers :
+
+This helps in listing dtrace style markers(SDT) present in user space
+applications through perf. Notes/markers are placed at important places by the
+developers. They have a negligible overhead when not enabled.
+We can enable them and probe at these places and find some important information
+like the arguments' values, etc.
+
+How to add SDT markers into user applications:
+We need to have this header sys/sdt.h present.
+sys/sdt.h used is version 3.
+If not present, install systemtap-sdt-devel package (for fedora-18).
+
+A very simple example:
+
+$ cat user_app.c
+
+#include <sys/sdt.h>
+
+void main () {
+ /* ... */
+ /*
+ * user_app is the provider name
+ * test_probe is the marker name
+ */
+ STAP_PROBE(user_app, test_mark);
+ /* ... */
+}
+
+$ gcc user_app.c
+$ perf list sdt ./a.out
+./a.out:
+%user_app:test_mark
+
+A different example to show the same:
+- Create a file with .d extension and mention the probe names in it with
+provider name and marker name.
+
+$ cat probes.d
+provider user_app {
+ probe foo_start();
+ probe fun_start();
+};
+
+- Now create the probes.h and probes.o file :
+$ dtrace -C -h -s probes.d -o probes.h
+$ dtrace -C -G -s probes.d -o probes.o
+
+- A program using the markers:
+
+$ cat user_app.c
+
+#include <stdio.h>
+#include "probes.h"
+
+void foo(void)
+{
+ USER_APP_FOO_START();
+ printf("This is foo\n");
+}
+
+void fun(void)
+{
+ USER_APP_FUN_START();
+ printf("Inside fun\n");
+}
+int main(void)
+{
+ printf("In main\n");
+ foo();
+ fun();
+ return 0;
+}
+- Compile it and also provide probes.o file to linker:
+$ gcc user_app.c probes.o -o user_app
+
+- Now use perf to list the markers in the app:
+# perf list sdt ./user_app
+
+./user_app :
+%user_app:foo_start
+%user_app:fun_start
+
+Also, we can see the SDT markers present in our system in the usual binaries.
+First, scan the binaries using :
+# perf list sdt --scan
+
+Creating a cache of SDT markers...
+perf sdt cache created!
+ Use : "perf list sdt"
+ to see the SDT markers
+
+After the sdt cache file is created, use perf list to view the markers :
+# perf list sdt
+
+%rtld : init_start
+%rtld : init_complete
+%rtld : map_failed
+%rtld : map_start
+%rtld : lll_futex_wake
+...
+...
+%libgcc : unwind
+%libvirt : rpc_server_client_auth_allow
+%libvirt : rpc_server_client_auth_fail
+%libvirt : rpc_server_client_auth_deny
+
+Using "perf list sdt <file-name>" after this will update the entries related to that
+<file-name> in the cache.
+
+Also, this link provides important info regarding SDT notes:
+http://sourceware.org/systemtap/wiki/UserSpaceProbeImplementation
+
+This link shows an example of marker probing with Systemtap:
+https://sourceware.org/systemtap/wiki/AddingUserSpaceProbingToApps
+
+- Markers in binaries :
+These SDT markers are present in the ELF in the section named
+".note.stapsdt".
+Here, the name of the marker, its provider, type, location, base
+address, semaphore address.
+We can retrieve these values using the members name_off and desc_off in
+Nhdr structure. If these are not enabled, they are present in the ELF as nop.
diff --git a/tools/perf/Documentation/perf-list.txt b/tools/perf/Documentation/perf-list.txt
index 6fce6a6..f146e53 100644
--- a/tools/perf/Documentation/perf-list.txt
+++ b/tools/perf/Documentation/perf-list.txt
@@ -8,7 +8,7 @@ perf-list - List all symbolic event types
SYNOPSIS
--------
[verse]
-'perf list' [hw|sw|cache|tracepoint|pmu|event_glob]
+'perf list' [hw|sw|cache|tracepoint|pmu|event_glob|sdt]

DESCRIPTION
-----------
@@ -108,6 +108,11 @@ To limit the list use:

. 'pmu' to print the kernel supplied PMU events.

+. 'sdt' to print the SDT markers present in dsos and binaries. An additional
+ argument of filename will instruct perf to look for SDT markers only in that
+ file. If that file is already present in the cache and if there is any change,
+ then the entries for that file will be updated in the cache.
+
. If none of the above is matched, it will apply the supplied glob to all
events, printing the ones that match.