[PATCH 2/2] perf probe: Fix offline module name missmatch issue

Ravi Bangoria
2016-04-26T14:27:00Z

Perf can add a probe on kernel module which has not been loaded yet.
Current implementation finds module name from path. But if filename
is different from actual module name then perf fails to register
probe while loading module because of mismatch in names. For example,
samples/kobject/kobject-example.ko is loaded as kobject_example.

Before applying patch:

$ sudo ./perf probe -m /linux/samples/kobject/kobject-example.ko foo_show
Added new event:
probe:foo_show (on foo_show in kobject-example)

You can now use it in all perf tools, such as:

perf record -e probe:foo_show -aR sleep 1

$ cat /sys/kernel/debug/tracing/kprobe_events
p:probe/foo_show kobject-example:foo_show

$ insmod kobject-example.ko

$ lsmod
Module Size Used by
kobject_example 16384 0

Generate read to /sys/kernel/kobject_example/foo while recording data
with below command
$ sudo ./perf record -e probe:foo_show -a
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.093 MB perf.data ]

$./perf report --stdio -F overhead,comm,dso,sym
Error:
The perf.data.old file has no samples!

After applying patch:

$ sudo ./perf probe -m /linux/samples/kobject/kobject-example.ko foo_show
Added new event:
probe:foo_show (on foo_show in kobject_example)

You can now use it in all perf tools, such as:

perf record -e probe:foo_show -aR sleep 1

$ sudo cat /sys/kernel/debug/tracing/kprobe_events
p:probe/foo_show kobject_example:foo_show

$ insmod kobject-example.ko

$ lsmod
Module Size Used by
kobject_example 16384 0

Generate read to /sys/kernel/kobject_example/foo while recording data
with below command
$ sudo ./perf record -e probe:foo_show -a
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.097 MB perf.data (8 samples) ]

$ sudo ./perf report --stdio -F overhead,comm,dso,sym
...
# Samples: 8 of event 'probe:foo_show'
# Event count (approx.): 8
#
# Overhead Command Shared Object Symbol
# ........ ....... ................. ............
#
100.00% cat [kobject_example] [k] foo_show

Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.vnet.ibm.com>
---
tools/perf/util/probe-event.c | 19 +++++--------------
1 file changed, 5 insertions(+), 14 deletions(-)

diff --git a/tools/perf/util/probe-event.c b/tools/perf/util/probe-event.c
index d58de20..26803e1 100644
--- a/tools/perf/util/probe-event.c
+++ b/tools/perf/util/probe-event.c
@@ -642,32 +642,23 @@ static int add_module_to_probe_trace_events(struct probe_trace_event *tevs,
int ntevs, const char *module)
{
int i, ret = 0;
- char *tmp;
+ char *mod_name = NULL;

if (!module)
return 0;

- tmp = strrchr(module, '/');
- if (tmp) {
- /* This is a module path -- get the module name */
- module = strdup(tmp + 1);
- if (!module)
- return -ENOMEM;
- tmp = strchr(module, '.');
- if (tmp)
- *tmp = '\0';
- tmp = (char *)module; /* For free() */
- }
+ mod_name = find_module_name(module);

for (i = 0; i < ntevs; i++) {
- tevs[i].point.module = strdup(module);
+ tevs[i].point.module =
+ strdup(mod_name ? mod_name : module);
if (!tevs[i].point.module) {
ret = -ENOMEM;
break;
}
}

- free(tmp);
+ free(mod_name);
return ret;
}

--
1.9.1